Essentials of Environmental Toxicology

Essentials of Environmental Toxicology

Essentials of Environmental Toxicology

The Effects of Environmentally HazardousSubstances on Human Health

ProfessorSchool of Allied Health Professions

Associate Clinical Professor of Pathology and Human AnatomySchool of Medicine

Loma Linda UniversityLoma Linda, California

W. William Hughes

ESSENTIALS OF ENVIRONMENTAL TOXICOLOGY: The Effects of EnvironmentallyHazardous Substances on Human Health

Copyright © 1996 Taylor & Francis. All rights reserved. Printed in the United States of America. Exceptas permitted under the United States Copyright Act of 1976, no part of this publication may be reproducedor distributed in any form or by any means, or stored in a database or retrieval system, without priorwritten permission of the publisher.

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This book was set in Times Roman by Innodata Publishing Services Division. The editors wereCatherine Simon, Sharon M.Twigg, and Elizabeth Dugger. Figure design by Louise Ceccarelli. Coverdesign by Michelle Fleitz. Interior book design by Bonny Gaston. Printing and binding by The MackPrinting Group.

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Library of Congress Cataloging-in-Publication Data

Hughes, W.William.Essentials of environmental toxicology: the effects of environmentally hazardous substances on

human health/W.William Hughes.p. cm.

1. Environmental toxicology. I.Title.RA1226.H84 1996 96–14283615.9′02—dc20 CIP

ISBN 0-203-36279-9 Master e-book ISBNISBN 0-203-37539-4 (Adobe eReader Format)ISBN 1-56032-469-4 (case)ISBN 1-56032-470-8 (paper)

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To Asa Thoresen…a gentle and great man

vii

Preface xi

Introduction to Environmental Toxicology 1What Is Environmental Toxicology? 3Terminology 3History of Toxicology 4Subdisciplines of Toxicology 7Ecological Concepts 10Relevance of Environmental Toxicology to the Human Species 12Structural Levels of Organization 13Review Questions 16

Toxicological Concepts 19Toxicity 20Toxicokinetics and Toxicodynamics 21Classification of Toxicants 21Determination of Toxicity 23Determining the Doses to Test 25Variables Affecting Toxicity 25Review Questions 26

Dose-Response Relationships 29Dose and Response Relationships 30Dose-Response Graphs 30Statistical Considerations of Dose-Response 32

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Contents

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3

viii Contents

Interpreting Dose-Response Data 36Review Questions 38

Absorption of Toxicants 43Interaction of Toxicants with Cells 45Cell Membrane Structure 45Processes of Cellular Absorption 47Cellular Uptake of Toxicants 48Routes of Absorption 49Review Questions 57

Distribution and Storage of Toxicants 59Distribution of Toxicants 60Factors Affecting Distribution of Toxicants to Tissues 63Storage of Toxicants 65Review Questions 68

Biotransformation and Elimination of Toxicants 71Biotransformation of Toxicants 72Biotransformation Reactions 73Location of Biotransformation Reactions 73Factors Affecting Biotransformation 75Phase I Reactions 76Phase II Reactions 76Elimination of Toxicants 78Additional Routes of Elimination 81Review Questions 82

Target Organ Toxicity 85Introduction to Target Organ Toxicity 87Hematotoxicity 87Hepatotoxicity 89Nephrotoxicity 92Neurotoxicity 94Dermatotoxicity 96

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Contents ix

Pulmonotoxicity 99Other Examples of Target Organ Toxicity 102Review Questions 102

Teratogenesis, Mutagenesis, and Carcinogenesis 105Introduction to Teratogenesis, Mutagenesis, and Carcinogenesis 107Replication 107Transcription and Translation 108Teratogenesis 111Mutagenesis 117Carcinogenesis 120Review Questions 122

Environmental Toxicants 125Introduction to Environmental Toxicants 127Pesticides 127Plastics 133Metals 133Organic Solvents 137Other Environmental Toxicants 139Review Questions 140

Risk Assessment 143Introduction to Risk 144Risk Assessment 144Risk Management 146Safety 146Conclusions 147Review Questions 147

Appendix: Resources in Environmental Toxicology 149Glossary 153Index 165

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The effects of environmentally hazardoussubstances on human health are an im-portant and timely subject. My goal wasto produce an introductory text similarin style to those found in the aged disci-plines such as anatomy, biology, botany,and physiology—a text that emphasizedthe general state of environmental toxi-cology by concentrating on importantprinciples and avoided the controversiesinherent within any discipline (not major-ing in the minors).

This text is written for students enrolledin environmental science and environmen-tally hazardous materials certificate, as-sociate, and baccalaureate degree pro-grams. It has been my experience, havingtaught environmental science and environ-mental toxicology during the past 16years, that these students have diversebackgrounds. The text has been writtenwith an awareness that many of you arejust beginning post-high-school studies,some already have advanced degrees, andothers are making career changes. It isassumed that the reader has a high-schoolfamiliarity with biology and chemistry,but no prior knowledge of environmen-tal toxicology.

Essentials of Environmental Toxicol-ogy takes a superficial look at what inreality is a very large and complex bodyof knowledge. Any endeavor to distill adiscipline into a few hundred pages of textis laden with an enormous responsibilityto avoid misrepresenting data for the sakeof simplification—I take full responsibil-ity for any dogma.

Each chapter begins with a list ofbehavioral objectives and keywords. Thekeywords appear in bold type and aredefined in the text. Chapters are internallyorganized—introductory principles andexamples are given before applications.Figures and tables are designed to illus-trate and organize important conceptspresented in the text. Each chapter con-cludes with review questions that may beused by the student for self-testing.

Thanks to Jonna Hughes (mom), mypersonal and also professional proof-reader, whose capacity to use a red pencilis only exceeded by her kindness to fam-ily and friends. I am indebted to KathrynDowling, PhD, Jon Kindschy, REHS,REA, RHSP, and Dennis Woodland, PhD,for providing scientific reviews. Thankyou to Dr. Joyce Hopp, Dean, and Dr. Edd

Preface

xii Preface

Ashley, Associate Dean, School of AlliedHealth Professions (LLU), for encourage-ment and resources; Jerry Daly, Directorof Media Services (LLU), for facilities andresources, and Louise Ceccarelli, Super-visor of Computer Graphics (LLU), forartistic design and numerous illustrations.Completion of the manuscript was facili-tated by the individual efforts of JanFisher, Shirley Graves, Stacey Hughes,Gunter Reiss, Betsy Pavlick, BeckyPendergrass, Derek Reid, Lynn Steil, andAlan Swarm—thank you!

At Taylor & Francis I thank RichardO’Grady, Acquisitions Editor, for believ-ing in the worthiness of my goal, and forproviding technical support—especiallyprompt Internet replies to my queries;Carolyn Ormes, Development Editor, formidcourse corrections in style and dead-line reminders; Bonny Gaston, Manufac-

turing Manager; Catherine Simon andSharon M.Twigg, Production Editors; andElizabeth Dugger, Copy Editor.

Asa once remarked that, in his nativeNew Zealand, a man was considered suc-cessful if he built a house, had a son, andwrote a book. I built the house; have threewonderful daughters (Stacey, Summer,and Courtney) and a lovely, supportivewife (Marilyn); and wrote the book…henever told me if two out of three counted!However, my goal is achieved if studentswho read this book gain an understand-ing of how toxicants in the environmenthave their ultimate impact on the healthof all organisms in the ecosystem—includ-ing our own species.

Billy HughesLoma Linda [email protected]

1

� Define environmental toxicology

� Describe the prehistory and history oftoxicology

� Distinguish descriptive, mechanistic,and regulatory disciplines oftoxicology

� Recognize the multidisciplinaryapproaches to environmentaltoxicology

� Summarize the relevance ofenvironmental toxicology to thehuman species

antidotesatmospherebiospherecellsclinical toxicologydescriptive toxicologyEbers papyrusecosystemenvironmental toxicologyetymologyforensic toxicologyhazardous wastehydrosphereindustrial toxicologyinfinite dilutionlithospheremacromoleculesmechanistic toxicologymoleculesmorbiditymortalityOrfilaorgan systemorganellesorgansParacelsus

Introduction toEnvironmental

Toxicology

Chapter

1

K eywordsO bjectives

2 Essentials of Environmental Toxicology

phytotoxinpoisonsregulatory toxicologytissuetoxic

toxicantstoxicitytoxinsvenomxenobiotics

K eywords (continued)

Introduction to Environmental Toxicology 3

What is EnvironmentalToxicology?

Environmental toxicology is the study ofthe poisons around us. A general defini-tion of environmental toxicology wouldinclude the hazardous effects that thesepoisons have on human health. Specifically,“environment” comes from the Frenchword environ, which means “around,” andmens is Latin for “mind.” The word “toxi-cology” comes from the Greek wordtoxikon, a poisonous substance into whicharrowheads were dipped, and the suffix -logy from the Greek word logos, whichmeans the study of, or treatise.

Some toxicology terms appear to besimilar but should be used with specificityto allow for accurate communication. Forexample, poisons are substances that inrelatively small doses act to destroy lifeor seriously impair cellular function.There are a variety of poisons, many ofwhich occur naturally in plants and ani-mals or as minerals. There are also man-made poisons, which are the direct resultof laboratory synthesis. Toxins are poi-sonous substances produced by plants(phytotoxins), animals (zootoxins), orbacteria (bacteriotoxins); a substance istoxic when it acts to destroy or impaircellular function. Toxicity is the state ofbeing poisonous. The term venom refersto poisonous substances secreted by cer-tain animals, such as bees, spiders, andsnakes. When substances produce symp-toms that are popularly referred to as in-toxication (or poisoning) they are referredto as toxicants. There are naturally oc-curring toxicants, as well as toxicants thatresult from technological advances involv-ing the manufacture and use of chemicals

in industry and agriculture. Xenobiotics(Greek xenos, a stranger; -biotic, pertain-ing to life) may include substances, suchas toxicants, that are not naturally pro-duced within an organism.

Terminology

Learning the vocabulary of environmen-tal toxicology allows for effective and ac-curate communication. Etymology is thestudy of word origins. The majority of en-vironmental toxicology terms have theirorigins in classical Greek (G.) and Latin(L.). In addition to scientific terminology,these “dead” languages are also the sourceof many common words found in the En-glish language.

Delving into a new discipline, such asenvironmental toxicology, requires thatyou learn the vocabulary along with thedefinitions. A knowledge of how wordsare formed can substantially reduce theamount of time spent in learning themeaning of new words. In general, scien-tific terms may contain three parts: a pre-fix, combining form, and suffix. To de-fine a new term, start on the right anddefine the suffix, then move to the left, orbeginning of the term, and define the pre-fix or combining form. For example, theword phytotoxin is made up of the com-bining form phyto, which comes from theG. phyton, a “plant,” and toxin from theG. toxikon, which means “poison”—phy-totoxin then is “poison from a plant.”

A good scientific or medical dictionaryis an invaluable aid when learning a newdiscipline. If you take the time to learnthe parts of words you will be surprisedat your ability to define new words


without having to look them up in a dic-tionary.

History of Toxicology

The prehistoric use of animal venoms (L.venenum, venom) and plant poisons (L.potio, potion) is evident from archaeologi-cal and cultural anthropological studies.Ancient cultures had a working knowl-edge of many naturally occurring toxinsthat were used as medicinals, in hunting,and for war. Today, there are indigenousnative peoples who still use naturally oc-

curring poisons and toxins for huntingand for medicinal purposes.

One of the oldest written records of theearly use of toxins is a series of eight Egyp-tian papyri dating from 1900–1200 B.C.The Ebers papyrus (Figure 1–1), whichdates from 1500 B.C., contains directionsfor the collection, preparation, and admin-istration of more than 800 medicinal andpoisonous recipes. Some have obviousmedicinal value, such as the use of opiumto alleviate pain. The list also includesnames of many potions of dubious medi-cal value. Of interest to toxicologists are

Figure 1–1. A small portion of the Ebers papyrus. This Egyptian document, writtenabout 1500 B.C., contains the recipes for more than 800 prescriptions. It also describesabout 700 drugs of animal, vegetable, and mineral origin. (English translation courtesyof Dr. Gunter Reiss.)


recipes for poisons like hemlock, whichwas extracted from the dried unripe fruitof the plant Conium maculatum(Figure 1–2).

The use of plant and animal toxins bythe Greeks was common. Dioscorides(A.D. 50–100), a Greek army physicianwho served in the court of Nero, the Ro-

man emperor, is responsible for an earlyattempt to classify poisons. His classifi-cation of more than 600 plant, animal,and mineral poisons as being toxic ortherapeutic is sufficiently valid to still beused today. The Greeks used poisons asthe state method of execution. Socrates,Demosthenes, and Cleopatra were all vic-

Figure 1–2. Poisonous hemlock can be extracted from the dried unripe fruit (circled)of Conium maculatum. (Original artwork by Lynn Steil.)


tims of poisoning, albeit for different rea-sons, including an execution and two sui-cides, respectively. Stories exist that thediscovery of antidotes, which are agentsto neutralize the effects of a poison, wasfacilitated by giving known toxins to con-demned criminals followed by the admin-istration of possible antidotes. Whencriminals survived these potentially lethalexperiments, new antidotes could beadded to the list.

The Romans (A.D. 50–400) made useof poisons for executions and assassina-tions, political or otherwise. By the fourthcentury A.D. the use of poisons hadreached epidemic proportions. DidAgrippina kill Claudius to “clear the way”for Nero to become emperor of Rome? DidNero subsequently kill Brittanicus, Clau-dius’ natural son, with a soup à la arsenic?

The Islamic empire, following the deathof Mohammed in A.D. 632, was stronglyinfluenced by Greek medicine. Avicenna(A.D. 980–1036), a master of many disci-plines, was considered to be an Islamicauthority on poisons and their antidotes.

Ancient Chinese literature containsabundant references to the medicinal valueof numerous plants and the poisonousproperties of certain fish. Emperor ShenNung (ca. 2700 B.C.) is reported to haveexperimented with poisonous as well asmedicinal plants. Indeed, a “cure” for stu-pidity involved the use of poisons fromnewts and salamanders.

Hindu medicine in India from 800 B.C.to A.D. 1000 makes references to poisonsand antidotes, such as for snake bites. Sig-nificant written works on medicine arefound in the Ayurveda. Although the Hin-dus are believed to have borrowed somemedicines from the Greeks, there are in-

dications that the Greeks had medicinalsof known Hindu origin.

Throughout the Middle Ages, poisonswere used to gain political and social, aswell as financial, advantage. In Italy, theBorgia family, including Cesare, his half-sister Lucretia, and their father Pope Al-exander VI reportedly gained wealth as aresult of their timely use of arsenic in wine.The term “lucre” (L. lucrum, gain), origi-nally used to describe riches or wealth, isused now in a humorously derogatorysense. In A.D. 1198, Maimonides, a Span-ish rabbi, wrote Poisons and Their Anti-dotes, which was a first-aid guide to thetreatment of poisonings.

In general, the scholarship of the Mid-dle Ages, from the ninth to fifteenth cen-turies, was based more on dogma thanon empirical evidence. The German phy-sician Paracelsus (1493–1541), a productof the Renaissance, brought the study ofmedicine and science to a new high (Fig-ure 1–3). The role of experimentation, therelationship between dose and therapeu-tic, as compared with toxic, responses tochemicals, and the specificity with whichdifferent doses of chemical agents producewell-defined toxic or therapeutic effectsare included in his writings—“What isthere that is not a poison? All things arepoison and nothing without poison. Solelythe dose determines that a thing is not apoison.” These early contributions formthe basis of what is now the science oftoxicology.

Of particular interest to environmen-tal toxicology are the writings of the Ital-ian physician Ramazzini. Diseases ofWorkers, published in 1713, deals withailments that result from exposure to toxicchemicals in the workplace.


The beginnings of toxicology are usu-ally traced to Orfila (1787–1853). Withhis 1815 book, A General System of Toxi-cology, or, A Treatise on Poisons, Drawnfrom the Mineral, Vegetable, and AnimalKingdoms, Considered as to Their Rela-tions with Physiology, Pathology, andMedical Jurisprudence, this Spanish phy-sician unknowingly established toxicologyas a separate and distinct scientific disci-pline (Figure 1–4).

Subdisciplines of Toxicology

Each day toxicology impacts your life. Atrip to your family physician may resultin the use of chemical agents to aid in di-agnosis, such as contrast agents used to

enhance radiographic images. Or you maybe prescribed a pharmaceutical agent thatwill prevent or treat disease. Excessiveamounts of these substances can pose adanger to your health. The vegetables youeat may contain chemicals, both thoseused initially to promote pollination orgrowth and those added later to prolongshelf life. Animals slaughtered to providemeat may have been treated with chemi-cals to promote growth. If you check un-der the kitchen sink or in the garage youprobably will find pesticides used to killinsects in the garden or molds growingon the shower tile. Each breath of air youtake or glass of water you drink can po-tentially contain toxicants from a varietyof industrial, automotive, agricultural,household, or natural sources.

Figure 1–3. The German physician Paracelsus (1493–1541), whose writings promotedthe essence of modern toxicology. (From Paracelsus: Philosophia Magna. Birckmann,Cologne, 1567.)


Figure 1–4. Title page and a selected Contents page from an 1816 translation of theSpanish physician Orfila’s 1815 book. (Courtesy of Library of the College of Physiciansof Philadelphia.)


Modern toxicology is composed ofthree subdisciplines. The first, descriptivetoxicology, involves toxicity testing ofchemicals. Initially, the determination asto whether or not a chemical is toxic must

be made before safety and regulatory is-sues can be addressed. Toxicity testingusually takes place using experimentalanimals. Second, mechanistic toxicologyexam ines the biochemical processes by

Figure 1–4. (continued)


which identified toxicants have an impacton the organism. Although descriptivetoxicologists continue to identify agentsof toxicity, the exact mechanism by whichmany toxicants have their action on theorganism awaits continued study. Last,regulatory toxicology is concerned withassessing the data from descriptive toxi-cology and mechanistic toxicology in anattempt to determine the legal uses of spe-cific chemicals, as well as the risk posedto the ecosystem by the marketing of thosechemicals.

Many disciplines contribute to an un-derstanding of toxicology (Table 1–1). Ofparticular interest, clinical toxicology ex-amines the effects of toxicants on indi-viduals and the efficacy of treatment forsymptoms related to intoxication. Foren-sic toxicology is concerned with the medi-cal and legal questions relating to theharmful effects of known or suspected

toxicants, and industrial (or occupational)toxicology studies the disorders found inindividuals who have been exposed toharmful materials in their place of work.

The scope of this book is environmen-tal toxicology, which deals with the im-pact of known or suspected toxicants onthe ecosystem, including the humanpopulation…the health hazards posed bythe poisons around us. Although this textfocuses on the effects of toxicants on thehuman species, remember that our eco-system is complex, and potentially allforms of life, both plant and animal, maybe affected by toxic substances.

Ecological Concepts

The biosphere is that region of our planetthat contains living organisms (Figure 1–5). Although planet Earth is quite large,

Table 1–1. Selected disciplines that contribute to a morecomplete understanding of toxicology


the region where life can be found is justa thin veneer on the earth’s surface, usu-ally involving only a few meters of thelithosphere and a few kilometers of theatmosphere and hydrosphere. The ecosys-tem is a self-regulating community of ani-mals and plants interacting with one an-other (biotic interactions) and with theirnonliving environment (abiotic interac-tions). Our ecosystem does not exist in avacuum. Every activity, process, and in-

teraction influences the ecosystem—muchlike the ripples that result when a stone isthrown into a pond go out and disturbthe pond. When biotic or abiotic factorsin the ecosystem are disturbed, they in turnwill influence the interacting populationsto either grow or decline in their num-bers. These are often referred to as posi-tive or negative feedback mechanisms,respectively.

Since time is a fundamental variable to

Figure 1–5. The biosphere includes those regions of the atmosphere, hydrosphere, andlithosphere where living organisms are found.


everything that happens in an ecosys tem,the time-dependent rates for changes tobiotic and abiotic interactions in the eco-system are important questions to con-sider—how fast the “ripples” travel. Ourconcept of time affects the questions weask and the solutions we propose. A veryshort time may involve the instantaneouschanges that last less than seconds, suchas when lightning discharges or chemicalreactions take place. Human time is usu-ally the span of time in which most of usthink, and it is in this unit of time that wemake most of our observations about theecosystem. “Long-term” planning withinthe constraints of the human time framerarely exceeds 20 years. Historical timeinvolves intervals of time that are too longto be studied by individuals. It is depend-ent on records provided by earlier gen-erations. Geological time refers to long-term changes within the ecosystem.Within this time frame the processes thatshape our earth and influence the struc-ture of the ecosystem are measured. Thedrifting of continents, mountain building,and geological cycles of erosion and depo-sition of sediments are all processes thatrequire long periods of time. Then thereis deep time or stellar time—the time inwhich the universe exists, stars begin andend, and planets are formed.

Not surprisingly, our “long-term” plan-ning is at odds with the stability of our eco-system. The changes we precipitate areoccurring at rates that are too fast, toooverwhelming, for an ecosystem shapedover millions of years. The fact that envi-ronmental toxicology has developed intoa significant discipline indicates that wehave exceeded the self-regulatory ability ofour ecosystem. To evaluate the profound

impact we have made on the environment,including the relevance of environmentaltoxicology to our survival as a species, re-quires an understanding of (1) the sourcesof toxicants, (2) environmental cycles thattransport toxicants, (3) the modes by whichthese toxicants enter and affect the humanbody, and ultimately (4) the degree towhich society defines safety and what risksare acceptable as related to our exposureto toxicants.

Relevance of EnvironmentalToxicology to the Human

Species

The human species has had a significantimpact on the ecosystem. Apparently,early humans were better able to coexistwith other animal and plant populations,most likely due to their small populationsize and reduced demands on the ecosys-tem. However, with the recent rapid in-crease in the size of the human popula-tion (including advances in industry andtransportation, and economies based oncontinued growth) our once benign inter-action with the ecosystem has changedinto one where the demand for resources,such as food, water, and habitable space,is exceeding the supply.

An end product of the consumption ofthese resources is a tremendous amountof waste. For many decades infinite dilu-tion was a common solution to the prob-lem of waste disposal. Vast oceans (hydro-sphere), land (lithosphere), and air (atmos-phere) (Figure 1–5) were the “buckets”in which “drops” of potentially toxicwastes were diluted—indeed, waste dis-posal was viewed as just that—a “dropin the bucket.” It was further thought that


the toxic wastes would be of no conse-quence or harm to human health since the“buckets” were so vast. Somehow we for-got that eventually we would be exposedto the cumulative effect of the “drops” oftoxic substances as we eat, breathe, andsleep in the “buckets.”

Hazardous waste is defined as wastethat, because of its biological, chemical,or physical characteristics, or quantity orconcentration, may pose a danger of mor-bidity (disease) and mortality (death) toorganisms (Figure 1–6). To illustrate themagnitude of the problem, in the UnitedStates alone over 4 billion tons of wasteis generated each year from mining, agri-culture, industry, and city sewage sources.On an average, each person contributesover 4 pounds of domestic solid wasteeach day. Of the approximate 274 mil-lion metric tons (1 metric ton=2,200 lb)of this waste, which is subject to regula-tion as hazardous waste by the Environ-mental Protection Agency (EPA), only anestimated 10% is disposed of in an envi-ronmentally safe manner.

With over 5 million natural and man-made chemicals and over 80,000 syntheticchemicals currently being used in industry,agriculture, household, and other applica-tions, the potential for exposure to hazard-ous waste is a concern. The health hazardsfor individuals who are exposed to hazard-ous wastes, as well as other toxic sub-stances, when they are disposed of in anunsafe manner, poses a serious problem.This is especially true when cause-and-ef-fect relationships are established betweencertain wastes and diseases. For example,based on worldwide epidemiological datathe World Health Organization (WHO)estimates that 90–95% of all cancers are

“environmentally related”—an environ-ment we have disturbed is now afflictingour own population with morbidity andmortality.

Although contact with toxic substancesmay come as a result of occupational, ac-cidental, or intentional exposure, there aresome contacts or exposures over whichwe have little or no control. The sayingthat you can run but you can’t hide is cer-tainly true for aspects of the atmosphere,hydrosphere, and lithosphere on which weare dependent for our survival. We mayunknowingly breathe air, drink water, oreat food that was polluted with toxicantshundreds or thousands of miles away.

The data of environmental toxicologyshould prompt us to stop or limit thesources of those substances that threatento harm plant and animal species in theecosystem. As a result of descriptive,mechanistic, and regulatory toxicologicalstudy, we will be better able to provide forthe future of our species. It is with anawareness of these factors that environ-mental toxicology has its relevance to thehuman population.

Structural Levels of Organization

Nature can be organized into different lev-els of structural complexity, from sub-atomic particles to the ecosphere (Figure1–7). Carbon (C), hydrogen (H), oxygen(O), nitrogen (N), calcium (Ca), potassium(K), and sodium (Na) are a few of the at-oms essential to living organisms. Whentwo or more atoms join together, mol-ecules are formed. The molecules may besmall (e.g., amino acids, simple sugars) orthey may combine to form larger mol-ecules called macromolecules, such as


Figure 1–6. From waste to disease. The paths by which wastes, including toxicants,move into the biosphere where they produce morbidity. (From C.E.Kupchella and M.C.Hyland, Environmental Science. Allyn and Bacon, 1989. Reprinted by permission.)


proteins. Cells are composed of complexassemblages of atoms, molecules, andcomplex molecules.

Cells are the basic unit of structure andfunction in a living organism. All of thefunctional or physiological processes inorganisms ultimately take place at thelevel of the cell. Within the cell are small

structures called organelles that carry onspecific activities.

Cells may be organized into units thattogether perform a similar function. Theseassemblages of cells are termed a tissue.There are four distinct tissue types in thehuman body: epithelial tis sue, which cov-ers the body and lines ducts and vessels;

Figure 1–7. Levels of structural and functional organization.

Figure 1–7. Levels of structural and functional organization.


connective (or support) tissue, such asblood, bone, and collagen; muscle tissue,with smooth, skeletal, and cardiac types;and nerve tissue, which includes neurons.

Organs result when groups of differ-ent tissues unite to form structures thatperform a specific function. Two or moreorgans may combine to form an organsystem. For example, the use of food re-sources by the human body is accom-plished by numerous organs (thegastrointestinal system), each of whichfunctions in sequence to permit ingestion(e.g., oral cavity), mechanical and chemi-cal digestion (e.g., stomach), absorptionof nutrients (e.g., intestines), and elimi-nation (e.g., anus).

Toxicants produce toxic effects by in-teracting with the molecules on or nearthe surface of, or within, the cell. The in-teractions in turn cause reversible or irre-versible cellular damage by affecting pro-teins associated with the cell membrane(e.g., receptors), interfering with a cell’senergy production (e.g., metabolism),binding to molecules within the cell (e.g.,enzymes), or causing certain cells to die.Although environmental toxicologistsmay observe the gross effects of atoxicant on the whole organism, remem-ber that the cumulative effect of the dis-ruption of structure and function at thelevel of the cell is ultimately responsiblefor organi-smal morbidity and mortality.

1. A general definition of this term would include a study of the hazardous effectsthat the poisons around us have on human health:

A. Descriptive toxicologyB. Environmental toxicologyC. Forensic toxicologyD. Mechanistic toxicologyE. Regulatory toxicology

2. Which term has its origin in the Greek word for “stranger”?

A. PoisonB. ToxinC. ToxicantD. VenomE. Xenobiotic

3. This Greek physician served in the Roman emperor Nero’s court and is responsiblefor classifying more than 600 plant, animal, and mineral poisons as being toxic ortherapeutic.

A. AvicennaB. Dioscorides

Review Questions


C. MaimonidesD. RamazziniE. Shen Nung

4. Identify the source of this statement: “All things are poison and nothing withoutpoison. Solely the dose determines that a thing is not a poison.”

A. The AyurvedaB. The Ebers papyrusC. MaimonidesD. OrfilaE. Paracelsus

5. Which area of toxicology is concerned with assessing the risk involved in themarketing of chemicals and their legal uses?

A. Descriptive toxicologyB. Forensic toxicologyC. Industrial toxicologyD. Mechanistic toxicologyE. Regulatory toxicology

6. “Long-term” planning within the constraints of the human time frame rarely exceeds20 years.

A. TrueB. False

7. It is estimated that 10% of hazardous wastes are disposed of in an environmentallysafe manner.

A. TrueB. False

8. In the human body, physiological processes ultimately take place at which level?

A. CellB. TissueC. OrganD. Organ systemE. Organism

9. List five disciplines that contribute to a more complete understanding of toxicology.

10. Construct a diagram that shows the relationship between hazardous wastes, andmorbidity and mortality.

19

Chapter

2


Toxicological Concepts

� Define toxicity

� Discuss the different types of toxicity

� Describe toxicokinetics andtoxicodynamics

� Explain how toxicants are classified

� Outline the steps involved in toxicitytesting

absorptionacute toxicitybiotransformationchronic toxicitydelayed toxicitydistributioneliminationend effectimmediate toxicityin vitroin vivolinear dose sequencelocal toxicitylogarithmic dose sequencestoragesystemic toxicitytoxicity testingtoxicodynamicstoxicokinetics


Toxicity

Toxicity, the state of being poisonous, isalso a general term used to indicate adverseeffects or symptoms produced by poisonsor toxicants in organisms. Toxicity willvary according to both the duration andlocation of exposure to the toxicant, as wellas the species-specific responses of the or-ganism. Four distinct types of toxicity char-acterize the duration and location of thepoisonous state. Acute toxicity involves asudden onset of symptoms that last for ashort period of time, usually less than 24hours. The cellular damage that producesthe symptoms associated with acute tox-icity is usually reversible, resulting in re-covery by the organism from the adverseeffects brought on by the toxicant. Chronictoxicity results in symptoms that are of along, continuous duration. The permanentnature of chronic toxicity is due to the ir-reversible cellular changes that have oc-curred in the organism. If cellular destruc-tion and the related loss of function are se-vere, the organism may die.

Local toxicity occurs when the symp-toms are restricted to the site of initialexposure to the toxicant. However, whenthe adverse effects occur at sites far re-moved from the initial site of exposurethe term systemic toxicity is used. Theability for toxicants to be absorbed at onesite and distributed to a distant region,such as an organ, results from transpor-tation within the organism via the bloodor lymphatic circulatory systems.

Exposure to carbon tetrachloride(CCl4), an organic solvent used in indus-try, provides an example of the differenttypes of toxicity. At high concentrationfor a short period of time, exposure to

carbon tetrachloride vapors may result intoxicity and could involve minor eye andthroat irritations. Upon cessation of thisshort-term, acute exposure to the vapors,the symptoms associated with this localtoxicity will stop. However, if the expo-sure to carbon tetrachloride again involvesa short period of time, but the toxicant isnow absorbed through the skin or oraland ocular mucosa, it may enter thebloodstream and be transported via theblood to the brain. Once in the brain thetoxicant produces symptoms, such as de-pression of the central nervous system(CNS), that may result in loss of con-sciousness. If the duration of exposure isshort or acute, then cellular damage isreversible. Repeated exposure to high con-centrations of either the vapor or liquidforms of carbon tetrachloride is capableof producing chronic and systemic toxic-ity, which is irreversible. Pathologies as-sociated with chronic toxicity include kid-ney and liver damage, as well as severeCNS depression, which can lead to death.

Toxicity is also classified according tothe timing between exposure to thetoxicant and the first appearance of symp-toms associated with toxicity. Immediatetoxicity results when the symptoms oc-cur rapidly within seconds or minutes fol-lowing exposure to the toxicant. Withimmediate toxicity, the relationship be-tween causative agents or toxicants andthe pathologic symptoms or toxicity ismore easily established. However, sometoxicants may take years to produce tox-icity. This delayed toxicity adds to the dif-ficulty in establishing the cause-and-effectrelationship. For example, diethy-lstilbestrol (DES) is a nonsteroidal drugprescribed for women during pregnancy

Toxicological Concepts 21

to prevent miscarriage. It is now knownthat daughters born to mothers who tookDES are at risk for developing vaginal andcervical cancers during adolescence. In thisexample the timing between in utero ex-posure to the toxicant and the first ap-pearance of symptoms associated withtoxicity may exceed 10 years.

It is important to recognize that toxic-ity results from exposure to specifictoxicants and that the terms acute andchronic may also be used to describe theduration of exposure (e.g., acute exposureand chronic exposure). Evidence showsthat acute and chronic exposure to manytoxicants will parallel acute and chronictoxicity; however, it should be emphasizedthat in some cases acute exposure can leadto chronic toxicity. Finally, the terms thatdescribe the type of toxicity may be usedin combination, depending on the dura-tion and location of toxicity and the tim-ing between toxicant exposure and toxic-ity. An understanding of toxicity (i.e.,acute, local, and immediate as comparedto chronic, systemic, and delayed) shouldassist in characterizing the effects thattoxicants have on organisms.

Toxicokinetics andToxicodynamics

Toxicokinetics is the study of five time-de-pendent processes related to toxicants asthey interact with living organisms. Theseprocesses are: absorption, how toxicantsenter the organism; distribution, how toxi-cants travel within the organism; storage,how some tissues preferentially harbor atoxicant; biotransformation, how toxi-cants are altered (or detoxified) by chemi-cal changes in the organism; and elimina-

tion, how toxicants are removed from theorganism (Figure 2–1). An understandingof the time-dependent behavior of a toxi-cant as related to its absorption, distribu-tion, storage, biotransformation, and elimi-nation is necessary to explain how toxi-cants are capable of producing local or sys-temic toxicity, acute or chronic toxicity,and immediate or delayed toxicity.

Toxicodynamics examines the mecha-nisms by which toxicants produce uniquecellular effects within the organism (Fig-ure 2–1). As expected, if toxicants exerttheir influence at the level of the cell, themechanisms will involve cellular compo-nents. Included in the mechanisms of toxicaction are alterations to the cell’s plasmamembrane, organelles, nucleus, cyto-plasm, enzyme systems, biosynthetic path-ways, development, or reproduction.Whether reversible or irreversible cellu-lar injury occurs will depend on the dura-tion of exposure as well as the specifictoxicokinetic properties of the toxicant.

Classification of Toxicants

Many classification schemes for toxicagents have been proposed (Table 2–1).Dioscorides classified substances using thegeneral characteristics of whether theywere toxic or therapeutic. Additionally,the source of the toxicant has long beenrecognized as a means of classification.An early scheme by Orfila classified sub-stances as being of animal, vegetable, ormineral origin. No single classificationsystem can be expected to adequately dis-tinguish all known toxicants. As moredata related to toxicants becomesavailable there will undoubtedly be morecharacteristics that can be used for


classification purposes. It is important topay careful attention in the selection andpresentation of a classification system orcombination of systems so they will be in-formative and appropriate for the in-tended audience. For example, a highly

technical mechanism of action classifica-tion may be meaningless to a general non-scientific audience. Ultimately, the valueof any classification system is its abilityto adequately convey comparative, rela-tive, or absolute information.

Figure 2–1. Simplified schematic representation of the toxicokinetic and toxicodynamicprocesses that connect exposure to a toxicant to the resulting toxicity.

Table 2–1. Some of the ways commonly usedto classify toxicants


Determination of Toxicity

Since toxicity is the state of being poison-ous, it is imperative that cause-and-effectrelationships between substances (in thiscase, suspected toxicants and the result-ant toxicity) be established. The descrip-tive toxicologist’s role is to identify andestablish the cause—effect or, more spe-cifically, the toxicant—toxicity relation-ship. Toxicity then is determined when,on the administration of a substance, anobservable and well-defined end effect isidentified. Paracelsus recognized the valueof cause-and-effect relationships and thespecificity with which different doses ofchemical agents produced well-definedtoxic or therapeutic effects.

Toxicity testing involves four steps. First,a test organism must be selected. Plants oranimals can be used. Algae, bacteria, mice,rats, rabbits, or nonhuman primates areoften selected as the test organisms. In vivo(in life) studies use the whole organism fortoxicity testing. Humans, for moral andethical reasons which are culturally de-fined, are normally not chosen as the testorganisms. Current in vitro (in glass or testtube) studies do not use the whole organ-ism but instead make use of cultured cellsor tissue cultures, providing an attractivealternative in terms of cost and ethics toin vivo toxicity testing. It is important toconsider the applicability of the test organ-ism as related to generalizations that willundoubtedly result from the toxicity test;that is, will the conclusions based on aplant, bacterium, or rat provide the rel-evant information as related to humans?

Second, the response (end effect) to beobserved and recorded must be selected.The response needs to be easily observable

and quantifiable. Of the many possibleresponses, some that are commonly usedinclude changes in the total number of cellsin a bacterial colony, the presence or ab-sence of biochemical products produced bycultured cells, changes in cell morphology,number of tumors produced, alterations insleep patterns, and changes in growth anddevelopment of an organism (Figure 2–2).For in vivo studies, the death of the experi-mental organism is the end effect.

Third, a selection of the duration of thetest or exposure period is necessary (Ta-ble 2–2). The duration may range from afew seconds to years, depending on thetype of test being performed. Eye irritanttests may only take a few seconds, whereasreproductive studies may take years, par-ticularly when multiple generations areexamined.

Fourth, doses to be tested are selected.For in vivo studies the dose is expressedas the weight in milligrams (mg) of the sub-stance being tested per kilogram (kg) ofbody weight of the experimental organism.This is written as mg/kg. Although a state-ment of absolute amounts of the substancebeing tested may appear to be a better wayof quantifying the dose, it is not, since tox-icity is related to the size of the organism.For example, 100 mg in a 250-g rat is verydifferent from 100 mg in a 5,000-g mon-key (Table 2–3). For in vitro toxicity test-ing, the weight in milligrams (mg) of thesubstance being tested per milliliter (mL)of medium containing the cells expressesthe dose, written as mg/mL.

Once the test organism, responses tomonitor, exposure period, and series ofdoses to test have been selected, toxicitytesting can proceed. Only after the selec-tion of these parameters and subsequent


Figure 2–2. Examples of responses observed and measured during toxicity testing: (A)growth and differentiation of cells in tissue culture, (B) quantitative and qualitativechanges in urine and blood, (C) formation of tumors, and (D) construction of webs byspiders.

Table 2–2. Selected descriptive toxicity tests as requiredfor U.S. registration and estimated costs

aCosts will vary depending on replications, duration of test, etc.


experimentation can the relationship be-tween dose and response be established andthe genius of Paracelsus’ statement be ap-preciated: “All substances are poisons;there is none which is not a poison. Theright dose differentiates a poison and a rem-edy.” Everything—water, oxygen, andchocolate included—may be considered atoxicant if the dose is great enough.

Determining the Doses to Test

The ability to accurately determine whichdoses are responsible for producing a spe-cific end result is critical. If the selecteddoses all produced the predetermined re-sponse, then questions relating to the mini-mum dose required to produce the selectedresponse cannot be answered. This is ofparticular concern when toxicity testing isdone to determine the minimum dose re-quired to produce a response. For this rea-son the series of doses selected usually willbe in a logarithmic dose sequence insteadof a linear dose sequence. For example, in-stead of testing linear doses of 1, 2, 3, 4,5, 6, 7, 8, 9, and 10 mg/kg, the logarith-mic dose sequence of 0.01, 0.1, 1.0, 10,

100, and 1,000 mg/kg will be tested. Loga-rithmic doses have an advantage over lin-ear doses in their ability to maximize therange of doses being tested, while minimiz-ing the possibility of overlooking a smalldose that may represent the responsethreshold or minimum dose at which theend effect will first be observed. Estimatesof the range of doses to test for a suspectedtoxicant are often made from previousexperience, including toxicity test resultsfrom similar chemical substances, or arange-finding subchronic study.

Variables Affecting Toxicity

A toxicant’s ability to interact with cellu-lar structures to produce morbidity andmortality depends on both the toxicantbeing tested and the chosen experimentalorganism. Characteristics of the toxicantinclude the intrinsic factors of the chem-istry of the toxicant itself, as well as theconcentration of the toxicant. The chem-istry of the toxicant is usually well definedand limited in a chemical sense—that is,the toxicant will behave in a predictablemanner.

Table 2–3. A comparison of weight, dosage, and dose for selectedanimals used for in vivo toxicity testing (454 g=1 lb)


In contrast to the toxicant’s predictablechemical behavior, the organism’s re-sponse to the toxicant will depend on anumber of variables related to the ratesof absorption, distribution, storage, bi-otransformation, and elimination. Thesevariables are different for each species.Species specificity is evidenced in the ef-fects of thalidomide, a sedative drug usedin humans in the late 1950s to treat morn-ing sickness associated with pregnancy, toproduce the same toxicity in different or-ganisms. This drug is capable of crossingthe placental barrier to produce congeni-tal malformations of fetal limbs, such asamelia (absence of limbs) and phocome-lia (presence of seal-like flippers). How-ever, not all mammalian species areequally affected by the drug, as seen in

mice and rats, which are resistant, andhamsters and rabbits, which show vari-able effects.

In addition to species differences, theremay be gender differences, with males andfemales exhibiting different responses toa toxicant. Age plays a role, as evidencedby different responses to toxicants byyoung, middle-aged, and older individu-als. Nutritional status, particularly a lackof essential vitamins and minerals, can leadto impaired cellular function and rendercells vulnerable to toxicants. Disease statesmay also affect the organism’s interactionwith toxicants. Finally, time of day of ex-posure may be important, as hormones andenzyme levels are known to fluctuate dur-ing the course of a day (i.e., circadianrhythms).

Review Questions

1. Which is not a correct statement about acute toxicity?

A. Involves a sudden onset of symptoms.B. Symptoms typically last less than 24 hours.C. Results in cellular damage that is irreversible.D. The organism usually recovers from the adverse effects.E. May involve local or systemic toxic responses.

2. With this form of toxicity it is often difficult to establish a cause-and-effectrelationship:

A. Acute toxicityB. Delayed toxicityC. Immediate toxicityD. Local toxicityE. Systemic toxicity

3. Toxicokinetics is the study of all but which one of the following?

A. AbsorptionB. Biotransformation


C. DistributionD. EliminationE. Mechanism of toxicity

4. Of the many ways commonly used to classify toxicants, the categories of stimulants,inhibitors, and blockers best characterize which classification scheme?

A. ChemistryB. GeneralC. Poisoning potentialD. Mechanism of actionE. Target organ

5. Which one of the following represents a correct quantification of a dose as used inan in vivo toxicity test?

A. 10 mgB. 10 mg/animalC. 10 mg/kgD. 10 mLE. 10 mg/mL

6. Which represents a logarithmic dose sequence?

A. 1, 2, 3, 4, 5, 6, 7 mg/kgB. 1, 5, 10, 15, 20, 25 mg/kgC. 10, 20, 30, 40, 50 mg/kgD. 0.1, 1, 10, 100, 1,000 mg/kgE. 0.1, 0.2, 0.3, 0.4, 0.5 mg/kg

7. _____________ examines the mechanism by which a toxicant has its unique cellulareffect within the organism.

8. Toxicity is determined when, on the administration of a substance, an observableand well-defined _____________ is identified.

9. Why is the correct selection of a test organism critical to a toxicity test?

10. Discuss variables (e.g., age, gender, disease states) affecting toxicity and toxicitytesting.

29

Chapter

3Dose-ResponseRelationships


� Explain the difference between causaland associative relationships

� Discuss the role of epidemiology inestablishing associative relationships

� Describe the relationship between doseand response

� Interpret frequency and cumulativedose-response curves

� Recognize subthreshold, threshold, andceiling effect doses

� Summarize effective, toxic, and lethaldoses

� Define potency, efficacy, mixed orreversed toxicity, and margin of safety

associative relationshipscausal relationshipsceiling effectcumulative dose-responsegraphdose-response relationshipED50

effective dose (ED)efficacyepidemiological studiesfrequency dose-response graphLD50

lethal dose (LD)margin of safetymixed toxicityNo Observable Effect Level

(NOEL)normal distributionpotencypotentreversed toxicitysubthreshold dosesTD50

threshold doseThreshold Limit Value (TLV)toxic dose (TD)


Dose and ResponseRelationships

A dose-response relationship exists whena consistent mathematical relationship de-scribes the proportion of test organismsresponding to a specific dose for a givenexposure period. Although a dose-re-sponse relationship may seem easy to es-tablish, a number of assumptions needconsideration.

The first assumption is that the observedresponse is caused by the substance admin-istered. A causal relationship must be es-tablished between the dose administeredand the observed response. It should beremembered that causal relationships arevery different from associative relation-ships. Direct cause-and-effect linkages in-volving single variables, rather than asso-ciative linkages with two or more variables,are needed to establish the dose-responserelationship. Retrospective epidemiologi-cal studies (i.e., prevalence of disease anddeath in a population) often conclude thatan “associative” relationship exists be-tween two observations. This may promptefforts to establish an exact causal relation-ship, but a dose-response relationship can-not be concluded until a definitive causalrelationship is demonstrated. Medicalrecords (e.g., death certificates) are invalu-able aids in establishing linkages betweenspecific diseases or causes of death and theirassociative relationship to numerous vari-ables (Figure 3–1).

Second, the magnitude of the responseis assumed to be directly related to themagnitude of the dose. This assumptiongoes beyond the first assumption that theobserved response is caused by the sub-stance administered. In this assumption, a

mechanism of action is proposed that in-volves the cell and the myriad of moleculeswith which the substance being tested caninteract to produce the observed response.It is assumed that there is a relationshipbetween the dose administered and theeventual concentration of the substance asit interacts at the level of the cell. A descrip-tive toxicologist can usually determine thatthe first assumption is correct for a giventoxicant; however, the mechanistic toxi-cologist ascertains the correctness of thissecond and more difficult assumption.

The third dose-response relationshipassumption states that it is possible to cor-rectly observe and measure a response.The ability to define and observepathologies associated with toxicity isdependent on the depth of understandingof cellular anatomy and physiology. It isimpossible to select and subsequentlymeasure responses for which related cel-lular structures and processes are un-known. This assumption stresses the valueof the contributions from basic scienceresearch. Disciplines such as cytology, ge-netics, molecular biology, and cellularphysiology establish the cellular structuraland functional norms against whichpathologies resulting from toxicity can becompared.

Dose-Response Graphs

The graphic presentation of dose and re-sponse data permits an environmentaltoxicologist to readily determine impor-tant dose-response relationships. Further-more, the graphs enable differenttoxicants to be compared. In a dose-re-sponse graph the horizontal axis (X axisor abscissa) always represents the dose in

Dose-Response Relationships 31

Figure 3–1. Death certificates provide an important source of information used in ret-rospective epidemiological studies.


a logarithmic scale using mg/kg units(Figure 3–2A and B). The vertical axis (Yaxis or ordinate) represents the in vivo orin vitro response. Proper labeling, whichindicates the response being measured andthe unit used for the series of doses beingtested, is necessary for accurate commu-nication of the dose-response relationship.To avoid errors associated with misread-ing fractional doses, decimal points shouldbe preceded by a zero (e.g., 0.1 mg/kg,instead of .1 mg/kg).

The “response” axis in a dose-responsegraph may be presented as a frequency-response or a cumulative-response. A fre-quency dose-response graph plots the per-centage of organisms responding to agiven dose (Figure 3–2A). These graphscan usually be recognized by their “bell-shaped” appearance. The cumulativedose-response graph represents the cumu-lative sum of responses from lower tohigher doses (Figure 3–2B). The frequencyor percentage of organisms responding tothe lowest dose is added to the percent-age responding to the next dose, which isthen added to the third dose, and so on.The line on these graphs appears “sigmoi-dal,” a name that comes from the Greekletter sigma, “s.” Cumulative dose-response graphs are often seen in toxicitystudies related to environmentaltoxicology.

Statistical Consideration ofDoes-Response

Any data that results from measurementson a sample (statistic) has a sampling dis-tribution. It is assumed that the data gen-erated by dose-response experimentation

will follow a Gaussian or normal distri-bution. When a normal distribution ispresent, the resulting frequency-responsegraph will appear bell-shaped, whereas thecumulative-response graph will appearsigmoidal. Although the lines on these twographs appear to be different, in realitythey are just different graphic presenta-tions of the same data.

Responses observed in actual test or-ganisms are assumed to be representativeof the total or universal population ofpotential test organisms. The validity ofthis assumption is questionable whensmall numbers of test organisms are used.On the other hand, it is not cost-effectiveto perform dose-response experimentswhen large numbers of test organisms areused. Somewhere between too many andtoo few test organisms a decision must bemade as to the minimum number of testorganisms (N) needed to establish a sta-tistically valid dose-response conclusion.To make this decision requires informa-tion about the response variability withinthe population of potential test organisms,the desired statistical strength of any con-clusions that may result, and availableresources (including cost of tests, time fortesting, available personnel, and labora-tory space). By paying careful attentionto research design, toxicologists can avoidthe erroneous conclusions that may resultfrom the use of too few test organismsduring toxicity testing.

When the response measurements arenormally distributed it is observed thatthe greatest number (frequency) of testorganisms will exhibit the response at adose somewhere between the lowest andhighest doses tested. This is visually evi-dent in the apex of the bell-shaped line


Figure 3–2. Dose-response graphs: (A) frequency-response graph and (B) cumulative-response graph showing subthreshold dose (STh), threshold dose (Th), and ceilingeffect.


on a frequency dose-response graph orin the middle flat region of the line on acumulative dose-response graph. A spe-cific point located in these regions repre-sents the mean (X

–) or average response

and is equal to the sum of all responses(�X) divided by the number of responses(N), or X

–= �X/N

When the responses of test organismsfollow a normal distribution there are al-ways a few organisms in which the pre-determined response will occur at a verylow dose, as well as a few organisms thatwill not exhibit the response until a veryhigh dose is given. These “supersensi-tive” or “hypersusceptible” and“hearty” or “resistant” test organismsare graphically represented on the sidesof the bell and at the beginning and endof the sigmoidal line, respectively. Thedistance of these “outliers” from themean (average) response is best stated byuse of a statistic called the standard de-viation (SD). A large or small SD value isable to convey valuable informationabout the dose-response relationship in atest population.

For starters, ±1 SD accounts for 67%of test organism responses, ±2 SD ac-counts for 95%, and ±3 SD represents99% of test organism responses. Assumethat the mean response is constant fortwo toxicants. If the SD for toxicant A isvery large and the SD for toxicant B isvery small, it can be concluded that thereis a wide range of doses over which thetest organisms responded to toxicant Aas compared with the small range ofdoses over which the test organisms re-sponded to toxicant B (Figure 3–3).Numbers such as the mean and standarddeviation are useful; however, changes in

the shape of the bell-shaped line or in thesigmoidal line of the cumulative dose-re-sponse graph allow for rapid visual char-acterization and comparison oftoxicants.

Three features characterize the sigmoi-dal line on a cumulative dose-responsegraph. First, there is a dose at which thefirst test organism will respond (Figure 3–2B). This is referred to as the thresholddose, which can be seen on the graph asthe left-side beginning of the sigmoidalline. Subthreshold doses are representedto the left of this point. At these doses noresponses were observed. The followingare often used to refer to this beginningregion of the cumulative dose-responsegraphs: No Observable Effects Level(NOEL), No Observable Adverse EffectLevel (NOAEL), Suggested No AdverseResponse Level (SNARL), Lowest Observ-able Effect Limit (LOEL), and ThresholdLimit Value (TLV).

At progressively higher doses, the ini-tially curved sigmoidal line begins tostraighten out (Figure 3–2B). This secondregion of the graph represents the dosesat which the majority of test organismswere observed to exhibit the response. Ofinterest is the cumulative 50% level, rep-resenting the dose at which the mean re-sponse occurred. Third, the right side ofthe line on a cumulative dose-responsegraph may be seen to once again curveand then become almost horizontal or flat(Figure 3–2B). This region represents thehigher doses at which the remaining fewtest organisms finally exhibited the pre-determined end effect. This region is saidto exhibit the ceiling effect, since an in-crease in dose produces little or no in-crease in response.


Figure 3–3. (A) Frequency dose-response graph and (B) cumulative dose-response graphshowing data from two toxicity studies that have the same mean doses (X–) but differentdistributions (i.e., standard deviations).


The cumulative 100% level is indicatedon the right side, at the point where thegraph line stops.

Interpreting Dose-Response Data

Depending on the observable response se-lected for the toxicity test, dosages maybe characterized as being effective, toxic,or lethal. An effective dose (ED) is evi-dent when the desirable response is ob-served at the dose tested. A toxic dose(TD) represents the dose at which toxic-ity is present in test organisms. When le-thality is the selected response, a lethaldose (LD) represents the dose resulting inthe death of the test organism. LD is al-ways used when lethality is selected as theobservable response, even though lethal-ity is in itself a response and could be re-ferred to as ED. Note that therapeuticdose, although used in pharmacology, isconsidered an effective dose and shouldbe signified by ED, not TD.

To facilitate the interpretation of datafrom a single dose-response study or whencomparing data from two or more dose-response studies, it is useful to examine thedose at which a specified cumulative per-centage of test organisms exhibit the ED,TD, or LD. A numerical subscript is addedto denote the cumulative percentage of thetest organisms that exhibit the predeter-mined response. For example, the ED50,TD50, or LD50 is indicative of the dose atwhich 50% of the test organisms wereobserved to exhibit the effective, toxic, orlethal response. Although 50% is oftenused for comparing toxicity, other cumu-lative percentages are also used such asED99, TD10, or LD01. Remember, valid com-

parisons between dose-response data fromtwo or more toxicity studies require thatthe same cumulative percentage be used.In other words, don’t compare the dose atwhich an LD01 occurs for toxicant A withthe dose at which an LD50 occurs fortoxicant B.

Determining relative toxicity is impor-tant. When comparing two toxicants, theone with the smaller ED50, TD50, or LD50

is considered to be the more potent (Fig-ure 3–4A). This means that the observedresponse occurred at a lower dose, or asmaller dose of toxicant A as compared totoxicant B produced the same response.Potency then is a relative concept for com-paring toxicants and, provided that thesame response cumulative percentages areused (e.g., ED01, ED05, or ED99), statementsabout potency are informative and may beused to classify toxicants. Of additionalvalue is the term efficacy. A toxicant is saidto have a higher efficacy when the dose-response relationship continues over agreater range of doses (Figure 3–4B). Sometoxicants are capable of evoking a responsein 100% of the test organisms over a shortrange of doses; however, other toxicantswill continue to produce responses at evenhigher doses.

On occasion, the sigmoidal lines on cu-mulative dose-response graphs from twoor more toxicity studies will be seen to in-tersect or cross, revealing a mixed or re-versed toxicity relationship (Figure 3–4C).This occurs when one toxicant is not con-sistently more potent over the range ofdoses tested as compared to anothertoxicant—the dose-response curves cross.For example, the LD10 for toxicant A oc-curs at a higher dose than for toxicant B,but the LD50 for toxicant A is lower than


Figure 3–4. Toxicity relationships. (A) Potency: toxicant A is more potent than toxi-cant B; (B) efficacy: toxicant B is more efficant than toxicant A; (C) mixed or reversedtoxicity: comparing the LD10’s, toxicant B is more potent than toxicant A, but whencomparing LD50’s, toxicant A is more potent than toxicant B; and (D) margin of safety:determining the ratio of effective doses to lethal doses, for example, LD01/ED99 or LD50/ED50.


for toxicant B. Mixed toxicity relationshipsare not detected if only a single cumula-tive dose-response percentage is used forcomparison (e.g., ED01, TD10, or LD50). Forthis reason it is of value to examine cumu-lative dose-response data and graphs overthe entire range of test doses.

Margin of safety expresses the magni-tude of the range of doses between anoneffective or minimally effective dose(e.g., NOEL or LOEL) and a lethal dose(LD) (Figure 3–4D). The margin of safetyis determined from the results of two tox-icity studies, such as an ED study and anLD study. Dose-response data from the EDstudy is valuable as it will indicate the doseat which minor, acute, or reversible signsof toxicity are produced and thereby

establish a threshold dose. Next, TD or LDdose-response studies are performed toestablish the doses at which toxicity maybe potentially chronic, irreversible, or le-thal. A ratio between selected LD and EDvalues is used to express the margin ofsafety, such as LD01/ED99 or TD50/ED50. Thelarger the ratio, the greater the margin ofsafety. These ratios are useful in determin-ing what constitutes an acceptable environ-mental exposure. For pharmaceuticals, alarge LD01/ED99 ratio is also desirable, sinceit indicates that the therapeutic value of adrug can be obtained at relatively low dosesas compared to the much higher doses atwhich the drug will be lethal. This is im-portant, especially when the potential foroverdose is a concern.

Review Questions

1. Type of relationship that exists when a constant mathematical relationship describesthe proportion of test organisms responding to a specific dose for a given exposureperiod:

A. AssociativeB. Dose-responseC. EpidemiologicalD. ExposureE. Toxicity

2. Which is not a true statement about epidemiology?

A. It is of value in determining associative linkages.B. It involves study of the prevalence of disease and death in a population.C. It may involve retrospective studies.D. It is used to establish dose-response relationships.E. It involves the study of two or more variables.

3. Identify the one false statement about cumulative dose-response graphs:

A. The graphs permit different toxicants to be compared.B. The horizontal axis represents the dose.


C. The vertical axis represents the cumulative responses.D. The graphs typically have a “bell-shaped” appearance.E. When writing fractional doses the decimal should be preceded by a zero (i.e., 0.1).

4. When examining a frequency dose-response graph, where would you find “hardy”or “resistant” test organisms?

A. At doses represented by NOEL.B. At the center of the bell-shaped curve at the mean dose.C. At subthreshold doses.D. At threshold doses.E. On the extreme right side where the graph line stops.

5. On comparing two chemicals it is noticed that chemical A has a smaller LD50 thanchemical B. This means that:

A. A has a greater efficacy than B.B. A and B exhibit a mixed toxicity relationship.C. B is safer than A.D. The therapeutic value of A is greater than B.E. A is more potent than B.

6. Approximately what percentage of a normally distributed population would beincluded in ±2 SD?

A. 2%B. 33%C. 67%D. 95%E. 99%

7. The dose at which three-fourths of the test organisms were observed to exhibittoxicity:

A. LD25

B. LD75

C. LD50

D. TD25

E. TD75

8. List and briefly describe the assumptions related to establishing a dose-responserelationship.

9. Define and diagram on the following graph the terms and abbreviations: LD10,LD50, threshold dose, and NOEL.


10. Toxicant A has already been plotted on the following graph. Plot data for toxicantB on the graph, then answer the following questions.

A. Based on the LD10, which toxicant is more potent?B. Based on the LD50, which toxicant is more potent?C. At what dose do these two toxicants have the same percentage lethality?


43

Chapter

4

� Describe the ways in which toxicantsinteract with cells

� Recognize how the molecularcharacteristics of toxicants affectentrance into a cell

� Explain human anatomy as related tointegumentary, respiratory, anddigestive systems

� Summarize integumentary, respiratory,and digestive routes of toxicantabsorption

absorptionactive transportadenosine triphosphate (ATP)alveolar regioncell membraneconcentration gradientdigestive systemendocytosisepithelial cellsepitheliumexocytosisfacilitated diffusionhydrophilichydrophobicintegumentary systemlipid solublelipophilicminute volume respiration(MVR)mucociliary escalatornasopharyngeal regionnonpolaroccluding cell junctionsOverton’s Rulespartition coefficientpercutaneous

eywordsKO bjectives

Absorption ofToxicants


phagocytosisphospholipidphospholipid bilayerpinocytosispneumocytes

Poiseuille’s Lawpolar moleculesrespiratory systemsemipermeable membranesimple diffusiontracheobronchial region


Absorption of Toxicants 45

Interaction of Toxicantswith Cells

In general, toxicants exert their effectswhen they interact with cells. This cellu-lar interaction may occur: (1) on the sur-face of the cell, (2) within the cell, or (3)in the underlying tissues and extracellu-lar (interstitial) space (Figure 4–1). Al-though there are four different types oftissues in the human body (epithelial, con-nective, muscle, and nerve), it is a collec-tion of epithelial cells that forms the epi-thelium that covers the surface of the body(i.e., skin), and forms the lining of the lu-men (or inside walls) of the respiratoryand digestive systems.

Chemical characteristics of both thetoxicant and cell membrane determinewhether any interaction occurs on thesurface of the cell or whether the barrierwill be effective in keeping the toxicantout of the organism. Under normal con-ditions, the contacts between adjacentepithelial cells will not permit the passageof substances. This is due to the presenceof occluding cell junctions, which areformed by intramembranous proteins ar-ranged to “stitch” the membrane of adja-cent cells together.

Cell Membrane Structure

A fundamental question in biology is:How do substances get into and out of acell? Since the cell is the ultimate level atwhich toxicants have their impact, theanswer to this question is relevant: notonly for toxicants, but also for substancesneeded for cell survival, such as nutrients(glucose), gases (O2, CO2), electrolytes

(Na+, K+, Cl-), and molecules produced bythe cell for export (enzymes, hormones,structural proteins).

A knowledge of the structure of epi-thelial cells, particularly the characteris-tics of the cell membrane, or plasma mem-brane, is important when considering whysome toxicants move through the barrierwith relative ease while other toxicantsfind entrance into the body difficult orimpossible.

The cell membrane is composed ofphospholipid molecules (Figure 4–2). Asthe term phospholipid implies, there aretwo components to the molecule: phos-phates and lipids. The phosphate head isa region that is hydrophilic. This meansthat this portion of the molecule prefersassociating with water (hydro-, water; -philic, attraction for or love of). In con-trast, the lipid tail is a hydrophobic re-gion, which is repelled by water. Addition-ally, this region is said to be lipophilic, orattractive to lipid-soluble substances.

The cell membrane is like a sandwich,composed of two layers of phospholipidmolecules. A typical cell membrane isabout 10 nm (10 one-billionths of a me-ter) thick. The term phospholipid bilayerprovides a good description of the appear-ance of the “sandwich.” In the middle ofthe phospholipid bilayer is a region whereadjacent lipid tails are located. The phos-phate heads are found on the inner andouter surfaces of the cell membrane wherethese regions are exposed to water, theubiquitous solvent found in living organ-isms. The phospholipid bilayer forms asemipermeable membrane that surroundsthe cell. The membrane is termed semi-permeable since it permits some moleculesto move across, while at the same time


Figure 4–1. A typical cell, showing organelles, cytoplasm, and nucleus, and the siteswhere toxicants have their impact. (From L.C.Junqueira, J.Carneiro, and R.O.Kelly,Basic Histology, 7th edition. Appleton & Lange, 1992. Reprinted by permission.)


stopping or impeding the progress of othermolecules.

Process of Cellular Absorption

Substances use a number of different pas-sive (spontaneous) and active (energy-re-quiring) transport mechanisms to gain en-trance into a cell. The most commonlyused process is simple diffusion. In thisprocess the molecule relies on its concen-tration gradient to enter the cell. This pro-cess is passive, as opposed to active, sinceno cellular energy is used to “power” thetoxicant across the cell membrane. Re-member, diffusion is the movement of asubstance (in this case a toxicant) from aregion of high concentration into a regionof low concentration and, depending onthe direction of the concentration gradi-ent, substances will continue to move intoor out of a cell until equilibrium isreached. In the absence of a concentra-

tion gradient, no net movement of sub-stances will occur.

A second process used to cross the cellmembrane is facilitated diffusion. In thisprocess molecules become bound to spe-cific carrier proteins found on the outersurface of the cell membrane. The moleculeis then passed, or passively transported, bythe membrane protein into the cell. The“energy” for transport is derived from thepotential energy stored in the concentra-tion gradient, not from cell energy input.Facilitated diffusion is a well-knownmechanism in the transport of nutrients,such as glucose, across the cell membrane.Facilitated diffusion is thought to play onlya minor role in the transport of toxicantsinto the cell; however, it does serve as animportant transport mechanism for theelimination of toxicants or theirmetabolites from the cell following absorp-tion via other mechanisms.

Third, active transport, as a means toenter the cell, involves the consumption of

Figure 4–2. Cell membrane structure. (From B.Davey and T.Halliday, editors, HumanBiology and Health: An Evolutionary Approach. Open University Press, 1994. Reprintedby permission.)


cellularly-produced energy, such as adeno-sine triphosphate (ATP). Whereas passiveand facilitated diffusion make good use ofconcentration gradients, active transportenables the cell to transport moleculesagainst, or up, their concentration gradi-ent. Although not a major route of toxicantentry into the cell, active transport, likefacilitated diffusion, does play a vital rolein the elimination of toxicants or theirmetabolic intermediates from a cell.

It is impossible for many large mol-ecules and particulates to go into or leavethe cell via passive or active transportmechanisms. Instead, these macromol-ecules enter and exit the cell by two dif-ferent processes called endocytosis andexocytosis, respectively. During endocy-tosis the cell membrane will flow aroundand engulf the macromolecules that arein close proximity to the cell. Once en-gulfed, the paniculate, now with its cellmembrane covering, will invaginate orturn inward to form a vesicle. The vesiclewill detach from the adjacent cell mem-brane and become part of the cytoplasm.Phagocytosis (cellular eating) and pino-cytosis (cellular drinking) are two typesof endocytosis. Phagocytosis, performedby special white blood cells, is responsi-ble for removing particulates from thesmall sacs (alveoli) in the lung.

Cellular Uptake of Toxicants

Two features characterize toxicants thatmake use of simple diffusion to enter a cell.First, they are nonpolar or lipid soluble.Nonpolar means that they have a neutralmolecular charge distribution, unlike po-lar molecules, which have a positive ornegative charge. And lipid soluble indicates

that the toxicant will dissolve in lipids, suchas would be found in the middle of thephospholipid bilayer. Second, they havelow molecular weights—that is, they aresmall, usually with a molecular weight ofless than 600 (MW<600). There is an in-verse relationship between the molecularweight of a chemical and its ability to movethrough the cell membrane. Typically, anonpolar, lipid-soluble toxicant will diffuseacross a cell membrane much more rap-idly than a polar, water-soluble toxicant ofthe same size.

Overton’s Rules, as applied to toxicants,summarize the general relationship be-tween polarity and solubility: (1) the per-meability of cell membranes to small,nonpolar molecules is directly proportionalto the lipid solubility of the toxicant; and(2) the permeability of cell membranes topolar molecules is inversely proportionalto the molecular size of the solute. Water,with its small molecular size and high po-larity, is an obvious exception to these rulessince it readily crosses the cell membrane.

It is often useful when comparing theabsorptive behavior of toxicants to deter-mine their relative solubility in lipids andalso in water. This is referred to as thepartition coefficient, and is defined as theratio of the toxicant’s solubility in anonpolar solvent, such as chloroform(CHCl3), hexane (C6H14), or octanol(C8H17OH), to its solubility in the ultimatepolar solvent, water (H2O).

Recognize that toxicants don’t alwaysenter a cell to exert their toxic influence.Many toxicants are capable of interact-ing with other molecules associated withthe outer cell membrane surface, such asreceptor proteins, recognition proteins,channel proteins, transport proteins, and


electron transfer proteins (Figure 4–2).Those toxicants that do enter the cell canpotentially interact with a number of dif-ferent cellular components, including thenucleus, organelles, cytoskeletal proteins,and the cytoplasm, to produce functionaland structural anomalies that lead to tox-icity (Table 4–1).

Routes of Absorption

Absorption is the process by which toxi-cants cross the epithelial cell barrier. De-pending on the nature of the toxicant,dose, duration, and type of exposure, atoxicant may limit its contact to the outersurface of the epithelial cell barrier, orcross the cell membrane, enter the cell, andpossibly move completely through the celland into the underlying lymphatic or car-diovascular divisions of the circulatory

system. There are three primary routes ofabsorption: (1) percutaneous (integumen-tary system), or through the skin; (2) therespiratory system; and (3) the digestivesystem. Under accidental circumstanceswhere there is an unnatural interruptionto the integrity of the barrier (e.g., lacera-tions, punctures, chemical or electricalburns), absorption can also take place inthe exposed tissues found beneath the epi-thelium, such as subcutaneous fat andmuscle.

Each route of absorption has its ownspecial type of epithelial cells that unite toform specific tissues. These unique cell andtissue characteristics present the potentialtoxicant with a different set of structuraland functional features that must be over-come to gain entrance into the body. Thereare also route-specific structures, such ashair follicles in the skin, that actually fa-cilitate the absorption of some toxicants.

Table 4–1. Major cellular structures and their functions


An awareness of anatomical and physi-ological characteristics associated witheach route of absorption is important as afirst step in understanding how toxicantsenter the body.

Percutaneous Route

The integumentary system is the largestorgan system in the human body. Al-though the skin is the most obvious or-gan of the integumentary system, this sys-tem also includes hair, fingernails and toe-nails, and mammary glands. Skin playsan important role to (1) provide a barrieragainst the entrance of toxicants, (2) pro-tect against the harmful effects of ultra-violet radiation, (3) prevent the entranceof microorganisms, (4) assist in thebiotransformation or metabolic detoxifi-cation of toxicants, (5) eliminate toxicantsor their metabolites via sweat or otherglandular secretions, (6) regulate bodytemperature, and (7) house sensory recep-tors for temperature, pressure, and pain.

The complexity of skin becomes appar-ent when you consider that each squarecentimeter (cm2) of skin contains approxi-mately 150 nerve endings, 80 sweatglands, 40 sensory receptors, and 15 oilglands, all of which require a supply ofblood provided by a meter’s length ofsmall blood vessels.

Skin is composed of the epidermis, der-mis, and hypodermis (Figure 4–3). Theepidermis is an outer protective region inwhich the pigment layer (melanocytes),stratum germinativum, and stratum cor-neum are located. The stratum corneumis typically 15–20 cells thick but will varyaccording to race, age, sex, physical stateof individual, climatic changes, and other

factors. New epithelial cells arise in the stra-tum germinativum and migrate outwardto become the stratum corneum. Togetherthese two cell zones form stratified squa-mous epithelium. The hardened or kerati-nized stratum corneum, along with extra-cellular lipids, provides the main protec-tive barrier to water loss or entrance andto toxicant entrance.

The dermis is composed of connectivetissue that is highly vascular (having nu-merous blood vessels). It is in the dermisthat oil and sweat glands, hair follicles,and sensory receptors are found. The un-derlying hypodermis is composed of con-nective tissue and adipose tissue, whereabout half of the body’s fat storage isfound. This fatty region is often the siteof lipid-soluble toxicant storage.

The combined thickness of the epider-mal and dermal zones varies in differentareas of the body. In areas that receive con-stant abrasion, such as the elbows, knees,and palms, skin may be 1 mm thick,whereas in less exposed areas like the eye-lid and antecubital space (in front of theelbow), skin is only 0.2 mm thick.

Several routes of absorption are possi-ble through the skin. The most commonis the cutaneous adsorption of a toxicantfollowed by passive diffusion through theepidermis into the dermis where thetoxicant might enter a blood vessel. Pas-sage into the dermis is enhanced if thetoxicant enters a sweat gland or hair folli-cle. Since these structures originate in thedermis and penetrate through the epider-mis, this route effectively bypasses the pro-tective barrier provided by the epidermis.

How quickly a toxicant diffusesthrough the epidermis is affected by anumber of factors, including dose, length


of exposure, lipid solubility, and skin lo-cation. Toxicants that are small, nonpo-lar, and lipid-soluble will diffuse most rap-idly. Diffusion is also accelerated when theskin has been pretreated with organic sol-vents, such as chloroform (CHCl3),methanol (CH3OH), or dimethylsulfoxide ([CH3]2SO). Enhanced epider-mal permeability is thought to result fromthe removal of extracellular lipids by thesesolvents.

Finally, when toxicants become local-ized in the epidermis, local toxicity, ratherthan systemic toxicity, is the likely result.This is because the epidermis is avascular

(having no blood vessels). Without atransport mechanism, toxicants cannot bedistributed to other areas of the bodywhere systemic toxicity may result.

Respiratory System Route

The respiratory system is composed of thenasopharyngeal, tracheobronchial, andpulmonary anatomical regions (Figure 4–4). Each region contributes a unique func-tional component that prohibits or limitsthe ability of toxicants to enter the body.In addition to the tissue type previouslynoted in the epidermis of the skin (i.e.,

Figure 4–3. Diagram of human skin. (From M.C.Willis, Medical Terminology: TheLanguage of Health Care. Williams & Wilkins, 1996. Reprinted by permission.)


stratified squamous epithelial tissue), thelumen of the respiratory system also con-tains ciliated columnar epithelium andciliated cuboidal epithelium, the appear-ance of which resembles the names. Un-like in skin, the stratified squamous epi-thelium in the respiratory system isnonkeratinized, and it becomes less fre-

quent farther down the respiratory pas-sageways. Due to its nonkeratinized form,the stratified squamous epithelium in therespiratory system is less effective in serv-ing as a barrier. Mucus-secreting cells,smooth muscle, cartilage, and immunecells can be found in specific regions ofthe respiratory system.

Figure 4–4. Diagram of the (A) respiratory system and (B) alveolus. (From V.C. Scanlonand T.Sanders, Essentials of Anatomy and Physiology, 2nd edition. F.A. Davis, 1995.Reprinted by permission.)


Anatomically, the nasopharyngealregion includes the nares (nostrils), na-sopharynx, oropharynx, laryngophar-ynx,and larynx. It is in this region that air firstenters the respiratory system. As air movesthrough the nasopharynx it is cleaned,humidified, and thermally adjusted. Hairsand mucus in this region are effective intrapping particulates greater than 5 µm, indiameter, which prevents them from enter-ing the lower regions of the respiratorysystem.

The tracheobronchial region is com-posed of the trachea (or windpipe), bron-chi (singular bronchus), and bronchioles.The two bronchi result from the initial bi-furcation (split) of the trachea. Thebronchioles represent additional bifurca-tions, with the smallest bronchioles beingthe result of about 16 separate bifurcations.These highly branched (bifurcated) andnarrow air passageways increase the avail-able surface area upon which toxicants caninteract. Mucus on the luminal surfaces ofthese cells is effective in trapping smallparticulates (2–5 µm in diameter) and wa-ter-soluble toxicant gases.

In the tracheobronchial region the im-portance of the cilia on the columnar epi-thelial cells becomes evident. Each cell maycontain over 200 cilia and each individualcilium is only 6–10 µm long. With coordi-nated motion these cilia move to “sweep”the mucus, along with trapped particulatesand gases, up and away from the delicatealveolar tissues where gas exchange takesplace. This combined action of the mucusand cilia is called the mucociliary escala-tor. If a toxicant impairs the normal func-tioning of the mucociliary escalator, sub-stances trapped in the mucus will no longerbe transported out of the lower regions of

the respiratory system. The consequenceswill be prolonged exposure of the epithe-lium to toxicants, accumulation of mucusin the respiratory passageways (which de-creases the cross-sectional area), and pos-sibly impaired function of the alveoli. Keepin mind that small changes in the cross-sec-tional area of the respiratory passagewaysleading to the alveolar region profoundlyreduce the flow of air (i.e., to the 4thpower), as defined by Poiseuille’s Law:

In the alveolar region, small terminal bron-chioles give rise to respiratory bronchiolesand their associated alveoli (singular alveo-lus), which continue to bifurcate an addi-tional six or seven times. The end result is400–1, 200 million alveoli in healthy adulthuman lungs. Although the additional bi-furcations do not decrease the cross-sec-tional area of the passageways, they do sig-nificantly increase the surface area acrosswhich gas exchange takes place to about70–80 m2. It is possible for non-water-soluble gases to reach the alveoli, as willparticulates less than 1µm in diameter.Once in the alveoli the toxicant may initi-ate an immune response involving phago-cytizing white blood cells, interact with thesurface of, or enter, special lung cells calledpneumocytes, or pass through pneum-ocytes to enter the cardiovascular systemor interstitial (extracellular) space.

In addition to factors such as chemicalcharacteristics of the toxicant, dose, andlength of exposure, the amount of atoxicant that can enter the body using therespiratory route will depend on aparameter unique to the respiratory


system—the minute volume respiration(MVR). The MVR is the tidal volume(amount of air breathed in on each respi-ratory cycle) multiplied by the respirationsper minute. For example, under normalconditions about 0.5 L is inhaled with eachrespiration and under resting conditions anormal adult will breathe 12 times perminute, in which case the MVR=0.5 L/resp×12 resp/min=6.0 L/min. Under con-ditions of physical exercise the MVR canrise significantly; this in turn will increasecontact of the toxicant with the respiratorytissues.

The respiratory system, with its closeanatomical and physiological associationwith the cardiovascular system, is one ofthe prime sites for the absorption and dis-tribution of toxicants. The cells lining thelumina of the respiratory system are highlysusceptible to toxicants, both particulatesand gases. Pneumocytes, which form thedelicate alveoli, are capable of rapidlytransporting toxicants directly into thepulmonary blood circulation for distribu-tion to the rest of the body.

Digestive System RouteThe digestive system includes the mouth,oral cavity, esophagus, stomach, small in-testine, large intestine, rectum, and anus,as well as accessory organs such as the pan-creas and liver (Figure 4–5A). Four distinctzones are found in the digestive system: (1)mucosa, (2) submucosa, (3) muscularis,and (4) serosa (Figure 4–5B and C). De-pending on the location in the digestive sys-tem, the mucosa lining the lumen can be atough, abrasion-resistant, nonkeratinized,stratified squamous epithelium (as in themouth and esophagus) or a simple colum-nar epithelium (as in the small intestine),

which functions well in absorption and se-cretion. The mucosa is avascular and insome regions (small intestine) has numer-ous projections called villi, each of whichwill have about 2,500 microvilli protrud-ing from its surface (Figure 4–5D). Again,as previously evidenced in the highlybranched alveolar region of the lung, thevilli and microvilli serve to increase the ab-sorptive surface area of the small intestine.

The submucosa contains abundantblood vessels, lymphatics, and nerves. Inthis region toxicants that have enteredthrough the mucosal barrier can enter theblood supply to be transported to otherbody regions. The muscularis is the thirdlayer inward from the luminal surface. Thislayer contains the involuntary (or smooth)muscle that produces the rhythmicperistaltic contractions that mix and movefood through the digestive system. Theoutermost covering of the digestive tubeis called the serosa. Composed of fibrousconnective (or collagenous) tissue, the se-rosa serves to encase the other three zones.

Absorption can take place across themucosal lining anywhere along the entirelength of the digestive system. However,the time food and potential toxicants arein the mouth and esophagus is usually tooshort to be a major site of toxicant entry.The stomach, where food may remain forabout 2 hours, is the site where mechani-cal digestion occurs and where hydrochlo-ric acid (HCl), gastric enzymes, and bac-teria help to chemically break down food.

Most absorption of food and toxicantstakes place in the small intestine. Pinocy-tosis is one way substances can enter thesystemic circulation via the lymph vessels,which are abundant in the submucosa thatunderlies the mucosa. This route of ab-

55

Figure 4–5. Diagram of the digestive system (A), and general anatomic structures asviewed in cross section of (B). (From V.C.Scanlon and T.Sanders, Essentials of Anatomyand Physiology, 2nd edition. F.A.Davis, 1995. Reprinted by permission.)


Figure 4–5. Diagram of the digestive system (A), and general anatomic structures asviewed in cross section of (B). (From V.C.Scanlon and T.Sanders, Essentials of Anatomyand Physiology, 2nd edition. F.A.Davis, 1995. Reprinted by permission.)


sorption is not as direct as it may seem.Toxicants must enter and move throughthe mucosal epithelium and continue intothe submucosa, where they enter the lym-phatic system. Lymph is then transportedthrough lymph ducts and vessels up to theregion of the heart, where two large lymphvessels, the common thoracic duct andright lymphatic duct, drain into the car-diovascular system near the aorta. Simi-lar lymphatic transport may also occur

with the integumentary and respiratoryroutes of absorption.

The large intestine is the final region ofthe digestive system. This region lacks villiand is not considered a major site for ab-sorption of toxicants. However, this regiondoes function to remove liquid from chyme(food that is mixed with digestive “juices”such as HCl and enzymes), thereby pro-ducing the more solid form of indigestiblewaste called feces.

Review Questions

1. Where do toxicants exert their effects in the body?

A. On the surface of cell membranes.B. Within cells.C. In spaces between cells.D. A and BE. A, B, and C

2. All of the following are true statements about cell membranes except:

A. They are composed of phospholipid molecules.B. The phosphate “head” is the hydrophilic region.C. The lipid tails are found on the inner and outer surfaces of the phospholipid bilayer.D. The middle of the phospholipid bilayer is lipophilic.E. The phospholipid bilayer forms a semipermeable membrane.

3. Which transport mechanism relies only on a concentration gradient to enter thecell?

A. Active transportB. EndocytosisC. PhagocytosisD. PinocytosisE. Simple diffusion

4. Defined as the ratio of the toxicant’s solubility in a nonpolar solvent to its solubilityin water.

A. AbsorptionB. Concentration gradientC. Facilitated diffusion


D. Overton’ s RulesE. Partition coefficient

5. Diffusion of a toxicant through the epidermis is affected by:

A. Length of exposureB. Lipid solubility of toxicantC. Skin locationD. A and BE. A, B, and C

6. Which is not a true statement about the tracheobronchial region of the respiratorysystem?

A. It is composed of trachea, bronchi, bronchioles, and alveoli.B. Columnar epithelial cells lining this region contain cilia.C. Mucus in this region is effective in trapping small particulates (2–5 µm in diameter).D. It is characterized by the mucociliary escalator.E. The smallest of bronchioles in this region are the result of about 16 separate

bifurcations.

7. Which route of entry involves crossing the mucosal zone prior to entering thesubmucosal zone, which contains abundant blood vessels, lymphatics, and nerves?

A. Digestive system routeB. Percutaneous routeC. Respiratory system routeD. A and BE. A and C

8. The absorption of toxicants by the respiratory system is affected by:

A. Chemical characteristics of the toxicantB. Length of exposureC. Minute volume respirationD. A and BE. A, B, and C

9. Diagram the anatomical structures associated with the route of absorption in eachof the following: percutaneous, respiratory system, and digestive system.

10. What molecular characteristics would the ideal toxicant possess to maximizeentrance into the body?

59

Chapter

5


Distribution andStorage of Toxicants

� Identify the ways toxicants aredistributed in the body

� Recognize the relationship between aspecific route of absorption and therelated pathways for distribution of atoxicant

� Describe the factors affectingdistribution of toxicants to tissues

� Define volume of distribution

� List the sites for toxicant storage

� Discuss how storage influences thehalf-life of a toxicant

adipose tissuealbuminarterial vesselsblood flowblood flow/mass ratioblood plasmablood-brain barriercapillariescardiac outputdistributionerythrocytesheartinterstitial fluidintracellular fluidleukocyteslymphlymph capillarieslymph nodeslymphatic systemlymphocyteslymphoid tissueplacental barrierplasma proteinplateletsportal veinstoragevenous vesselsvolume of distribution (VD)


Distribution of Toxicants

Following exposure, the fate of a toxicantin the body will be affected by many pro-cesses. Recall that toxicokinetics is thestudy of five processes: (1) absorption, (2)distribution, (3) storage, (4) biotransfor-mation, and (5) elimination.

Distribution occurs when a toxicant isabsorbed and subsequently enters thelymph (L. lympha, clear water) or bloodsupply for transport to other regions of thebody. The lymphatic system is a part of thecirculatory system and drains excess fluidfrom the tissues (Figure 5–1). Included inthe lymphatic system are lymph capillar-ies, lymph nodes, aggregations of lymphoidtissue (tonsils, spleen, and thymus), andcirculating lymphocytes (one of the fivedifferent types of white blood cells).

The cardiovascular part of the circula-tory system includes the heart, arterial ves-sels, venous vessels, capillaries, and the cir-culating medium called blood (Figure 5–2). Blood is composed of three cellularcomponents: (1) erythrocytes (red bloodcells or RBCs), (2) leukocytes (white bloodcells or WBCs), and (3) platelets(thrombocytes), all of which are suspendedin a yellowish, noncellular fluid calledblood plasma. Although both the lym-phatic and cardiovascular circulatory sys-tems are capable of distributing toxicants,the blood in the cardiovascular system isresponsible for most transport.

A number of features must be consid-ered when determining whether or not atoxicant will be distributed to tissues indistant regions of the body. These featuresinclude duration of exposure, dose, thechemical characteristics of the toxicant,and the presence of lymphatic or blood

vascular components. The precise locationwhere the toxicant enters the bloodstreamis important because once a toxicant gainsentrance into the organism, the othertoxicokinetic processes—such as storage,biotransformation, and elimination—willaffect the concentration of the toxicantin the lymph or blood. When thesetoxicokinetic processes occur soon afterthe entrance of the toxicant into the bloodsupply, or immediately “downstream”from the point of entry, then blood levelsof the toxicant may be diminished oreliminated, thus reducing or eliminatingtoxicity.

For example, toxicants absorbedthrough the digestive system may enter thecardiovascular circulatory system directlyor they may take a circuitous route in-volving lymphatic circulation. Toxicantsthat directly enter the bloodstream in thehighly vascularized submucosa of the di-gestive system are initially transported bythe portal vein to the liver, a primary sitefor biotransformation or metabolic de-toxication. Blood from the liver is trans-ported to the heart, where it first entersthe pulmonary circulation and then ispumped to the systemic circulation.

Toxicants that enter the lymphatics as-sociated with the digestive system movewith the lymph through afferent (prenodal)lymph vessels to lymph nodes, then on toefferent (postnodal) lymph vessels andlarger vessels called lymph trunks. The twolymph trunks, right and left, ultimatelyempty into the venous blood supply asso-ciated with the right and left jugular veins,respectively, then return to the heart to bepumped through the systemic circulation.

Toxicants absorbed through the respi-ratory system may enter directly into the

Distribution and Storage of Toxicants 61

pulmonary circulation and, along withoxygen, may be transported from thelungs to the heart via the pulmonary vein.The heart pumps the toxicant and oxy-genated blood into the systemic circula-tory pathway, where it is distributed tothe tissues of the body. Other inhaled toxi-cants, such as particulates, move into thespaces between cells, where they are trans-ported at a slow pace along with thelymph in lymphatic vessels.

Finally, toxicants absorbed throughthe skin may enter the peripheral bloodsupply for distribution to tissues far re-moved from their original route of entryinto the body. The toxicant may then in-teract with these distant cells to producetoxicity. Keep in mind that additionalprocesses that affect toxicokinetics, suchas storage, biotransformation, and elimi-nation, may lower the concentration ofthe toxicant.

Figure 5–1. Lymphatic system. (From B.Davey and T.Halliday, editors, Human Biol-ogy and Health: An Evolutionary Approach. Open University Press, 1994. Reprintedby permission.)


Figure 5–2. Circulatory system. (From B.Davey and T.Halliday, editors, Human Biol-ogy and Health: An Evolutionary Approach. Open University Press, 1994. Reprintedby permission.)


Factors Affecting Distribution ofToxicants to Tissues

The distribution of toxicants to tissues isdependent on several factors: (1) physicaland chemical properties of the toxicant,(2) concentration of the toxicant in theblood and in the tissues (i.e., concentra-tion gradient), (3) volume of blood flow-ing through a specific tissue, (4) tissuespecificity or preference of the toxicant,and (5) presence of special “barriers” toslow down toxicant entrance.

Properties of the Toxicant

The same physical and chemical charac-teristics that determined a toxicant’s ini-tial absorptive behavior also determine itslikelihood of being distributed to tissuesin the organism. Distribution to tissuesrequires the toxicant to once more inter-act at the level of the cell. This time, in-stead of interacting with the cells that formthe “absorptive barrier,” its interaction iswith the cells that form the tissues of thetarget organ. Molecular weight and po-larity are again of concern, with thesmaller, nonpolar toxicants gaining pref-erential entrance.

Concentration Gradient

The concentration gradient between theamount of the toxicant in the blood ascompared to the tissues will depend on anumber of features. Following absorptiona toxicant will be diluted by the fluid vol-ume present in the organism. This fluid isfound in three separate sites: (1) bloodplasma, which accounts for about half ofthe blood volume—depending on theindividual’s sex and age, blood volumes

in humans range from 4 to 6 L, and typi-cally will account for 7–9% of the totalbody weight; (2) interstitial fluid volume,the fluid between cells, which makes upabout 13% of the total body weight; and(3) intracellular fluid, the fluid inside cells,which accounts for about 40% of the to-tal body weight. Dilution, and the result-ing concentration of a toxicant in any ofthese three sites, is an important consid-eration, since the passive diffusion of thetoxicant into or out of these sites will bedetermined by the toxicant’s concentra-tion gradient. The apparent volume ofdistribution (VD) is used to define the vol-ume of body fluids in which a toxicant isdistributed. The following expression isoften used:

The apparent VD does not indicate inwhich of the three fluid volume sites atoxicant is distributed. Hence, if a toxi-cant is distributed only in the plasma fluid,a low VD would result; however, if a toxi-cant is distributed in all sites (bloodplasma, interstitial and intracellular flu-ids) the VD will be greater. The validity ofthe apparent VD can be further compro-mised by toxicants that undergo rapidstorage, biotransformation, or elimina-tion. Additionally, some toxicants bind toplasma proteins (e.g., albumin) circulat-ing in the blood. Binding to plasma pro-teins “removes” the toxicant from addedinteraction with cells. This removal effec-tively reduces the concentration of thetoxicant in the plasma or VD, since only“free,” non-protein-bound toxicants willinteract with cells.


Blood Flow

The volume of blood flowing through spe-cific tissues or organs in the human bodyis an important factor affecting distribu-tion. Two parameters affect the accumu-lation of a toxicant in an organ: (1) thevolume of blood flowing through the or-gan, and (2) the size or mass of the organ.A combination of these two, the bloodflow/mass ratio, permits the comparisonof toxicant accumulations in different or-gans (Table 5–1). The percentage of thetotal cardiac output received by each ofthe body organs is also important, sinceorgans that receive larger blood volumescan potentially accumulate more of agiven toxicant. Cardiac output equals thevolume of blood pumped per heart beat(stroke volume) times the heart rate (beatsper minute). During exercise, cardiac out-put increases as a result of a combinationof a larger stroke volume and an increasein heart rate.

Body regions that receive a large per-centage of the total cardiac output or havehigh blood flow/mass ratios (i.e., mL/kg)include the liver, kidneys, heart muscle,and brain. Blood flow to the brain is con-

stant, even during exercise. Approxi-mately half of the total cardiac output willbe sent to the liver (27.8%) and kidneys(23.3%). This is not surprising, as theseare the major organs involved in the elimi-nation of toxicants or their metabolites.Skeletal muscle and skin have intermedi-ate blood flow/mass ratios. However, dur-ing strenuous exercise blood flow to skel-etal muscles may increase from less than750 mL/min to more than 10,000 mL/min(or 10 L/min). This increase in blood flowmay subject skeletal muscles and otherorgans to greater toxicant exposure andaccumulation.

Bone and adipose tissues have relativelylow blood flow/mass ratios. This is im-portant to remember, since these areas alsoserve as primary storage sites for manytoxicants, especially those that are fatsoluble as well as those that readily asso-ciate (or complex) with minerals com-monly found in bone.

Affinity of Toxicants for SpecificTissues

Some tissues are “attractive” to specifictoxicants and, in spite of the rather low

Table 5–1. Comparison of mass, blood flow, and percentage of total cardiac outputfor selected body regions

Note. After P.Bard, editor, Medical Physiology, 11th edition. C.V.Mosby, 1961. aColumn total does notequal 100 due to founding of region values.


flow of blood to these tissues, they willpreferentially accumulate a given toxicantto disproportionately high concentrations.A good example is adipose tissue, such assubcutaneous fat, which is poorly per-fused by blood but is an attractive tissuefor lipid- or fat-soluble toxicants. Oncedeposited in these storage tissues, toxi-cants may remain for long periods of time,due to their solubility in the tissue andthe relatively low blood flow.

Structural Barriers to ToxicantEntrance

During distribution, the passage of toxi-cants from capillaries into tissues or or-gans in the body is not uniform. Someorgans (e.g., brain, placenta, testes) havespecialized barriers that make it difficultfor toxicants to diffuse into their cells.For example, the brain is protected bythe blood-brain barrier. This barrier isformed by specialized glial cells called as-trocytes (“star cells”), which possessmany small thread-like branches. Nu-merous “end-feet” on the branches ofeach astrocyte attach to the outer surfaceof the endothelial cells that line the capil-laries, forming a barrier that separatesthe endothelium of the capillary from theneurons of the brain. Lipids in theplasma membrane of astrocytic end-feetimpede the diffusion of water-solubletoxicants into the brain. In addition,tight junctions between adjacent endot-helial cells limit the passage of water-soluble molecules. Although the term“blood-brain barrier” indicates a ratherimpenetrable barrier, the “barrier” onlyserves to slow down the rates at whichtoxicants cross into brain tissue.

The placental barrier is another impedi-ment to chemical substances. Besides pro-viding for the nutritional, gas exchange,and excretory needs of the developing fe-tus, the placenta also protects the fetusfrom toxicants absorbed and subsequentlydistributed in the maternal circulation.The barrier is formed from cell layers be-tween the maternal and fetal circulatoryvessels in the placenta. Lipids present inthe plasma membranes of these additionalcells serve to limit the diffusion of water-soluble toxicants. As with the blood-brainbarrier, the placental barrier only slowsdown the diffusion of toxicants frommaternal circulation into fetal circulation,reducing exposure of developing fetal tis-sues to toxicants.

Storage of Toxicants

Once distribution occurs, toxicants canundergo other toxicokinetic processessuch as storage, biotransformation, andelimination. Storage results when toxi-cants accumulate in specific tissues or be-come bound to circulating plasma pro-teins (Figure 5–3). Both mechanisms re-duce the concentration of the “free” toxi-cant in the blood plasma.

Plasma Protein Storage

Albumin is the most abundant circulat-ing plasma protein and the most commonplasma protein to which toxicants arebound. The amount of a specific toxicantthat will become bound to plasma pro-teins varies widely (0–99%). Toxicantsbound to plasma proteins are “stored”;even though the toxicant-plasma proteincomplex may continue to circulate


through organs, the complex will not in-teract as only the free toxicant can inter-act with the cell.

Chemical bonding of toxicants toplasma proteins may be accomplishedthrough covalent (i.e., electron sharing)or noncovalent (e.g., ionic) mechanisms.

Replacement reactions, in which thetoxicant is “kicked off” the plasma pro-tein by another molecule with a higheraffinity or attraction, are a concern.Once “kicked off,” the now “free”toxicant will increase its concentrationcirculating in the blood. If this occurs

Figure 5–3. Common storage locations for toxicants in the body.


rapidly it can pose serious problems re-lated to toxicity.

Storage in Bone

Bone is composed of proteins (e.g., col-lagen, glycosaminoglycans, andproteoglycans) and the mineral salt hy-droxyapatite (Ca10(PO4)6(OH)2). Al-though bone appears to be rather static,it is not—bone is alive, and will bleed sinceit contains blood vessels. Osteoblasts arethe cells responsible for bone formation,and osteoclasts are the cells that reabsorbbone. During bone formation, osteoblastsproduce extracellular proteins and alsocreate a chemical environment that favorsthe natural or biogenic precipitation of hy-droxyapatite.

During the normal extracellular proc-esses that lead to the formation of bone,a number of elements and compoundsmay become involved in chemical substi-tution reactions. Fluoride (F-) may be sub-stituted for hydroxyl (OH-), and strontium(Sr) or lead (Pb) may be substituted forcalcium (Ca). Once substituted, elementsor compounds become incorporated intothe bone matrix. Under normal conditionsbone is continually being recycled throughresorptive and depositional processes. Onan average, the minerals in bone are recy-cled every 7–10 years—this means thatany chemicals locked in the matrix willeventually be released from storage toreenter the circulatory system.

Storage in the Liver

The liver is good at concentrating toxi-cants—it has a large blood flow/ mass ra-tio, it receives the largest percentage of

the total cardiac output, and its hepato-cytes (i.e., liver cells) contain cytoplasmicproteins that bind to numerous chemicals,including toxicants. Not only is the livera primary storage site, but it is also thesite where most toxicant biotransforma-tion takes place.

Storage in the Kidneys

The bilaterally placed kidneys have thehighest blood flow/mass ratio of all theorgans, more than four times greater thanthat of heart muscle. The large volume ofblood flowing though the kidneys prefer-entially exposes these organs to toxicants.Storage in the kidneys, unlike bone, is notconfined to mineral matrices, which haveslow turnover rates. Instead, soft tissueassociated with the nephron (the func-tional unit of urine formation) is exposedto the toxicant or its metabolites.

Storage in Fat

The storage of triglycerides (neutral fat) isa major function of adipose tissue. Subcu-taneous adipose tissue is the site for abouthalf of all stored neutral fat in the body.Additional fat can be found around the kid-neys, in the intestinal omenta (membranesthat hold the stomach and intestines in theirproper anatomical positions), in the geni-tal areas, between muscles, behind the eyes,and in folds on the surfaces of the heartand intestines. As with storage in bone, thetriglycerides deposited in these sites arecontinually exchanged with the blood andmay be redeposited in other adipose tissuecells.

Given that many toxicants are lipophilic,they will readily penetrate cell membranes


and become concen trated in adipose tis-sue. Storage results when toxicants un-dergo physical dissolution in the neutralfats found in adipose tissue. Once depos-ited, toxicants may be released, as a result

of normal processes of exchange, for dis-tribution to other sites where they may beredeposited, biotransformed, or elimi-nated.

Review Questions

1. The distribution of toxicants to distant regions of the body is influenced by:

A. Duration of exposureB. Chemical characteristics of the toxicantC. Location of toxicant entrance into the bloodstreamD. A and BE. A, B, and C

2. Which sequence best represents the pathway by which toxicants move through thelymphatic system to enter the bloodstream?

A. Enter lymph → lymph nodes → afferent lymph vessels → efferent lymph vesselslymph trunks → enter venous blood supply

B. Enter lymph → afferent lymph vessels → lymph nodes → efferent lymph vesselslymph trunks → enter venous blood supply

C. Enter lymph → lymph trunks → lymph nodes → afferent lymph vessels → efferentlymph vessels → enter venous blood supply

3. The portal vein is associated with which organ?

A. LungB. LiverC. KidneyD. Small intestineE. Spleen

4. The validity of the apparent VD can be compromised by:

A. Toxicants that undergo rapid storageB. The rapid elimination of a toxicantC. Binding of the toxicant to plasma proteinsD. A and BE. A, B, and C

5. Under normal conditions approximately half of the total cardiac output will besent to:

A. Bone and adipose tissuesB. Brain and kidneys


C. Kidneys and liverD. Liver and spleenE. Skeletal muscle and brain

6. Which is not a true statement about the blood-brain barrier?

A. It is formed by special glial cells called astrocytes.B. The cell membranes of astrocytic “end-feet” impede the diffusion of lipid-soluble

toxicants into the brain.C. Tight junctions between adjacent endothelial cells limit the passage of hydrophilic

molecules into the brain.D. The barrier functions to slow down the rate of toxicant entrance into brain tissue.E. The barrier separates the circulatory system from neurons in the brain.

7. Storage of toxicants in the body results when toxicants:

A. Accumulate in specific tissuesB. Become bound to circulating plasma proteinsC. Are “kicked off” the plasma proteins by replacement reactionsD. A and BE. A, B, and C

8. The liver is a primary site of toxicant storage because:

A. It has a large blood flow/mass ratio.B. Hepatocytes contain cytoplasmic proteins that bind to toxicants.C. Toxicants are readily incorporated into the bone matrix.D. A and BE. A, B, and C

9. Discuss factors that affect the distribution of toxicants to tissues.

10. Diagram the sites of toxicant storage in the body.

71

Chapter

6


� Explain the role of biotransformationin toxicokinetics

� Describe how biotransformationfacilitates the elimination of toxicantsor their metabolites from the body

� Distinguish between phase I and phaseII biotransformation reactions

� Define bioactivation or toxication

� Identify the tissues responsible forbiotransformation reactions

� List the factors affectingbiotransformation in humans

� Summarize the role of elimination intoxicokinetics

� Describe processes occurring in thekidney, liver, and lung as related to theelimination of toxicants

anabolismbioactivationbiological half-life (T1/2)biotransformationcatabolismconjugateconjugation reactionscytochrome P-450cytosolic enzymesdetoxicationenterohepatic loopfecal excretionglomerular filtrationglucuronidationhydrolysisintestinal excretionionized moleculesmetabolismmicrosomal enzymesnephronsoxidationphase I biotransformationphase II biotransformationreabsorptionreductionsecretionsulfate conjugationtoxication

Biotransformationand Elimination of

Toxicants


Biotransformation of Toxicants

Recall that lipophilic toxicants, particu-larly those that are nonpolar and have lowmolecular weights, are readily absorbedthrough the cell membrane. These lipo-philic toxicants, easily absorbed and po-tentially systemically distributed, are dif-ficult to eliminate from the body in theiroriginal chemical form. The very chemi-cal nature that facilitated their absorptioninhibits their elimination—they are lipo-philic. With continued absorption and noelimination, many lipophilic toxicantswould accumulate within the body. To beeliminated, these lipophilic toxicants mustbe metabolized or biochemically changedto metabolites (i.e., altered toxicants) thatare hydrophilic. In contrast, water-solubletoxicants are generally eliminated fromthe body in their original chemical form.

Metabolism is the sum of biochemicalchanges occurring to a molecule withinthe body. These chemical changes occurwithin the cell. Anabolism is the sum ofbiochemical changes that “build up” com-plex molecules (e.g., proteins). Catabolismis the sum of biochemical changes that“break down” complex molecules (e.g.,degradation of glucose). Metabolic proc-esses may take place “free” in the cyto-plasm or be “restricted” to specificorganelles found within the cell.

The human body has the capacity toeliminate most toxicants either in theiroriginal chemical form or as a metabolite.The major routes of elimination (feces andurine) are well known. Biochemical proc-esses or metabolic pathways used by thebody to facilitate elimination of toxicants,as well as molecules or their metaboliteswhich occur naturally in the body, are not

always appreciated. Biotransformation isthe process by which both endogenous(formed within) and exogenous (formedoutside) substances that enter the body arechanged from hydrophobic to hydrophilicmolecules to facilitate elimination fromthe body.

Biotransformation is responsible forchanging naturally occurring lipophilicmolecules into hydrophilic metabolites thatare more readily eliminated from the body.A good example is the fate of hemoglobin,the oxygen-carrying iron—protein com-plex in red blood cells. Under normal con-ditions hemoglobin is metabolized to bi-lirubin, one of a number of hemoglobinmetabolites. Bilirubin is toxic to the brainof newborns and, if present in high con-centrations, may cause irreversible braininjury. Biotransformation of the lipophilicbilirubin molecule in the liver results in theproduction of a water-soluble (hydrophilic)metabolite excreted into bile and elimi-nated via feces.

Typically biotransformation producesfour changes that facilitate elimination oftoxicants: (1) the resulting metabolites, oraltered toxicant molecules, are chemicallydistinct from the original toxicant; (2) themetabolites are usually more hydrophilicthan the original toxicant; (3) the hy-drophilic nature of the biotransformedmetabolites reduces their ability to crossmembranes, thereby altering its distribu-tion to tissues; (4) there is reduced reab-sorption of metabolites by cells associatedwith the organs of elimination (kidneysand intestines).

The rate at which a toxicant is removedfrom the body is dependent on its rate ofbiotransformation in the body and rateof elimination from the body. The biologi-

Biotransformation and Elimination of Toxicants 73

cal half-life (T½) is the time required toreduce by half the quantity of a toxicantpresent in the body (e.g., plasma). Thebiological half-life provides a means forcomparing the residence times for differ-ent toxicants in the body. This informa-tion is useful when establishing “safe”exposure durations for toxicants.

Biotransformation Reactions

Most toxicants that enter body tissues arelipophilic. The chemical reactions respon-sible for changing a lipophilic toxicantinto a chemical form which the body caneliminate are termed phase I and phase IIbiotransformations. The goal of the phaseI and phase II biotransformation reactionsis to facilitate detoxication (i.e., detoxifi-cation), thus producing water-soluble me-tabolites that are more readily eliminatedby the urinary and biliary (pertaining toliver bile) systems (Figure 6–1).

At physiologic pH, a toxicant or itsmetabolites that are water soluble willundergo dissociation into ions or becomeionized. Ionized molecules are the mol-ecules that react in living systems. Theseionized molecules (e.g., toxic metabolites),with their positively or negatively chargedregions, are the molecules that are morereadily transported across cell membranes.

On occasion, biotransformation pro-duces intermediate or final metabolitespossessing toxic properties not found inthe original parent chemical, be it a natu-rally occurring endogenous chemical or axenobiotic. The terms bioactivation andtoxication refer to the sequence of chemi-cal reactions that produce intermediate orfinal metabolites that are more toxic (or

reactive) than the original parent chemi-cal. In some cases bioactivation producesa highly reactive metabolite that may in-teract with naturally occurring macromol-ecules within the cytoplasm, cell mem-brane, or nucleus (e.g., DNA).

Once distributed, the original chemi-cal may not exhibit any toxic propertiesand should be referred to as a xenobiotic,not a toxicant. An example is acetami-nophen, a commonly used analgesic (painkiller) and antipyretic (fever reducer).When prescribed doses are taken, thisdrug gives the desired therapeutic responsewith little or no resulting toxicity. This isbecause acetaminophen rapidly undergoesphase I and phase II biotransformationreactions and is subsequently eliminatedin the urine and feces. However, at highdoses, normal phase I and phase II bi-otransformation reactions are over-whelmed, and an additional biosyntheticpathway produces a reactive metabolitetoxic to the liver (hepatotoxic).

Location of BiotransformationReactions

Most tissues have a limited ability tobiotransform. For example, skin, testes,and placenta have a low capacity, whereasthe intestines, kidneys, and lungs have amedium capacity. The highest capacity forbiotransformation is in the liver. The liverreceives blood directly from the gas-trointestinal tract, where chemicals, nu-trients, and toxicants are absorbed. Blood,with its gastrointestinally derived “chemi-cal payload,” is eventually distributed toall other tissues. It is vital that the liverremoves potentially toxic chemicals fromthe blood prior to distribution; the “first


pass” means that blood from the gas-trointestinal tract is shunted directly to theliver via the portal vein, thereby increas-ing the possibility of immediate detoxica-tion.

The liver’s biotransformation capacityis not specific for toxicants. Rather, the liveruses phase I and phase II biotransforma-tion reactions that, in addition to the “nor-mal” work of biotransforming endogenouschemicals (e.g., bilirubin) and xenobiotics,

are also capable of chemically modifyingtoxicants to facilitate their eliminationfrom the body.

At the subcellular level the enzymes thatcatalyze biotransformation reactions oc-cur either free in the cytoplasm or boundto the membrane of the endoplasmic re-ticulum in hepatocytes as well as othercells in the body. Microsomal enzymes areassociated with phase I reactions. Theterm microsomal describes the “small

Figure 6–1. Schematic relationship among toxicants, phase I and phase II biotransfor-mations, products, and elimination from the body in relation to lipophilic and hydro-philic characteristics.


bodies” or vesicles that form whenhepatocytes or liver tissue is homogenized(blended) to form an acellular homoge-nate (pureed hepatocytes!). Within thehomogenate, small segments of endoplas-mic reticulum membranes with boundphase I enzymes spontaneously form smallvesicles.

The hepatocyte homogenate also con-tains soluble enzymes that catalyze bi-otransformation reactions. Thesecytosolic enzymes are non-membrane-bound and occur free within the cyto-plasm. Cytosolic enzymes are associatedwith phase II reactions.

Although small and often overlooked,microbes living in the intestine are capa-ble of biotransformation. The role that themore than 400 bacterial species play inthe biotransformation of xenobiotics isprobably equal to that of the liver.

Factors AffectingBiotransformation

How effective biotransformation is indetoxication of toxicants absorbed anddistributed in the human body dependson several factors, including age, gender,nutrition, disease, and time of day.Biotransformation characteristics associ-ated with different organisms are unique.Differences in the qualitative and quanti-tative properties of phase I and phase IIbiotransformation enzymes must be con-sidered. Since nonhuman species are of-ten used for toxicity testing, cautionshould be exercised when extrapolatingor generalizing conclusions from speciesother than our own.

In general, human fetuses and neonates(newborns) have limited abilities for

xenobiotic biotransformations. This is dueto inherent deficiencies in many, but notall, of the enzymes responsible forcatalyzing phase I and phase IIbiotransformations. The capacity for bi-otransformation develops rapidly through-out infancy and peaks during adolescenceand adulthood. In the aged (over age 65),the levels of many enzymes—includingthose related to biotransformation—havedeclined, predisposing them once again tothe effects of toxicants.

Experimental studies on rodents (e.g.,mice and rats) indicate that there are sig-nificant male and female differences in thecapacity to biotransform. For some of theenzymes that catalyze phase I and phaseII reactions, male rodents exhibit fivetimes the capacity to biotransform as com-pared to females. Gender differences arealso suspected to be present in humansand are likely related to variation in tis-sue mass, VD, enzyme concentrations,hormone levels, and protein binding. Ani-mal studies indicate that circadian (L.circa, about; dian, a day) rhythms influ-ence the rate of biotransformation ofxenobiotics. The concentrations of manychemicals and enzymes related to bi-otransformation are known to fluctuateduring the day.

Biotransformation is also affected bynutritional status, as evidenced innonhuman studies. Certainly cautionshould be exercised when extrapolatingresults from animal studies to humans. Itis not unreasonable to conclude that hu-mans could exhibit many of the same re-sponses as those observed in animals withdietary deficiencies in vitamins, minerals,and nutrients—resulting in a decline inbiotransformation rates.


Many diseases impair an individual’scapacity to biotransform xenobiotics, in-cluding toxicants. Hepatitis, an inflamma-tory liver disease, can reduce hepatic bi-otransformation to half of the normalcapacity. The liver is a key site of biotrans-formation, and its impairment poses seri-ous health consequences.

Phase I Reactions

During phase I biotransformation reac-tions a small polar group is either exposed(“unmasked”) on the toxicant or added tothe toxicant (Figure 6–2). The polar groupenhances the solubility of the toxicant inwater, which favors elimination. The re-actions are catalyzed (brought about) bynonspecific enzyme systems, the most im-portant of which is cytochrome P-450.

Cytochromes (cyto-, cell; -chrome,colored) are iron—protein complexes thattransport electrons (or hydrogen) bychanging the valency of iron (e.g., Fe++→Fe++++e- or Fe+++ +e-→Fe++). CytochromeP-450 gets it name from the observationthat, in its reduced state (i.e., Fe++), thisiron—protein complex has a maximumabsorbance of visible light at 450 nm (1nanometer = 10–9 meters; the part of thespectrum that is visible to the human eyeranges from violet at 390 nm to red at760 nm).

Phase I reactions usually involve: (1)oxidation, which occurs when thetoxicant loses electrons, (2) reduction,when the toxicant gains electrons, or (3)hydrolysis, a process that cleaves (splits)the toxicant into two or more simplermolecules, each of which then combineswith a part of water (i.e., H+ and OH-) atthe site of cleavage.

Toxicants undergoing phase I biotrans-formation will result in metabolites suffi-ciently ionized, or hydrophilic, to be eitherreadily eliminated from the body withoutfurther biotransformation reactions re-quired or rendered as an intermediatemetabolite ready for phase II biotransfor-mation. Some intermediate or finalmetabolites may be more toxic than theparent chemical.

Phase II Reactions

On completion of a phase I reaction, thenew intermediate metabolite producedcontains a reactive chemical group (e.g.,hydroxyl, -OH; amino, -NH2; or car-boxyl, -COOH). For many intermediatemetabolites the reactive sites, which wereeither exposed or added during phase Ibiotransformation, do not confer suffi-cient hydrophilic properties to permitelimination from the body. These metabo-lites must undergo additional biotransfor-mation, called a phase II reaction.

During phase II reactions (Figure 6–3 )a molecule provided by the body must beadded to the reactive site produced dur-ing phase I. Phase II reactions are referredto as conjugation reactions. These reac-tions produce a conjugate metabolite thatis more water-soluble than the originaltoxicant or phase I metabolite. In mostinstances the hydrophilic phase IImetabolite can be readily eliminated fromthe body.

One of the most popular moleculesadded directly to the toxicant or its phaseI metabolite is glucuronic acid, a moleculederived from glucose, a common carbo-hydrate (sugar) that is theprimary source of energy for cells.


Glucuronidation, the process of addingglucuronide to the toxicant or phase I me-tabolite, occurs primarily in hepatocytes,the functional cells of the liver. The re-sulting glucuronic acid conju gate is ex-creted into the bile, which then moves onto the intestine for elimination in the fe-ces. Typically, glucuronic acid conjugates

with MW> 350 are secreted in the bile,while those with MW<250 are secretedby the kidney.

Another phase II reaction is sulfate con-jugation, which takes place primarily inthe liver. Unlike glucuronic acid conju-gates that are eliminated in the bile, thehighly polar sulfate conjugates are read-

Figure 6–2. Representative phase I biotransformation reactions.

Figure 6–3. Representative phase II biotransformation reactions.


ily secreted in the urine. Other phase IIreactions may involve the addition of amethyl group (-CH3) or an amino acid,most commonly glycine. Sodium salicylate(aspirin) is eliminated as a glycine—sali-cylic acid conjugate in the urine.

Elimination of Toxicants

Toxicants or their phase I or phase II me-tabolites are eliminated from the body bymany different routes, including urine, fe-ces, exhaled air from the lungs, milk,sweat, saliva, and cerebrospinal fluid. Byfar the main route of elimination is theurine, produced by the kidneys. Second isfecal elimination, which involves excre-tion of xenobiotics into the bile by hepa-tocytes. Third is elimination via the lungs,where gaseous toxicants or their metabo-lites are exhaled during the respiratorycycle.

Urinary Elimination

As an organ of elimination the kidney al-lows for intimate contact between the cir-culatory system and the urinary system.Two kidneys, each weighing about 150 g,are present in adults. Each kidney con-tains about one million nephrons—thefunctional units of the kidney (Figure 6–4). Three distinct structural/functionalregions characterize each nephron: (1)Bowman’s capsule/filtration, (2) proximaltubule/reabsorption, and (3) distal tubule/secretion. These structural regions,straightened and laid end to end, wouldextend over 75 miles.

Filtration is the first process in urineformation. The glomerulus, with its densecapillary network, brings blood into closecontact with Bowman’s capsule in thenephron. Glomerular filtration resultswhen hydrostatic pressure within the cap-illaries forces small molecules, including

Figure 6–3. Representative phase II biotransformation reactions.


water, across the sieve-like filter and intoBowman’s capsule. Molecules with mo-lecular weights greater than 60,000,which include large protein molecules andred blood cells, remain in the capillary anddo not become part of the urinary filtrate.

In adults, the glomerular filtrate accu-mulates at a rate of about 125 mL/min or180 L/day. This volume of filtrate is notentirely unexpected, considering bloodflow to the kidneys is over 400 mL/100 g/min, and that the kidneys receive aboutone-fourth of the total cardiac output.Fortunately, processes occurring in“downstream” structures of the nephronreturn about 99% of the watery filtrate,along with important small molecules, tothe blood supply.

Reabsorption, the second process inurine formation, occurs in the proximal

convoluted tubule of the nephron. In thisregion most of the water lost duringglomerular filtration reenters the blood.All of the glucose, potassium, and aminoacids are reabsorbed, either by passive oractive transport mechanisms, into theblood in the proximal convoluted tubule.Reabsorption of water occurs by osmo-sis, a passive transport process in whichwater follows its own concentration gra-dient and moves from a region of highconcentration in the proximal tubule intoa region of lower concentration in thecapillaries surrounding the tubule.

Secretion, which occurs in the distalconvoluted tubule, is the last process inurine formation. Whereas reabsorption isresponsible for moving water and smallmolecules out of the urine and into theblood, secretion transports molecules out

Figure 6–4. Kidney and nephron anatomy. (From M.C.Willis, Medical Terminology:The Language of Health Care. Williams & Wilkins, 1996. Reproduced by permission.)


of the blood and into the urine. Secretedsubstances include potassium ions, hydro-gen ions, and some xenobiotics.

The very processes used to eliminateendogenous metabolites are the processesused to rid the body of toxicants ortoxicant metabolites entering the systemiccirculation. Urinary elimination may re-sult from glomerular filtration, or passiveor active tubular transport of the toxicantinto the urine. Some toxicants enter theurinary filtrate during glomerular filtra-tion. If unaffected by an additional proc-ess, such as reabsorption, the toxicant willremain in the urine and be eliminated fromthe body. This is particularly true forsmall, polar toxicants. Other toxicants toolarge to enter the urinary filtrate maymove via passive transport from the bloodacross capillary endothelial cells and ne-phron tubule membranes to enter theurine. Some protein-bound toxicants,which are too large to enter urine duringglomerular filtration, may be transportedinto the urine by tubular secretion.

Fecal Elimination

Ingested xenobiotics will either be unab-sorbed, and pass on through in the feces,or be absorbed, and subsequently enterthe circulatory system. For manyxenobiotics the former is a path that poseslittle hazard to the body; however, onceabsorbed and distributed, xenobiotics (in-cluding toxicants) will require urinary,fecal, or other routes of elimination. Notall toxicants will be eliminated from thebody by the same route that they entered.Toxicants or their metabolites may enterfeces as a result of intestinal or biliaryexcretion.

Intestinal excretion involves the trans-port of xenobiotics from the blood intothe intestinal lumen. Recall that the sub-mucosa of the intestines is highly vascu-larized and, once absorbed, nutrients andxenobiotics are rapidly distributed. Dur-ing intestinal excretion the reverse occurs.Xenobiotics passively diffuse through theendothelia of capillaries in the submucosa,then through mucosal cells and into theintestinal lumen to be eliminated in feces.

Fecal excretion is an important processleading to the elimination of toxicants andtheir metabolites. It involves not only thegastrointestinal system but also an acces-sory organ, the liver. Cells in the liver(hepatocytes) secrete bile. Hepatocytes arecapable of producing about 500 mL of bileper day. Biotransformed (phase I)toxicants, with possible phase II conju-gates, flow into small ducts called canal-iculi, which in turn flow into larger bileducts and the gallbladder for temporarystorage. To facilitate digestion, bile is re-leased. A bile duct connects the liver to theduodenum of the small intestine.

Bile is composed of water (97%), bilesalts (0.7%), inorganic salts (0.7%), bilepigments (0.2%), and trace amounts offatty acids, fats, cholesterol, and lecithin.In addition to transporting xenobiotics ortheir conjugates, bile has other essentialfunctions. For example, bile salts are re-sponsible for the emulsification of fats inthe duodenum. This facilitates fat diges-tion and absorption in the small intestine.

Not all toxicant- or metabolite-conju-gates transported in bile from the liver tothe intestines are eliminated in feces. Someare reabsorbed as they pass through thesmall intestine and will enter the circula-tory system for trans port via the portal


vein back to the liver. When phase II con-jugates become hydrolyzed (the toxicantis split from the endogenous molecule) the“free” toxicant, usually lipophilic, can bereabsorbed through the intestinal mucosa.This cycling of toxicants between the liverand intestine is termed an enterohepaticloop. Bile salts participate in their ownenterohepatic loop—over 90% arereabsorbed in the intestines for a returntrip to the liver.

Nutrient—and potentially toxicant—rich blood leaves the intestines and circu-lates through the liver (1.5 L/min) beforeflowing on to the heart and lungs, afterwhich it is pumped into the systemic cir-culation for distribution to tissues. Thecapacity to biotransform toxic substancesbefore they can be distributed to otherbody regions is of survival value.

Respiratory Elimination

The lungs are involved in the eliminationof xenobiotics that exist in a gaseousphase at body temperature. Simple diffu-sion results when the concentration (va-por pressure) of the xenobiotic dissolvedin capillary blood is greater than the gas-eous phase present in the alveoli. This rep-resents a concentration gradient, and thevolatile gas will diffuse down its concen-tration gradient until equilibrium acrossthe alveolar membrane is achieved. Gaseswith a low solubility in blood are morerapidly eliminated than those gases witha high solubility. In addition to vapor pres-sure and solubility, other factors such asrespiration rate and blood flow to thelungs determine the elimination of gaseousxenobiotics by the lungs.

Ethanol, a volatile alcohol, provides a

good example of a xenobiotic that is elimi-nated from the body by fecal, urinary, andrespiratory routes. After the ingestion ofalcoholic beverages, ethanol is rapidly ab-sorbed across the gastrointestinal mucosa.Ethanol then enters the systemic circula-tion and is distributed to the tissues, includ-ing the brain, kidneys, liver, and lungs.About 90% of the ethanol undergoes phaseI biotransformation to acetaldehyde andacetate in the liver. The remaining 10% iseliminated unchanged by the kidneys inurine or by the lungs during exhalation.

Additional Routes of Elimination

Saliva. Three pairs of salivary glands lo-cated within the oral cavity are capableof producing about 1.5 L of saliva per day.Xenobiotics that passively diffuse into sa-liva may, when swallowed, be availablefor absorption through the mucosa of thegastrointestinal system. Overall, salivaplays a minor role in the elimination oftoxicants. However, the elimination ofsome pharmaceuticals (drugs) into salivais responsible for the “drug taste” re-ported by some patients.

Sweat. Skin has about 80 sweat glands/cm2. The amount of water lost each dayin sweat varies widely and may amount tomore than 1 L/hour under strenuous workin hot temperatures. On average, about100 mL of water is lost per day. This rep-resents about 4% of water output from allsources (2,400 mL), such as water expiredin air (350 mL), water lost through skinby diffusion (350 mL) and sweat (100 mL),and water lost in urine (1,400 mL) and in-testines (200 mL). Sweat is responsible forthe elimination of many metals, includingcadmium, copper, iron, lead, nickel, and


zinc. Under normal conditions xenobioticsentering sweat through passive diffusionrepresent a minor elimination component;however, under conditions of greater sweatproduction, significant elimination mayresult.

Milk. During lactation, xenobioticspresent in the mother’s blood may also bedetectable in her milk. This is partly dueto the fat content of milk, which enhancesthe passage of lipophilic xenobiotics. Theconcentration of a given xenobiotic elimi-nated in milk depends on the chemicalcharacteristics of the xenobiotic, bloodflow to breasts, and amount of milk pro-duced. (Note: Breastfeeding should be dis-couraged when toxicants are present inthe mother’s milk, as the toxicants willlikely have an adverse effect on the infant.)

Nails. Fingernails and toenails are partof the integumentary system and are shedunder normal growth conditions. As thenail forms, xenobiotics (e.g., arsenic) maybecome incorporated into the horny ma-trix. This effectively removes thexenobiotic from the body as the nail isworn away.

Hair. Like nails, hair is a part of theintegumentary system and under normalgrowth conditions it is, over time, lostfrom the body. Xenobiotics (such as ar-senic, cadmium, and lead) that becomeincorporated in hair will eventually beeliminated from the body.

Skin. Desquamation, or loss of epithe-lial cells in the epidermis, permits the lossof xenobiotics through the skin.

Cerebrospinal Fluid. Toxicants that en-ter the central nervous system (CNS in-cludes the brain and spinal cord) may en-ter the cerebrospinal fluid (CSF). About150 mL of CSF is present in the ventriclesof the brain and in the tissues (i.e., sub-arachnoid space) that surround the brainand spinal cord. CSF is produced mostlyby the choroid plexus at a rate of 550 mL/day. The CSF turnover rate is about 3.7times/day, which contributes to a substan-tial flow of this fluid. CSF is absorbed intothe tissues surrounding the CNS where itenters the venous blood supply. Toxicantsin the CSF are actively transported into tis-sues that surround the brain or by passivediffusion through the blood-brain barrier.

Review Questions

1. Which is not a true statement about metabolites that result from biotransformation?

A. They are chemically distinct from the original toxicant.B. They have a reduced ability to cross membranes.C. They are usually more hydrophobic than the original toxicant.D. A and BE. A, B, and C

2. If exposure to a toxicant resulted in blood plasma concentration of 200 mg/mL,and the toxicant has a T½ of 4 hours, then how long would it take to reduce the bloodplasma level of the toxicant to 25 mg/mL?


A. 8 hoursB. 12 hoursC. 16 hoursD. 24 hoursE. More than 25 hours

3. Term used to describe the sequence of biotransformation reactions that produceintermediate or final metabolites that are more toxic than the original parent chemical.

A. BioactivationB. DetoxicationC. IonizationD. OxidationE. Reduction

4. Which sequence best represents the highest to lowest capacity for biotransformationto take place in different organs?

A. Skin→lungs→liverB. Lungs→liver→kidneysC. Placenta→testes→lungsD. Liver→kidneys→skinE. Liver→skin→lungs

5. Which is not a true statement about phase I biotransformations?

A. They involve cytosolic enzymes.B. They may be impaired by disease states.C. Nonspecific reactions are catalyzed by cytochrome P-450.D. They produce metabolites sufficiently ionized to be eliminated from the body.E. They may involve oxidation, reduction, or hydrolysis reactions.

6. Some protein-bound toxicants, which are too large to enter urine during glomerularfiltration, may be transported into the urine by tubular secretion.

A. TrueB. False

7. Which is (are) a true statement about fecal elimination of toxicants?

A. Unabsorbed toxicants will pass on through in the feces.B. Intestinal excretion involves the transport of toxicants from the blood into the

intestinal lumen.C. Fecal excretion is the most important process leading to the elimination of toxicants

and their metabolites.D. A and CE. A, B, and C


8. The lungs are involved in the elimination of toxicants that exist in a gaseous phaseat body temperature.

A. TrueB. False

9. Discuss additional routes of elimination other than urinary, fecal, and respiratorypathways.

10. How are phase I and phase II biotransformation reactions influenced by thefollowing: age, gender, nutrition, disease, and species?

85

Chapter

7


Target Organ Toxicity

� Define target organ toxicity

� Explain the basis for the specificity oforgan toxicity

� Contrast the toxicity mechanisms forhematotoxicity, hepatotoxicity,nephrotoxicity, neurotoxicity,dermatotoxicity, and pulmonotoxicity

� Describe examples of target organtoxicity

� Discuss the characteristic evaluativeprocedures for determining toxicity intarget organs

acute tubular necrosis (ATN)agranulocytopeniaallergic contact dermatitisanemiaanthracosilicosisanuriaarterial blood gases (ABGs)asbestosisberylliosisblood dyscrasiasblood urea nitrogen (BUN)bronchoscopycentral nervous system (CNS)chloracnecreatininedermatotoxicityerythropoietinforced vital capacity (FVC)glial cellsglomerular filtration rate(GFR)glycosuriahematotoxicityhematotoxinshematuriahemolytic anemiashepatotoxicityhypoxia


inulinirritant contact dermatitisleukemiamicrocytic hypochromic anemiamyelinnephritic syndromenephrotic syndromeneurotoxicitynerveneuronsnephrotoxicityneurotransmitterobstructive uropathiesoliguria

PAH clearancepancytopeniaperipheral nervous system (PNS)phototoxicitypneumoconiosespoietinsproteinuriapulmonary fibrosispulmonotoxicityradiopharmaceuticalsilicosisslow vital capacity (SVC)target organ toxicitythrombocytopenia


Target Organ Toxicity 87

Introduction to Target OrganToxicity

Time-dependent toxicokinetic processesrelated to absorption, distribution, stor-age, biotransformation, and eliminationwill determine how much of a toxicantwill be distributed to a specific target or-gan (e.g., kidney, liver, or lung) in thebody. Target organ toxicity is defined asthe adverse effects or disease states mani-fested in specific organs in the body. Tox-icity is unique for each organ, since eachorgan is a unique assemblage of tissues,and each tissue is a unique assemblage ofcells. Toxicity may be enhanced by distri-bution features that deliver a high con-centration of the toxicant to a specificorgan (e.g., large blood flow/mass ratio)or by inherent features of the cells andtissues of the organ that render it highlysusceptible to the toxicant, even at lowconcentrations.

Although the mechanisms responsiblefor organ toxicity are not always known,the observed differences in target organtoxicity are most likely due to structuraland functional differences in the cells thatmake up the tissues and organs. In otherwords, under the influence of a toxicant,each organ will manifest different diseasestates (i.e., toxicity), depending on thestructural and functional characteristicsof the cells present.

Cells differ in many ways, including theirenergy consumption (e.g., use of ATP), rateof cellular division, active and passivetransport characteristics, relationship tocell barriers (e.g., blood-brain barrier) andextracellular matrices (e.g., hydroxyapa-tite, collagen), presence of intracellularcomponents (e.g., contractile filaments in

muscle cells, microtubules in neurons), re-pair mechanisms, and biotransformationcapacity. Cellular specialization, in addi-tion to making “life” possible in a multi-cellular organism, also means that eachorgan will respond to a toxicant in a dif-ferent way.

When considering target organ toxic-ity, remember that: (1) not all organs areaffected to the same extent by a toxicant;(2) a single toxicant may have several tar-get organs; (3) several toxicants may havethe same target organ; (4) the highest con-centration of a toxicant is not alwaysfound in the target organ; and (5) the con-centration of a toxicant in a target organis the result of all toxicokinetic processes.

Hematotoxicity

Introduction

Although not self-evident, blood is classi-fied as a connective tissue. All blood cellcomponents (i.e., erythrocytes, leukocytes,and thrombocytes) have their origin in stemcells found in bone marrow (Figure 7–1).Poietins, or stimulating factors, regulate the“fate” of a stem cell—that is, whether itbecomes an erythrocyte (erythropoietin),leukocyte, or thrombocyte. Proper oxygentransport, immune function, and clot for-mation result when normal numbers oferythrocytes, leukocytes, and thromb-ocytes are present, respectively.

Toxicity Mechanisms

Hematotoxins (G. haimatos, blood) alterquantitative and qualitative characteristicsof blood cells to produce toxicsymptoms. Hematotoxicity occurs when


too many or too few of these differentblood components are present or structuralanomalies occurring in blood componentsinterfere with normal functioning.

For example, anemia occurs when thereare too few erythrocytes, which results ina reduced oxygen-carrying capacity of theblood. Too many white blood cells,leukemia, can be deadly. Not enoughthrombocytes, thrombocytopenia, mayresult in external or internal hemorrhaging(i.e., blood loss).

Qualitative changes in blood cell com-ponents can also result in disease. Micro-

cytic hypochromic anemia results whenerythrocytes have a low hemoglobin con-tent. Although adequate in quantity, thequality of these “small, less colored” redblood cells prevents them from carryingthe normal amount of oxygen.

Examples

Carbon monoxide (CO) alters the oxy-gen-carrying capacity of hemoglobin. Thequalitative change occurs because COpreferentially binds to hemoglobin, pre-venting the transport of O2. In effect, CO

Figure 7–1. Formation of the cellular components of blood from stem cells.


out-competes O2 for the available trans-port sites on hemoglobin molecules, lead-ing to hypoxia or anoxia, which is an in-adequate supply of oxygen to the tissues.Symptoms may include low blood pres-sure, fainting, dizziness, headache, weak-ness, and nausea.

Cyanide (HCN) and hydrogen sulfide(H2S) are capable of producing cytotoxichypoxia, a potentially lethal condition inwhich cells in the body cannot utilize O2

during normal cell metabolism associatedwith energy production. Oxygen in thevenous blood becomes abnormally high,since it is not being used by the cell. Hy-drogen sulfide, as encountered in oil fieldsand sewers, is recognized by its “rottenegg” odor.

Blood dyscrasias (i.e., disorders) can beinduced by toxicants. Dyscrasias usuallyresult in abnormal cellular components—too many of one blood cell type, too fewof another. Benzene (an organic solvent)is known to cause thrombocytopenia andleukemia. Agranulocytopenia, a decreasein the monocytes and lymphocytes (i.e.,agranular leukocytes), can be induced byDDT (an insecticide). Other chemicals,such as chlordane, can cause pancytope-nia, a reduction in all blood cells. Finally,hemolytic anemias result when erythro-cytes are destroyed by a toxicant, such asnaphthalene.

Evaluating Hematotoxicity

Five tests are commonly used to measurethe quantitative and qualitative aspects ofblood. Three of these—the red cell count,white cell count, and platelet count—arequantitative measurements of blood com-ponents. The other two, hemoglobin (Hb)

and hematocrit (HCT), are indicators ofthe oxygen-carrying capacity of blood.Collectively, these tests, plus others, aretermed a complete blood count (CBC) withdifferential (DIFF). The CBC/DIFF pro-vides information on the number, variety,percentages, concentrations, and quality ofblood components. Additionally, arterialblood gases (ABGs) may be measured todetermine the partial pressure of oxygen(PaO2), partial pressure of carbon dioxide(PaCO2), and the percentage of hemoglo-bin bound to oxygen or oxygen saturation(SaO2), all of which are important indica-tors of the blood’s ability to acquire andrelease oxygen to the tissues.

Hepatotoxicity

Introduction

Hepatotoxicity refers to toxic effects inthe liver. In addition to its important rolein detoxication, this largest gland in thebody also functions to synthesize manyproteins (e.g., plasma albumin, coagula-tion proteins), excrete bile, and metabo-lize fats, carbohydrates, and proteins.

The liver is particularly susceptible totoxic agents for two reasons: (1) afterabsorption, most toxicants that enter theblood flow through the liver (“first pass”)before being distributed to other systemicregions, including other organs; and (2)the liver is the primary site for biotrans-formation of toxicants, which exposes theliver to toxicants and their metabolites,some of which, as a result of toxicationor bioactivation, are more toxic than theoriginal chemical.

The functional unit of the liver is thelobule. Approximately 50,000–100,000


lobules are present in an adult liver. Eachlobule is about 1–2 mm in diameter andcontains an orderly arrangement ofhepatocytes, a hepatic venule, a hepaticarteriole, sinusoids, and a small bile ductcalled a canaliculus (Figure 7–2). As bloodflows through sinusoids in the lobule,from hepatic arterioles and portal venulesto the terminal hepatic venule, bile isformed by hepatocytes that lie betweenvascular sinusoids and the canaliculi. Bileflows within canaliculi in the oppositedirection of the blood flow.

Toxicity Mechanisms

Liver toxicants are typically characterizedas being cytotoxic or cholestatic (chole-,bile; -static, standing still). Cytotoxicmechanisms affect hepatocytes and are re-sponsible for different types of liver in-jury, including fatty liver, liver necrosis,and cirrhosis. Cholestatic mechanismsaffect the flow of bile. Intrahepaticcholestasis occurs when the flow of bile isblocked within the liver as it flowsthrough canaliculi, as well as bile ductules.

Examples

A number of organic chemicals are toxicto hepatocytes, including trichloroethyl-ene, carbon tetrachloride, anddichlorodiphenyltrichloroethane (DDT).The resulting toxicity may lead to necro-sis (the death of hepatocytes), inflamma-tion, or cirrhosis (the replacement of hepa-tocytes by fibrous tissue during the repairof damaged hepatocytes). Chronic etha-nol toxicity involves the accumulation ofexcess fat within hepatocytes, which canlead to fatty liver disease and cirrhosis.

The cholestatic mechanisms that leadto the blockage of bile are poorly under-stood. “Blockage” may result fromblocked transport mechanisms in the cellmembrane of hepatocytes, in which caselittle or no bile is transported into the ca-naliculi, or from precipitates and “bileplugs” that form within canaliculi. Bilesalts, steroids, and a-naphthylisocyanateare known cholestatic agents.

Evaluating Hepatotoxicity

Noninvasive liver tests are used to evalu-ate liver structure and function. Typicallythree types of tests are done. The first in-volves testing blood serum (i.e., the non-cellular portion) for particular enzymesknown to be present at specific levelswhen the liver is functioning properly, butthat may be elevated in a damaged liverdue to their release into the blood fromdamaged hepatocytes.

The second type of test, again per-formed on blood, examines liver functionfor its ability to remove routinely encoun-tered substances (e.g., bilirubin) or intro-duced substances (e.g., dyes) from theblood. Since the liver is responsible forproducing most of the factors involved inthe cascading sequence of reactionsneeded to produce a “fibrin clot,” the timefor clot formation can be examined as anindicator of liver function.

A third type of test, the liver scan, is usedto examine both liver anatomy (e.g., size)and function (e.g., flow of blood and bile).This nuclear test is performed by intrave-nously injecting a radiopharmaceutical, aradioactive chemical used in diagnostics.Pictures using noninvasive radioactiveimaging techniques are then taken to


Figure 7–2. Structure of a liver lobule. (From D.W.Fawcett, Bloom and Fawcett, ATextbook of Histology, 11th edition. W.B.Saunders, 1986. Reprinted by permission.)


detect the radiopharmaceutical as it circu-lates through the liver.

Nephrotoxicity

Introduction

Nephrotoxicity refers to the toxic effectsin the kidney. Remember, three processesoccurring in the kidney, glomerular filtra-tion, tubular reabsorption, and tubularsecretion, are responsible for the produc-tion of urine. Nephrotoxins are knownto influence each of these processes.

Toxicity Mechanisms

During filtration, two toxic responses maybe manifest. First, due to the large vol-ume of filtrate formed at the interface ofthe vascular glomerulus and Bowman’scapsule, toxicants may accumulate in thisanatomical region of the nephron. Second,this may in turn increase or decrease therate of filtration or alter characteristics ofthe glomerular apparatus (the filter). If theglomerulus is made more porous, or lessselective, substances normally excluded bythe filter will be able to cross and enterthe filtrate.

Other nephrotoxicity mechanisms af-fect both the qualitative and quantitativeaspects of the reabsorptive process. Theproximal tubule is responsible for the se-lective reabsorption of most of the saltsand water present in the filtrate. All aminoacids and glucose are reabsorbed and re-turned to the blood in this region. Anychanges induced by toxicants or theirmetabolites in the characteristics of thecell membranes that form the tubule can

profoundly affect the reabsorptive proc-ess.

Tubular secretion, the third and lastprocess of urine formation, is responsiblefor the active transport of substances fromthe blood (e.g., H+, K+, xenobiotics) intothe urine. Again, when toxicants or theirmetabolites alter these transport mecha-nisms, the nephron is unable to functionproperly and nephrotoxicity may result.

Examples

The pathologies associated with nephro-toxicity are dependent on the anatomicalregion of the nephron affected by the toxi-cant. Two major responses may be ob-served when the glomerular filtration ap-paratus is injured: nephrotic syndromeand nephritic syndrome. Although the pa-thologies for each are complex, nephroticsyndrome is usually characterized byheavy proteinuria (i.e., presence of pro-tein in the urine), whereas nephritic syn-drome is typically characterized by hema-turia (i.e., presence of blood cells in theurine). Some xenobiotics, such as lead andheroin, are linked to nephrotic syndrome,often resulting in heavy proteinuria.

The selectivity of the glomerular filtercan be altered by exposure to xenobiotics.In contrast to increased permeability toalbumin resulting from exposure to puro-mycin (an antibiotic), two other antibiot-ics—gentamycin and kanamycin—de-crease the rate of glomerular filtration.

As the glomerular filtrate flows throughthe nephron, renal tubules may be exposedto high concentrations of filtered toxicantsor their metabolites. Damage to the epi-thelial cells that line the tubules is respon-sible for producing acute tubular necrosis


(ATN). Heavy metals, antibiotics, and or-ganic solvents are known to cause ATN.

Tubular reabsorption is affected by cad-mium (Cd), lead (Pb), and mercury (Hg).Up to the age of 50, Cd normally accumu-lates in the human kidney—in fact, about10 times as much Cd accumulates in thekidney as in the liver. Cadmium is capableof producing glycosuria (loss of glucose inthe urine) and aminoaciduria (loss ofamino acids in the urine). Lead inhibits thereabsorption of glucose and amino acidsin the proximal tubule, also leading to gly-cosuria and aminoaciduria. Kidney failuremay result from exposure to inorganicmercury (Hg2+), a powerful tubularnephrotoxin. When oliguria (little urine)or anuria (no urine) is formed, the buildupof toxic wastes in the body can lead todeath.

Obstructive uropathies result when theflow of urine is prevented either byintratubular or extratubular pathologies.Ethylene glycol, a commonly used anti-freeze or coolant, is metabolized by thebody to calcium oxalate. This insoluble saltaccumulates in the proximal tubule, bothin the lumen and in the epithelial cells lin-ing the lumen, forming an intratubularobstruction to the normal flow of urine.

Evaluating Nephrotoxicity

A number of quantitative and qualitativetests are used to evaluate kidney function.Glomerular filtration rate (GFR) is definedas the amount of glomerular filtrate (mL)per unit of time (min). GFR can be deter-mined by intravenously administering inu-lin, a fructose polymer (MW=5,200). Thissugar polymer is readily distributed in theblood, does not become bound to plasma

proteins, is not metabolized or stored, ef-fortlessly enters the glomerular filtrate,and is not reabsorbed or secreted by thenephron. On entering the glomerular fil-trate, inulin becomes a permanent com-ponent of urine. By measuring the amountof inulin present in plasma (PI) and urine(UI), and urine volume (V) after a specificinterval of time, the inulin clearance (CI)can be used to estimate GFR by the fol-lowing formula:

GFR�(UI) (V)/PI=CI

Typical values have been inserted into thisformula to show its usefulness in deter-mining normal kidney function as relatedto glomerular filtration rates:

GFR�(30 mg/mL) (1.25 mL/min)/0.3 mg/mL=125 mL/min

When nephrotoxicity affects the glomeru-lar filtration apparatus, as may be evi-denced by a decrease in the normal GFRof 125 mL/min, secondary consequencesaffecting toxicokinetics are likely to oc-cur. For example, a decrease in GFR mayresult in a decrease in urinary eliminationor “clearance” of a toxicant or its metabo-lites. This would increase the T1/2 andcould lead to toxicity involving otherorgans.

The organic acid PAH (p-aminohippuricacid) is used to evaluate kidney function.Readily filtered, PAH is actively secretedinto the urine. It is almost completely“cleared” from blood plasma during onepass through the kidneys—so much so, thatPAH clearance is used to estimate the rateof plasma flow through the kidneys. To-gether, inulin and PAH studies provide in-formation about glomerular filtration andtubular secretion.


Two other indicators of kidney func-tion are blood urea nitrogen (BUN) andcreatinine. Urea (a potential endogenoustoxicant formed from the catabolism ofproteins) and creatinine (a product ofmuscle metabolism) are distributed in theblood. In adults, normally functioningkidneys will eliminate urea (25 g/day) andcreatinine (1.8 g/day) into urine. Elevatedserum BUN and creatinine levels are in-dicative of kidney dysfunction.

Neurotoxicity

Introduction

The nervous system is divided into the cen-tral nervous system (CNS) and peripheralnervous system (PNS). The CNS includesthe brain and spinal cord, which functionto process information and providememory. The PNS contains peripheralnerves that are involved with sensory andmotor control functions.

Neurons, more than a billion of them,are the characteristic cells found in boththe CNS and PNS. These cells gather in-formation (i.e., sensory), process informa-tion, provide memory, and initiate appro-priate responses (i.e., motor). Neurons arecomposed of three cellular regions: the cellbody, dendrites, and axon. A nerve in thePNS is a collection of individual motoror sensory neurons.

Additional support cells in the CNS,termed glial cells, provide a structuralframework and transport of nutrients(astrocytes), myelin production(oligodendrocytes), and immune function(microglia). The oligodendrocytes (CNS)and Schwann cells (PNS) are responsiblefor the production of myelin, a lipid-rich

“cell membrane wrapping” around axons.Myelin functions to insulate the axon, en-hancing the velocity of axonal conduction.

Neurons, either sensory or motor, arelinked together to form a communicationnetwork. Two different processes are re-sponsible for the propagation of a com-munication along a neuronal path. Thefirst process ensures that a signal is “elec-trically” transmitted along the length ofeach neuron’s axon, while the second“chemically” propagates the signal fromone neuron to the next.

Neurotoxins are known to alter neuronsin both the CNS and PNS, as well as theglial support cells in the CNS. Much of ourcurrent understanding of the nervous sys-tem’s structure and function has resultedfrom the experimental use of neuro-toxins—particularly neurotoxins affectingcell membrane proteins that function in celltransport and as membrane receptors.

Toxicity Mechanisms

Neurotoxins interfere with the communi-cation ability of neurons, impeding recep-tor or motor neuron signaling and CNSfunctioning. Neurons are able to propagatea signal due to an electrical gradient thatexists between the inside and outside of theaxonal cell membrane. The gradient is cre-ated by Na+/K+ pumps in the cell mem-brane. These pumps actively transport Na+

to the outside of the cell and K+ to the in-side of the cell, creating a potential differ-ence of about—70 mV (millivolts) acrossthe axonal cell membrane.

Within a single neuron, signal propaga-tion occurs when the electrical wave ofdepolarization runs the entire length of theaxon. Depolarization results when passive


transport channels open to permit Na+ torapidly enter the cell membrane, followedalmost instantaneously by additional pas-sive channels that open to allow K+ to exit.When passive or active transport mecha-nisms are slowed, blocked, or otherwiseimpaired, or if the membrane “leaks” ex-cessively, the potential difference cannot bemaintained and the neuron will be unableto propagate a signal down the length ofthe axon.

When signals reach the distal axonalregion, a second process called synaptictransmission is initiated. Synaptic transmis-sion is responsible for propagation of thesignal to the next neuron. In this process aneurotransmitter (chemical messenger), isreleased from the distal region (i.e., presy-naptic neuron). Once released (i.e.,exocytosis of vesicles), the neurotransmit-ter moves across the synaptic cleft (a mi-croscopically small space) to bind toreceptors on the membrane surface of thepostsynaptic neuron.

If sufficient neurotransmitters bind tothe postsynaptic membrane receptors, thepostsynaptic neuron will depolarize andthe electrical wave will be propagateddown the axon of the next neuron.Synaptic transmission involves a chemi-cal mode of transmission rather than theelectrical gradient responsible for propa-gating the signal in the axon.

Neurotoxins may bind to postsynapticreceptors, blocking synaptic transmission.Following synaptic transmission,neurotoxins may act to inhibit the “re-moval,” or degradation, of the recentlyreleased neurotransmitters, permittingcontinued synaptic transmission that maybe sufficient to repeatedly depolarize thepostsynaptic neuron.

Examples

Neurotoxicity results when toxicants in-terrupt the normal mechanisms of neu-ronal communication. Batrachotoxin, se-creted by frogs, destroys the electrical gra-dient by increasing the Na+ permeabilityof the axonal cell membrane. Ethanol de-presses CNS function by (1) inhibiting orstimulating a variety of transport chan-nels and (2) increasing membrane fluid-ity by altering the packing of moleculeswithin the membrane. Both processes actto depolarize the neuron, thereby decreas-ing signal transmission.

The insecticide DDT increases mem-brane permeability to Na+ in the presyn-aptic region, which leads to continualsignaling. Other insecticides (e.g.,malathion, diazinon) prevent the break-down of acetylcholine, a neurotransmitter,by binding to cholinesterase, an enzymeresponsible for catabolism.

Tetrodotoxin, found in puffer fish,blocks Na+ channels. Botulin toxin, abacteriotoxin, prevents the release of theneurotransmitter acetylcholine. Curare, aphytotoxin, is a neuromuscular blockingagent that prevents a motor neuron fromsignaling a muscle cell by blocking ace-tylcholine receptors on muscle cells.

A number of neurotoxins (e.g.,hexachlorophene and lead) damage themyelin sheath. This loss of insulation re-sults in signal “short circuiting” betweenadjacent neurons and slower neuronaltransmission velocities.

Evaluating Neurotoxicity

The CNS presents a challenge to directevaluation, since invasive diagnostic pro-cedures involving the brain and spinal


cord may be life threatening. Indirect ex-aminations provide information aboutneurological function, and the followingare helpful in CNS and PNS toxicity di-agnoses: (1) patient history, (2) mentalstatus (e.g., memory), (3) deficits in cra-nial nerve function (e.g., auditory nervefunction in hearing and balance), (4) sen-sory neuron function (e.g., pain, tempera-ture), and (5) motor neuron function (e.g.,coordination, gait, muscle strength, re-flexes, tremors).

Other information related to neurologi-cal structure and function may be obtainedfrom x rays, computerized axial tomogra-phy (CAT scans), magnetic resonanceimaging (MRI), electromyography (EMG),electroencephalography (EEG), peripheralnerve conduction velocity, and cerebros-pinal fluid (CSF).

Dermatotoxicity

Introduction

Dermatotoxicity describes the adverse ef-fects produced by toxicants in the skin. Re-call that skin is composed of the epidermis,dermis, and subcutaneous fatty tissue. Hairfollicles, oil and sweat glands, blood ves-sels, and sensory neurons are present. Skinis more than a protective covering. It func-tions to limit water loss, reduce the harm-ful effects of ultraviolet radiation, preventthe entrance of microorganisms, regulatebody temperature, and also biotransformand eliminate toxicants. Skin, with all itsrelated functions, is vulnerable to toxicity,because skin is often the first part of thebody to come into contact with a toxicantduring handling.

Toxicity Mechanisms

Toxic skin reactions are diverse and mayinvolve any one or a number of combina-tions of skin components. Irritant contactdermatitis results when toxicants elicit aninflammatory response in skin. Depend-ing on the exposure site, this form ofdermatotoxicity is manifest in the accu-mulation of watery fluid (edema), an in-crease in the amount of blood (hyper-emia), or, if serious, the loss of tissue (ul-ceration and necrosis). Although irritantcontact dermatitis is usually confined tothe site of contact, prolonged exposuremay result in systemic toxicity.

Some individuals show little or no re-sponse on exposure to a chemical; how-ever, on exposure at a later time they mayexhibit a delayed hypersensitivity reaction,sometimes severe, termed allergic contactdermatitis. This delayed sensitivity totoxicants involves the immune system, andit may take from a few days to many yearsfor individuals to become sensitized. Spe-cifically, the initial exposure sensitizes theimmune system (T lymphocytes) to “rec-ognize” the toxicant on later exposure.Again, the delayed dermatotoxicity or hy-persensitivity reaction may result in edema,hyperemia, or ulceration.

Phototoxicity, a form of light-induceddermatotoxicity, results when skin is over-exposed to ultraviolet light or from thecombination of exposure to specific wave-lengths of light and a phototoxic sub-stance. Symptoms associated withphototoxicity include erythema (sunburn),hyperpigmentation, premature skin aging,and cancer. Hyperpigmentation andhypopigmentation symptoms result fromchanges in melanocytes, the cells located


in the epidermis responsible for the pro-duction of melanin (a pigment that givescolor to skin and hair).

Dermatotoxic responses may occur inhair, and in sebaceous and sweat glands.The cells responsible for hair productionhave some of the highest mitotic (cellulardivision) rates in the body. Toxicants, in-cluding chemotherapeutic agents, that in-terrupt cellular division typically will in-duce hair loss. Some dermatotoxins stimu-late a proliferation of the epithelium sur-rounding sebaceous glands. Proliferatingepithelial cells plug the pilosebaceous (L.pilus, hair; L. sebaceus, oily) orifices, pro-ducing chloracne, a condition similar toacne vulgaris experienced by adolescents.

Examples

Irritant contact dermatitis (e.g., edema,erythema) may result from exposure to avariety of agents, including organic sol-vents, acids (pH less than 5.5), bases,plants (orange peel, nettles), detergents,and even water. Chronic exposure to ce-ment and chrome can result in serious ul-cerative conditions accompanied by ne-crosis.

Allergic contact dermatitis has beenlinked to exposure to antibiotics (e.g., peni-cillin), antihistamines (e.g., diphenhy-dramine), anesthetics, plants (e.g., poisonivy), tanning agents, metal compounds(e.g., chromates), numerous industrialagents, and rubber antioxidants. The lat-ter are commonly used in the manufactureof gloves, shoes, and undergarments.

Phototoxicity can result from acute andchronic exposure to ultraviolet radiation(e.g., “sunbathing”). Exposure to certainwavelengths of light coupled with simul-

taneous exposure to phototoxicants, suchas drugs (e.g., tetracycline), perfumes, polycyclic aromatic hydrocarbons, and dyes,can result in dermatotoxicity.

Sunlight, coal tar compounds, petro-leum oils, and heavy metals (e.g., arsenic,mercury) are known to producehyperpigmentation. Of therapeutic inter-est in the treatment of hypopigmentationare the psoralens (e.g., 8-methoxypsoralen). Vitiligo (white patchesof skin caused by an absence or decreasein melanin production) is often successfullytreated with PUVA (psoralen and ultravio-let-A) therapy. Together the phototoxicantand UV-A produce hyperpigmentation.

A decrease in melanin production, orhypopigmentation, can be produced as aresult of burns, chronic dermatitis, anddermatotoxicants such as p-tertiary butylphenol. The mechanism by which p-terti-ary butyl phenol functions as adepigmenting agent (i.e., melanotoxicant)is probably related to its structural like-ness to L-tyrosine, one of two amino acidprecursor molecules involved in melaninbiosynthesis. The resulting decrease inmelanin is likely due to the biosynthesisof a nonfunctional “melanin-like” mol-ecule, or as a result of a reduction of theenzymes that normally catalyze the reac-tion of L-tyrosine into melanin. The en-zymes are depleted or inhibited duringattempts to catalyze p-tertiary butyl phe-nol, and hence are unavailable to catalyzethe normal L-tyrosine to melanin reaction(Figure 7–3).

Acne-like conditions may be producedby a number of dermatotoxicants, includ-ing coal tar, greases, oils, and even cosmet-ics. However, a few specific halogenatedaromatic compounds are responsible for


producing chloracne (Figure 7–4). Amongthe more active chloracne-producingchemicals are polyhalogenated biphenyls,dibenzofurans, dioxins, and naphthalenes.These halogenated chemicals induce epi-thelial hyperplasia (i.e., increase the num-ber of epithelial cells), which blocks theopening to sebaceous glands.

Evaluating Dermatotoxicity

Usually the causative agents responsiblefor most irritant dermatitis are readilyidentified on questioning the patient or

reviewing the history of exposure. Whenquestioning is not helpful in isolating thecausative agent, the patch test (performedby a dermatologist) may be used in diag-nosis. In this test the dermatologist selectsthe contact allergens and concentrationsto be tested. The suspected allergen is ap-plied to the skin under a nonabsorbentpatch and is left for a specified period oftime. The presence of erythema and otherpathologies, on removal of the patch, sig-nifies a positive patch test result.

Figure 7–3. The substitution of p-tertiary butyl phenol for L-tyrosine in the biosyn-thetic pathway of melanin.


Pulmonotoxicity

Introduction

Pulmonotoxicity refers to the disease statesin the respiratory system brought about bythe inhalation of gases, vapors, liquid drop-lets, and particulates. Inhalational toxi-cants may affect nasopharyngeal, tracheo-bronchial, and alveolar regions.

The respiratory mucosal lining is highlysusceptible to toxic substances. Unlike thestratified keratinized epithelium present inskin, the epithelial cells lining the respira-tory system are not always stratified andare not keratinized, diminishing their bar-rier qualities. Different types of epithelialcells are present in different regions, suchas the stratified squamous epithelium in thepharynx and the ciliated columnar epithe-lium in the tracheobronchial region. Still

different epithelial cells, type I and type IIpneumocytes, make up the alveoli.

The varied pulmonotoxicities are a re-flection of the characteristic assemblageof the more than 40 cell types present inthe respiratory system. Ultimately, con-cern is paramount when toxic responsesdecrease the lung’s ability to exchangeoxygen and carbon dioxide across the al-veolar-capillary interface.

Toxicity Mechanisms

In addition to the systemic toxicity thatmay result when toxicants are absorbedthrough the respiratory system, the respi-ratory system itself exhibits responsesranging from minor local irritations andallergic responses to cellular damage, fi-brosis, and neoplasms (cancer). Manygases are known to irritate the epithelial

Figure 7–4. Chloracne, as seen on the face of 4-year-old Alice Senno (October 29,1976), resulted from exposure to dioxin released when a chemical factory exploded inSeveso, Italy, on July 10, 1976. (From AP Wirephoto. Reproduced by permission.)


cells lining the respiratory system. Irrita-tion may involve inflammatory responsesthat lead to a contraction of the smoothmusculature that surrounds the air pas-sageways (e.g., bronchioles) and to edema.These two conditions reduce the cross-sectional area of the passageway, limitingthe flow of air.

Some pulmonotoxicants “target” spe-cific cells in the respiratory system, suchas the ciliated columnar epithelial cells inthe tracheobronchial region or Clara cellsin the region of the terminal bronchioles.The resulting pulmonotoxicity reflects thedamage to these specific cell populations,which may lead to impaired function ofthe mucociliary escalator and necrosis,respectively—both of which decrease pul-monary function.

Asthma refers to a narrowing of the airpassages in response to a number of stimuli,including allergens, infections, exercise,and drugs. Of interest to toxicologists isoccupational asthma. More than 80 dif-ferent occupational asthma inducers havebeen identified. Occupational asthma de-velops when, on contact with the sub-stance, the smooth muscles surroundingthe bronchioles contract. This reduces thecross-sectional area of the bronchiole andrestricts the flow of air—remember Poi-seuille’s Law.

Diffuse alveolar damage (DAD), clini-cally termed adult respiratory distress syn-drome (ARDS), results when the cells lin-ing the alveoli (pneumocytes) and alveo-lar capillaries (endothelial cells) allow pro-tein-rich fluid to leak into the tiny spacesbetween the capillary and alveolus. Thisleads to the destruction of type Ipneumocytes, inflammatory responses,and eventual pulmonary fibrosis (the for-

mation of fibrous tissue in the lung),which may impair alveolar function.

Pulmonotoxicities resulting from theinhalation of mineral “dusts” are termedpneumoconioses. If, on inhalation, themineral particulates enter the alveoli, theymay stimulate the formation of pulmonaryfibrosis. This decreases or eliminates thefunctional capacity of the affected alveoli.

In the United States, carcinoma of thelung is the most common cause of deathfrom cancer. Carcinogens (cancer-causingchemicals) are causally linked to this formof pulmonotoxicity.

Examples

Among identified respiratory system irri-tants are ozone (O3), nitrogen dioxide(NO2), sulfur dioxide (SO2), chlorine (Cl2),and ammonia (NH3). A decrease in the ac-tion of the mucociliary escalator is ob-served on exposure to cigarette smoke,ozone, and sulfuric acid. Clara cells, lin-ing the terminal bronchioles, are highly vul-nerable to ipomeanol, a fungitoxin pro-duced by a mold that grows on sweet po-tatoes.

Occupational asthma has been causallylinked to careers in agricultural harvest-ing, animal handling, food preparation,and woodworking. Pharmaceuticals (e.g.,aspirin) and industrial chemicals (e.g.,toluene di-isocyanate) are also implicatedas pulmonotoxicants.

Pulmonary fibrosis, associated withDAD, is a pulmonotoxic response to theherbicide paraquat (1, 1'-dimethyl-4,4'dipyridilium). In addition to delayedtoxic effects in the liver and kidneys, thisweed killer produces interstitial alveolarfibrosis within 4–7 days following expo-sure. Paraquat can also be percutaneously


absorbed, with subsequent distribution tothe lungs. The higher concentrations ofparaquat found in the lungs, as comparedto other tissues, results from the activetransport of paraquat across the alveolarcell membrane.

Depending on the causative agent, pul-monary fibrosis as linked to pneumoco-nioses may result from exposure to asbes-tos fibers (asbestosis), silicon dioxide par-ticles (silicosis), coal dust that usually con-tains silica (anthracosilicosis), and beryl-lium (berylliosis). Although thepathogenesis of each pulmonotoxic dis-ease differs slightly, the common pathol-ogy is fibrosis. Once inhaled into the al-veolar sac, the “dust” is ingested bymacrophages (large “scavenger” immunecells, monocytes). Altered by the presenceof the “dust,” the macrophages stimulatesurrounding fibroblasts to secrete colla-gen, which leads to fibrosis.

There is a causal relationship betweenasbestos exposure and mesothelioma, aneoplasm involving the mesothelial cellsthat cover the lung (pleura). Not only isthe incidence of mesothelioma high forasbestos workers, it is also found in thewives of asbestos workers, most likely aresult of exposure when washing theirhusbands’ contaminated clothes.

Carcinoma of the lung kills more than100,000 persons in the United States eachyear. Approximately 90% of these lungcancers are a direct consequence of ciga-rette smoking. Lung neoplasms are alsolinked to exposure to arsenic, chromium,nickel, and coke oven emissions.

Evaluating PulmonotoxicityRespiration involves two separate but in-timately related processes, each of which

can be evaluated. The first of these pro-cesses includes the muscles of respiration,air passageways, compliance (elasticity) oflung tissues, and in general those struc-tures and functions that facilitate the ex-change of air between the lungs and theexternal environment. The total volumeof air contained in the lungs is termed slowvital capacity (SVC). This volume can bemeasured with a spirometer by perform-ing an SVC test. For the SVC test, sub-jects completely fill their lungs with a deepbreath, then blow into a small, handhelddevice (spirometer) that measures the vol-ume of air. A second test, the forced vitalcapacity (FVC) test, determines not onlythe SVC but also the volume of air ex-pelled as a function of time. Decreases inthe SVC or FVC usually indicate impairedpulmonary function (e.g., obstructions inair passageways).

Chest x rays, CAT scans, and MRIs arenoninvasive approaches to detecting pul-monary pathologies.

Additional information is obtained byvisually inspecting or sampling (e.g., bi-opsy) the larger passageways. Bronchos-copy is performed with a small fiber-op-tic instrument, a bronchoscope, insertedthrough the oral cavity and into the tra-cheobronchial region.

The second process ensures that the air(O2) that enters the lungs as a result of“breathing” actually enters the blood fordistribution to the tissues where it is usedduring cellular respiration. Arterial bloodgases (ABGs) are good indicators of O2

and CO2 transport across the alveolar-en-dothelial membranes. When consideringpulmonotoxicity, remember that normal“breathing” may not always result in suf-ficient “oxygenation” of tissues.


Other Examples of TargetOrgan Toxicity

Cellular specialization in multicellularorganisms has led to an array of different cell types. Virtually every organ in

the human body, because of the uniqueassemblages of cells and tissues, is ca-pable of exhibiting a different type of tar-get organ toxicity. Table 7–1 containsadditional examples of target organ tox-icity.

Table 7–1. Additional examples of target organ toxicity

1. Which is not a true statement about target organ toxicity?

A. It is defined as the adverse effects as manifested in specific organs of the body.B. Toxicity is unique for each organ.C. Toxicity may be enhanced by distribution features that deliver a high concentration

of the toxicant to the organ.D. A single toxicant may have several target organs.E. The highest concentration of the toxicant is always found in the target organ.

Review Questions


2. Examples of hematotoxicity include:

A. ThrombocytopeniaB. Microcytic hypochromic anemiaC. ProteinuriaD. A and BE. A, B, and C

3. Hepatotoxicity is routinely evaluated by:

A. Noninvasive testing of urine for particular enzymes associated with normal liverfunction

B. PAH clearance testC. Testing clot formation timesD. A and BE. A and C

4. The mechanisms of neurotoxicity may include:

A. An interruption in synaptic transmissionB. The binding of neurotoxins to postsynaptic receptorsC. The presence of nephritic syndromeD. A and BE. A, B, and C

5. Glial cells function to:

A. Transport nutrients to CNS neuronsB. Produce myelinC. Provide a structural framework in the CNSD. A and BE. A, B, and C

6. All of the following are noninvasive diagnostic tests used to assess neurologicalfunction except:

A. Cranial nerve testsB. Sensory function testsC. Motor neuron function testsD. Patient historyE. Cerebrospinal fluid tests

7. Some individuals show little or no response on exposure to a chemical; however,on exposure at a later time they exhibit this delayed hypersensitivity reaction.

A. Allergic contact dermatitisB. Chloracne


C. Chronic dermatitisD. HyperpigmentationE. Irritant contact dermatitis

8. Pulmonotoxicity that results when the cells lining the alveoli allow protein-richfluid to leak into the tiny spaces between the capillary and the alveolus:

A. AnthracosilicosisB. Carcinoma of the lungC. Diffuse alveolar damageD. Occupational asthmaE. Pneumoconiosis

9. Discuss target organ toxicity as found in the eye, heart, and reproductive system.

10. Define the following abbreviations: ABG, ARDS, ATN, BUN, CAT, CBC/DIFF,CSF, DAD, EEG, EMG, GFR, HCT, Hb, MRI, PAH, PUVA, FVC, and SVC.

105

Chapter

8


Teratogenesis,Mutagenesis, and

Carcinogenesis

� Define teratogenesis, mutagenesis, andcarcinogenesis

� Describe the relevance of replication,transcription, and translation toteratogenesis, mutagenesis, andcarcinogenesis

� Summarize the mechanism of actionfor teratogens, mutagens, andcarcinogens

� Discuss examples of known teratogens,mutagens, and carcinogens

agenesisAmes assayaneuploidyanticodonsatresiabase analoguesbase substitutioncancercarcinogenesiscarcinogenscellular divisioncentromerechromosomecodondeoxyribonucleic acid (DNA)developmental syndromesdiploiddivision failuresdysraphic anomaliesectopiaembryogenesisembryolethalityepigeneticfetal alcohol syndrome (FAS)frameshiftgenegenetic codegenotoxicgerm cells


haploidhistogenesishypoplasiainitiationkaryotypemeiosismetaphasemitosismonosomymorphogenesismutagenesismutagensnucleic acidsnucleotidesoogenesisorganogenesis

point mutationpolyploidyprocarcinogenpromotionpurinespyrimidinesreplicationribonucleic acid (RNA)somatic cellsspermatogenesisteratogenesisteratogensteratologytranscriptiontranslationtrisomy


Teratogenesis, Mutagenesis, and Carcinogenesis 107

Introduction to Teratogenesis,Mutagenesis, and Carcinogenesis

Toxicants, in addition to their effects ontarget organs, may react with or modifythe molecules of life. Once incorporatedinto these molecules, deoxyribonucleicacid (DNA) or ribonucleic acid (RNA),toxicants can have dramatic effects on theexposed individuals or their unborn off-spring.

The perpetuation of life depends on thereproduction of cells, or cellular division.This is true for somatic cells (i.e., bodycells), which reproduce over the life spanof an individual, and germ cells (i.e., gam-etes), such as ova and spermatozoa, whichform and subsequently join together dur-ing fertilization to facilitate continuationof the species. An understanding of thepathways by which genetic informationis perpetuated (replication) and expressed(transcription and translation) is essentialto comprehending the mechanisms of tera-togenesis, mutagenesis, and carcinogen-esis.

Teratogenesis (G. teras, monster; -gen-esis, origin) is the origin or production ofmalformed fetuses. Teratogens alter nor-mal cellular differentiation or growthprocesses, which results in a malformedfetus (birth defects). Mutagenesis (L.mutare, to change) is the production of amutation or change in the genetic code.On cellular division, these changes arepassed on to “daughter cells.” Carcino-genesis (G. karkinos, cancer) is the for-mation of cancer, including carcinomasand other malignant neoplasms. The sub-stances responsible for causing mutationsand neoplasms are termed mutagens andcarcinogens, respectively.

Unlike target organ toxicity that affectsexposed individuals, teratogens (andmany mutagens and carcinogens) leave alegacy of toxicity to future generations.

Replication

Mitosis

In humans, all somatic cells contain 23pairs of chromosomes. Each parent con-tributes one set (haploid or “n”) to thepair of chromosomes in a diploid somaticcell. The expression “2n” (diploid) refersto these 46 chromosomes (i.e., 23 pairs).

Mitosis, a type of cell division, resultsin the production of two daughter cellswith exactly the same chromosomenumber (2n) as in the original parent cell.Mitosis ensures that the genetic contentof each daughter cell will be identical tothe parent cell. This results from replica-tion, a process that duplicates the cell’sDNA.

Once replicated, each set of DNA be-comes tightly coiled, wound, and con-densed to form two chromatids. Thesetwo “sister” chromatids join in a regioncalled the centromere to form a bivalentor tetrad chromosome (i.e., colored bod-ies). The characteristic >< appearance ofhuman somatic bivalent chromosomes ac-tually represents the replicated DNA (i.e.,sister chromatids) connected at the cen-tromere, with each side of the chromo-some, > and <, delineating a single set ofthe DNA (i.e., chromatids). During mito-sis, bivalent chromosomes will separateat their centromere and, upon completion,each daughter cell will have “inherited”half of the ><.


Meiosis

The formation of male and female ga-metes, spermatogenesis and oogenesis, re-spectively, results from a second type ofcellular division termed meiosis. Unlikemitosis, meiosis produces daughter cellsthat possess only one set of chromosomes.The resulting haploid gametes have only23 chromosomes (n), half of the 23 pairsof chromosomes normally found in so-matic cells.

Unlike mitosis, meiosis guarantees thatthe gametes (i.e., daughter cells) producedwill not be identical in chromosomenumber or genetic content to the parentcell. The first of these differences ensuresthat at conception (i.e., sexual reproduc-tion), when the haploid ovum (n) and thehaploid spermatozoon (n) unite, the dip-loid number (2n) of chromosomes will berestored in the fertilized ovum. The sec-ond difference is the source of the geneticvariation so evident in all organisms us-ing sexual reproduction to perpetuatetheir species.

Karyotypes

Chromosomes are best viewed and pho-tographed during metaphase, a stage of mi-tosis (prophase → metaphase → anaphase→ telophase). Once photographed, chro-mosomes can be arranged as pairs in de-scending order of size (i.e., overall height,>< → x) and centromere placement. Cen-tromeres may be found in the middle, asin metacentric chromosomes (e.g., ><), ortoward the ends of sister chromatids, as inacrocentric chromosomes. Once arranged,the systematized array of chromosomes iscalled a karyotype or karyogram (G.karyon, nucleus). Karyotypes are an invalu-

able aid in diagnosing chromosomal ge-netic anomalies.

Transcription and Translation

DNA occurs as a double strand of nucle-otides (i.e., nucleic acid, sugar, and phos-phate) in the form of a spiral ladder ordouble helix. There are two families ofnucleotides, the pyrimidines (cytosine,thymine, and uracil), composed of a singlecarbon-nitrogen ring, and purines (adenineand guanine), composed of two carbon—nitrogen rings. Each rung or step of the lad-der results when two of four possiblenucleic acids (adenine, cytosine, guanine,and thymine) form a bond between thenucleotide sequences on each strand.Complementary base pairing always in-volves bonding between the nucleic acidbases adenine and thymine (A-T), and gua-nine and cytosine (G-C). The many differ-ent sequences of nucleic acid bases in eachstrand constitute the genetic code.

A gene represents a region or sequenceof bases that contains the “blueprint” fora protein. (Remember that proteins arecomposed of amino acids.) A gene mustcode for a specific sequence of amino ac-ids. Human DNA is composed of about50,000–100,000 genes, which contain atotal of about 6 billion base pairs.

With four different bases, and a codefor a single amino acid consisting of a se-quence of three bases called a codon, atotal of 64 (43) codons are possible. Thisis more than enough codes to representthe 20 amino acids available to “build”proteins (Figure 8–1). Codons also serveas start and stop signals in the process ofmaking proteins.


DNA never leaves the nucleus, and acopy of the DNA “blueprint” must bemade for export to ribosomes in the cyto-plasm where actual protein synthesis oc-curs. An additional nucleic acid, RNA,functions as the messenger molecule be-tween the nucleus and the cytoplasm.During a process called transcription, thedouble strands of DNA unzip along thebonds between complementary basepairs—the rungs on the ladder (Figure 8–2). This exposes the nucleotide sequenceon each strand.

Specific enzymes facilitate the construc-tion of a “copy” of each DNA strand,which means that new complementarybase pairs are formed in the unzipped re-gion. The RNA copy of DNA is almostidentical to the original unzipped nucle-otide sequence except for two differences.First, in RNA the nucleic acid uracil re-places thymine, so base pairing occursbetween RNA’s uracil (not thymine) andDNA’s adenine, and RNA’s adenine andDNA’s thymine. Second, the five-carbonsugar deoxyribose present in DNA (the

Figure 8–1. The genetic code. A sequence of three nucleotide bases in the messengerRNA (mRNA) codon for the code for a specific amino acid. Note that there is a total of64 (43) possible mRNA codons and that most amino acids are coded for by more thanone codon.


D in DNA) is just ribose in RNA (the Rin RNA). The newly formed RNA nucle-otide sequence separates from the DNAtemplate to form a single strand of RNA

designated as messenger RNA or mRNA(Figure 8–2).

Transcription completed, the DNA tem-plate is transcribed to mRNA, and the two

Figure 8–2. Overview of transcription (DNA → mRNA) and translation (mRNA→protein). (From V.C.Scanlon and T.Sanders, Essentials of Anatomy and Physiology, 2ndedition. F.A.Davis, 1995. Reprinted by permission.)


strands of DNA that were previously un-zipped zip back together to once again formthe double helix. The newly formed mRNAleaves the nucleus through nuclear poresto enter the cytoplasm, where it becomesthe blueprint for protein synthesis.

Messenger RNA (mRNA) and two ad-ditional forms of RNA, transfer (tRNA)and ribosomal (rRNA or ribosomes), al-ready present in the cytoplasm, are all in-volved in translation, the next phase ofprotein synthesis. Each form of RNA playsa role in translation: mRNA is the blue-print, rRNA is the factory (ribosome), andtRNA brings specific amino acids to theribosomal protein factory.

Translation begins when mRNA arrivesat the ribosome or rRNA (Figure 8–2).Since amino acids by themselves cannotinterpret the sequence of nucleic acidcodons present in mRNA, they must attachto anticodons present in tRNA. TransferRNA performs two functions: (1) it at-taches to a single specific amino acid thatcorresponds to the anticodon, and (2) itrecognizes the correct mRNA codon. Dur-ing translation, tRNA anticodons tempo-rarily bond to the mRNA codons. One byone, in a sequence specified by mRNA,amino acids are linked together by the ri-bosome to form a protein. Once formed,the protein leaves the ribosome to be usedby the cell or transported outside the cell.

Teratogenesis

Teratology is the study of developmentalanomalies in fetuses. Worldwide, congeni-tal anomalies are present in 2% of all new-borns. Although animal studies indicatenumerous chemical, physical, and biologi-

cal teratogens, there are relatively fewproven teratogens in humans. For ethicalreasons, direct toxicity testing involvingsuspected teratogens in humans is not anoption. Instead, researchers must rely onpopulation surveys, retrospective studies,and investigations of reported adverse ef-fects of suspected teratogens. Evidencefrom these approaches enables descriptivetoxicologists and epidemiologists to infer“associative” relationships.

Through elaborate and complex path-ways, the fertilized ovum (i.e., zygote)develops to form three germ layers—theectoderm, mesoderm, and endoderm. Onfurther differentiation, these germ layersgive rise to all the specialized parts of thebody (Figure 8–3). Teratogenesis resultswhen these pathways are blocked, slowed,or in other ways altered. The resultinganomalies involve in utero malformationsin the development of a body region, or-gan, or part of an organ.

The timing of exposure to teratogensis important because of the sequenceddevelopment of structures (e.g., CNS,heart) during early embryogenesis (forma-tion of the embryo). There is a narrowtime window during which exposure to ateratogen coincides with histogenesis (theformation of tissues) and organogenesis(the formation of organs). Exposure dur-ing this time window may result in: (1)malformation of the organ, (2) retarda-tion or delayed formation of the organ,or (3) death of the embryo or fetus.

The most critical period for teratogenicactivity is during the first 8 weeks of ges-tation, referred to as the embryonic stage.During this time there is maximal sensi-tivity to the development of morphologi-cal abnormalities in response to


teratogenic agents (Figure 8–4 ). Exposureto teratogens during the fetal stage (weeks9 through 38) rarely results in major er-rors of morphogenesis (G. morphe, formor shape). However, these later teratoge-nic insults may result in neoplasms (i.e.,cancer), organ or organ system dysfunc-tion, and behavioral and developmentalanomalies. Delayed functional maturationis evident in the CNS, which does not ma-ture until several years after birth.

Typically, the outcomes related to er-rors in morphogenesis are dose depend-ent. At high doses, teratogens cause seri-ous errors in morphogenesis. The sever-ity of these errors usually results inembryolethality, which is death of the fer-tilized ovum or the embryo during the first8 weeks (embryonic stage). An estimated15–25% of all conceptions result in spon-taneous abortion during the first 8 weeks.These percentages probably underestimate

Figure 8–3. The origin of specialized tissues and organs in the human body.

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teratogenic embryolethality, as embryonicdeath often goes unnoticed or presents asheavy menstrual bleeding.

Only over a narrow range of doses—not so high as to cause embryolethalityand not so low as to have no effect—teratogens have deleterious effects on his-togenesis or organogenesis. Examples in-clude: agenesis, the complete absence ofan organ (e.g., unilateral kidney agenesis);hypoplasia, reduced size of all or part ofan organ (e.g., microcephaly); dysraphicanomalies, failure of apposed structuresto fuse (e.g., spina bifida); division fail-ures (e.g., syndactyly, the fusion of fin-gers); atresia, incomplete formation of alumen (e.g., esophageal atresia); ectopia,presence of an organ outside its normalsite (e.g., ectopic heart, with the heartoutside the thoracic cavity); and develop-mental syndromes that involve multiple,but related, anomalies (e.g., fetal alcoholsyndrome).

Mechanisms of teratogenic action mustinvolve some form of cytotoxicity. Giventhe high rates of cellular division duringhistogenesis and organogenesis, any tera-togen that interferes with the processes ofcell division, replication, transcription,and translation may cause an error ofmorphogenesis. Descriptive toxicologistshave identified numerous teratogens, butthe biochemical mechanisms by whichthese agents have a teratogenic impact onhumans, as well as other species, for themost part remain unknown. The follow-ing three examples, two drawn from phar-maceuticals and one from dietary prefer-ence, illustrate the specificity of teratogen-esis. They also reveal the uncertainty ofour understanding of the mechanisms ofteratogenic action.

Thalidomide

In 1956, thalidomide was introduced inEurope as a sedative drug to alleviate thesymptoms of “morning sickness,” such asnausea and vomiting during the first tri-mester of pregnancy. The Food and DrugAdministration (FDA) refused to approvethe use of thalidomide in the United Statesuntil additional studies on its safety werecompleted. Meanwhile, evidence was ac-cumulating about the safety of the drug.Of particular concern was the sudden in-crease in limb deformities in Germany andEngland, which were subsequently linkedto the maternal use of thalidomide dur-ing the first trimester. The drug is nowknown to be a potent teratogen and thecausative agent for numerous errors inmorphogenesis involving limbs, ears, andthe heart.

Phocomelia (G. phoko, seal; G. melos,limb) and amelia (i.e., without a limb orlimbs), small or missing ears, and heartanomalies were among the teratogenic ef-fects observed in over 7,000 children bornto women who took thalidomide for thera-peutic purposes early in pregnancy. Expo-sures to even as little as one tablet duringthe crucial time windows when limbs, ears,and the heart are forming are known toresult in teratogenesis (Figure 8–5).

It is interesting that thalidomide doesnot exhibit teratogenic properties (i.e.,phocomelia or amelia) in toxicity tests inwhich mice and rats are the experimentalanimals. However, in primate species(such as marmosets) teratogenesis is ob-served (Figure 8–6). The implications ofgeneralizing conclusions from nonhumantoxicity testing are self-evident. After threedecades of extensive research, the precise


mechanism of toxicity is still unknown.Although thalidomide was withdrawnfrom the market in 1961, its pharmaceu-tical value in the treatment of HIV/AIDSis currently being investigated.

Diethylstilbestrol

Between 1940 and 1971 over 2 millionwomen were given diethylstilbestrol(DES), a potent synthetic estrogen(antiabortifacient) prescribed as a phar-maceutical treatment for high-risk preg-nancies. DES is associated with a numberof genital-tract anomalies present at birthand has also been implicated as a “de-layed” teratogen.

Unlike thalidomide, whose teratogeniceffects are manifest at birth, the delayedteratogenic actions of DES are observedin daughters, most commonly between theages of 17 and 22, whose mothers tookDES during pregnancy. The delayed tera-togenesis includes an uncommon neo-plasm (i.e., new growth, cancer), called aclear-cell adenocarcinoma, found almostexclusively in women exposed in utero toDES.

Although animal studies involving miceand rats show a dose-response relation-ship, the mechanism of teratogenesis isunknown. That teratogenic effects may beobserved two decades after birth is sober-ing. The importance of designing

Figure 8–5. Phocomelia in child exposed in utero to thalidomide, after her mothertook the drug during critical stages of limb bud development. (From E.Rubin and J.L.Farber, Eds., Pathology, 2nd edition. J.B.Lippincott, 1994. Reprinted by permission.)


long-term teratogenic studies again is self-evident.

Fetal Alcohol Syndrome

The consumption of alcoholic beveragesduring pregnancy results in a number ofmorphological and developmentalanomalies. Among the observable effectsare a thin upper lip, microcephaly (i.e.,small head), micrognathia (i.e., small jaw),an underdeveloped philtrum (i.e., depres-sion on upper lip), intrauterine growth re-tardation (IUGR), and CNS dysfunction.

These anomalies are termed fetal alcoholsyndrome (FAS).

Although the prevalence (existing cases)of FAS in Europe and the United States is1–3 per 1,000 live births, in somepopulations with high rates of alcoholismthe incidence (new cases) is 20–150 per1,000. The greatest danger results fromheavy alcohol consumption during thefirst trimester. Approximately 30–50% ofwomen consuming more than 450 mL (1pint) of “whiskey” per day will give birthto a child with FAS. In spite of extensiveresearch, the teratogenic mechanism re-mains unknown.

Figure 8–6. Thalidomide effects in fetal marmosets exposed to the drug between days38 and 52 of gestation. Left, unexposed; right, exposed. (From W.G.McBride andP.H.Vardy, Pathogenesis of Thalidomide Teratogenesis in the Marmoset (Callithrixjacchus): Evidence Suggesting a Possible Trophic Influence of Cholinergic Nerves inLimb Morphogenesis, Dev. Growth Differ., 25:361–373. Blackwell Science, 1983. Re-printed by permission.)


Mutagenesis

Mutagens may affect either germ cells orsomatic cells. Depending on the cell typesaffected, mutagenesis can have two verydifferent outcomes. Somatic cells, unlikegerm cells, are not the cells that contrib-ute genetic information to the next gen-eration. Mutations that occur in somaticcells only produce mutagenic, and oftencarcinogenic, effects in exposed individu-als. Future generations are affected onlywhen mutagens cause genetic damage togerm cells. Mutagens are capable of pro-ducing different types of genetic damage,ranging from small-scale point mutationsin a single gene to large-scale changes in-volving whole chromosomes.

Point Mutations

The “simplest” type of genetic damage re-sults when there is a mutation in the DNAsequence. Referred to as a point mutation,this mutation represents a change in thechromosome involving a single nucleotide(base) within the gene. These changes mayresult in the substitution, deletion, or in-sertion of a base.

A base substitution occurs when a nu-cleotide is substituted for a normally oc-curring base. If the substituted base doesnot alter the amino acid coded for in thatposition, then it will have no effect on theamino acid (Figure 8–7). This outcomeis possible since each amino acid iscoded for by more than one codon (seeFigure 8–1). Two additional outcomes,missense and nonsense, result when themutated triplet codon codes for a differ-ent amino acid or signals stop, respec-tively (Figure 8–7).

Although stated to be a “simple” typeof genetic damage, point mutations resultin complex mutagenic effects. Missense,resulting from base substitution, is respon-sible for sickle-cell disease. Substitutionof the nucleotide adenine for thyminechanges the codon for the sixth amino acidfrom CTC (glutamic acid) to CAC (va-line). This “simple” substitution alters theamino acid sequence present in two of thefour peptides that together formhemoglobin, producing RBCs that arestructurally and functionally abnormal.

The deletion or insertion of a nucleotideresults in an effect called a frameshift. Onthe deletion of a single nucleotide, the re-maining bases must shift one position tofill the space once occupied by the nowdeleted base. The insertion of a nucleotidemeans that all bases must shift to allowfor the added base. The effects of basedeletion and insertion on protein transla-tion are complex, since the translationreading frame has been shifted.Frameshifts may cause extensive missenseor immediate nonsense (Figure 8–7).Frameshifts also result when entire tripletcodons are deleted from or inserted intoa nucleotide sequence.

Several physical and chemical mutagensare responsible for point mutations (Ta-ble 8–1). Base analogues are moleculeswhose chemical structure closely resem-bles pyrimidines and purines. These mol-ecules (e.g., 5-bromouracil) may be sub-stituted for normal bases during DNAsynthesis (i.e., replication). The substi-tuted base will be transcribed to mRNAduring transcription. The base analoguesubstitution will have its ultimate impactduring translation (i.e., no effect, missense,or nonsense).


Figure 8–7. Examples of base mutations. Comparison of the normal codon and codedamino acid with mutations in which base substitution, deletion, and insertion haveoccurred.


Chromosome Aberrations

Large-scale mutations may affect both thestructure and number of chromosomes.Chromosome structure is altered whenchromatids break off or are unexpectedlyexchanged, centromeres fail to form, or“ring” chromosomes form. When thesemutations occur during gametogenesis,and the resulting gamete is involved in fer-tilization, the altered chromosome(s) canbe transmitted to offspring.

Numerical chromosome abnormalitiesare characterized as aneuploid or poly-ploid. Aneuploidy (without a true set) re-sults when there is one more, or one less,chromosome present. The terms trisomyand monosomy, as well as the correspond-ing numerical formulas 2n+1 and 2n–1,are used to indicate these aneuploid con-ditions. Recall that the diploid state con-tains 46 chromosomes; therefore, 2n–1=46–1=45, and 2n+1=46+1=47.

Polyploidy (many sets) occurs when anadditional set or multiple sets of chromo-

somes are present. (In humans, some livercells are normally tetraploid or 4n.) Theadditional chromosome sets result fromthe formation of diploid (2n), rather thanhaploid (n), gametes. For example, anabnormal diploid spermatozoon that fer-tilizes a normal haploid ovum results in atriploid (3n=69) zygote, or if both the sper-matozoon and ovum are diploid, a tetra-ploid (4n=92) zygote results. As evidencedby many agriculturally important poly-ploid plants, polyploidy is an importantsource of genetic variation. However,polyploidy in humans always results inembryolethality or death of the fetus. Inrare instances a neonate, in spite of de-fects present in nearly all organs, may sur-vive for a few days.

As with structural abnormalities, mul-tiple sets of chromosomes in gametes canpotentially be transmitted to offspring.Most likely underestimated, chromosomeaberrations contribute to the incidence ofspontaneous abortion and numerous ge-netic disorders (Table 8–2).

Table 8–1. The effects of selected physical and chemical mutagens


Ames Assay

Toxicity testing to determine which agentsare mutagens is costly andtimeconsuming. This is particularly truewhen the test organisms are mammals,which are expensive to maintain and re-quire longer experimental periods due tothe overall slower rates of cell division.To reduce the cost and time, mutagenic-ity studies use bacteria and mammaliancell cultures. The Ames assay, named af-ter Dr. Bruce N.Ames, is the most com-monly used test for mutagenicity. The testuses the bacterium Salmonellatyphimurium, which is cost-effective andtime-conserving since it has rapid cell di-vision. This assay also is valuable in iden-tifying carcinogens, as about 90% of allknown carcinogens exhibit mutagenicbehavior in the Ames assay.

The Ames assay begins with a special

strain of the bacteria that has a mutationin the gene coding for histidine synthesis.Since histidine is required for cell division,these bacteria will not multiply unless aback-mutation occurs in the histidine syn-thesis gene that again will permit histidinesynthesis. Bacteria are exposed to the sus-pected mutagenic agent in a histidine-freegrowth medium (agar). Also present in themedium are microsomes from a rat’s liver,should the test chemical require biotrans-formation to a reactive metabolite (i.e.,mutagen). Finally, the rate of bacterialmultiplication (i.e., colony growth) ismonitored. If the suspected agent is not amutagen, the bacterial colony will notgrow. If the agent is a mutagen, the result-ing mutations will permit histidine synthe-sis, and colony growth will take place.

Carcinogenesis

All cells possess an inherent rate of cellu-lar division, one in which the rate of nor-mal cell death is matched by the forma-tion of new cells. Cancer, simply stated,occurs when there is uncontrolled prolif-eration of cells. Worldwide, cancer ac-counts for 4,800,000 deaths per year, adistant third cause of death behind car-diovascular disease (12,000,000), and di-arrheal disease (5,000,000). In the UnitedStates, cancer is responsible for one-fifthof the total mortality, exceeded only bydeaths from cardiovascular disease andstroke.

Remember, the World Health Organi-zation (WHO) estimates that 90–95% ofall cancers are “environmentally related.”A few well-known examples are the rela-tionship between lung cancer and cigarettesmoking, cancer of the scrotum and

Table 8–2. Mutagenic effects onhuman chromosomes

Note. Normal sex chromosomes (23rd pair) inmales, XY; and females, XX. Arms (chromatids)of chromosomes are designated “p” (short arm)and “q” (long arm).


occupational exposure of chimney sweepsto soot, and bladder cancer and exposureto dyes (i.e., aromatic amines) in textileworkers.

Epidemiological observations support-ing these relationships are based on anumber of criteria, including the strengthof the association and the consistency ofthe association under different circum-stances. Even in epidemiological studies,the suspected causative agent must pre-cede the effect (i.e., chronological se-quence of the dose-response relationship),and an increase in dose must be and pro-

motion as related to carcinogenesis. par-alleled by an increase in response. Thespecific mechanisms of carcinogenesis arefor the most part poorly understood. Thisis due to the complex mechanisms in-volved in the development of cancer.

From experimental and epidemiologi-cal studies, we know that cancer resultsfrom a multistep process (Figure 8–8).Most frequently the complex process be-gins with a procarcinogen that isnonreactive. Only after the procarcinogenundergoes biotransformation (e.g.,bioactivation) does it become a carcino-

Figure 8–8. Proposed relationship among bioactivation, covalent binding, initiation,and promotion as related to carcinosenesis.


gen (i.e., reactive metabolite) that maycovalently bind to DNA. The interactionbetween a carcinogen and DNA can pro-duce the mutations leading to cancer for-mation.

According to the two-stage theory ofcarcinogenesis, in addition to bioactivationtwo other steps may be involved: initiationand promotion (Figure 8–8). Initiation in-volves a subtle alteration of DNA or pro-teins within target cells by the carcinogen.Promotion occurs when these altered cells,on exposure to a promoting agent, give riseto cancer. The promoting agent itself maybe carcinogenic or noncarcinogenic.

Depending on their mode of actionwithin the cell, carcinogens can be classi-fied as genotoxic or epigenetic. Genotoxiccarcinogens are DNA reactive; they actdirectly on DNA or the expression ofDNA occurring during translation.Genotoxic mechanisms include errors inDNA replication, point mutations, andaberrations in chromosome structure andnumber.

Epigenetic carcinogens are non-DNAreactive. These carcinogens do not directlyalter DNA; however, they are able to “af-fect” carcinogenic activity through numer-ous mechanisms. For example, epigenetic

cocarcinogens may potentiate (increase)the activity of genotoxic carcinogens.Other epigenetic carcinogens act to in-crease cellular activity, modify the activ-ity of hormones, or suppress the immunesystem, all of which may lead to cell pro-liferation (i.e., cancer).

Determining which agents are carcino-genic requires extensive testing. Initially,the Ames assay, as well as others, can beused to assess mutagenicity. Remember,about 90% of all known carcinogens ex-hibit mutagenic behavior in the Ames as-say. If a compound is determined to be amutagen, more costly and time-consum-ing carcinogenic studies using animalmodels are undertaken. Although thereare hundreds of suspected human carcino-gens, the list of actual or proven humancarcinogens is quite short. It is estimatedthat in less than 5% of all cancers thecause is attributable to occupation,whereas about three-fourths of cancersresult from diet (~50%) and tobacco(~25%). It is easy to see how rapidly wecan approach the WHO estimate that over90–95% of all cancers are “environmen-tally related”—diet, tobacco, and occu-pations are all controllable aspects of ourenvironment.

1. All of the following statements about mitosis are true, except:

A. It produces daughter cells with exactly the same chromosome number as the parentcell.

B. All daughter cells are haploid.C. Each daughter cell will be 2n.D. It is used by somatic cells to reproduce.E. It produces daughter cells that are identical in genetic content.

Review Questions


2. A karyotype:

A. Appears as a systematized array of chromosomes.B. Is an invaluable aid in diagnosing chromosomal anomalies.C. Results when chromosomes are viewed and photographed during telophase.D. A and BE. A, B, and C

3. Examine the following normal sequence of bases and resulting amino acid sequence:

Which answer best identifies the mutation in the base and amino acid sequence thatfollows?

A. Base substitution that has no effect.B. Base substitution that causes missense.C. Base substitution that results in immediate nonsense.D. Base deletion that produces a frameshift and immediate nonsense.E. Triplet insertion that results in missense.

4. Which is not a true statement about teratology?

A. Humans are often selected as the test organisms.B. Teratogens may produce major errors of morphogenesis.C. At low doses teratogens may produce embryolethality.D. A and BE. A, B, and C

5. Thalidomide, diethylstilbestrol, and ethanol are all examples of:

A. CarcinogensB. MutagensC. Promoting agentsD. TeratogensE. Translators

6. The greatest sensitivity of organs and organ systems to teratogens is:

A. Prior to fertilization.B. During the preimplantation period.


C. During the first 8 weeks.D. During the second and third trimester.E. During the neonatal period.

7. Which term best describes a karyotype that contains 45 chromosomes?

A. AneuploidB. MonosomyC. TetraploidD. TriploidE. Trisomy

8. Base analogues are molecules whose chemical structure closely resembles pyrimidinesand purines.

A. TrueB. False

9. Diagram and discuss the relationship between bioactivation, covalent binding,initiation, and promotion as related to carcinogenesis.

10. Outline the sequence of events occurring during transcription and translation.

125

Chapter

9


EnvironmentalToxicants

� Define environmental toxicants

� Recognize the contribution ofenvironmental toxicants to worldwidemorbidity and mortality

� Discuss representative categories ofenvironmental toxicants, includingexamples

� Describe the mechanisms of toxicitywithin categories of environmentaltoxicants

Agent Orangealiphatic alcoholsalkylbenzenesanticoagulantsaromatic hydrocarbonsarsenicbenzeneberylliumbiological magnificationbipyridylscadmiumcarbamatescarbon disulfidecarbon tetrachloridecataractogenicchelating agentschlorinated aliphaticschloroformchlorophenoxy compoundschromiumdiabetogenicsdinitrophenoldioxanedioxindithiocarbamateselectromagnetic fieldsenvironmental toxicantsethyl alcohol


ethylene glycolfluoroacetic acidfungicidesglycolsherbicideshexachlorobenzeneinsecticidesionizing radiationleadMee’s linesmercurymethyl alcoholnickelnickel itchnonbiodegradable

nonphotodegradablenorbormideorganochlorinesorganomercurialsorganophosphatespesticidesphthalimidesplasticspyrinimilradiumrodenticidesthermoplasticsthermosetting plasticsvasoconstrictorswarfarin


Environmental Toxicants 127

Introduction to EnvironmentalToxicants

Environmental toxicants are agents in oursurroundings that are harmful to humanhealth. Some substances, such as air andwater pollutants, are recognized for theirtoxicity. No one would knowingly breathair polluted with sulfuric acid or drink wa-ter containing pesticides. However, othertoxic agents equally harmful to humanhealth (food additives and contaminants,bacteriotoxins, fungitoxins, phytotoxins,household products, and industrial chemi-cals) often go unrecognized for their seri-ous toxicity.

Although man-made products (e.g.,industrial chemicals) are thought to ex-hibit greater toxicity, historical data sug-gests that naturally occurring substancesare a larger concern to human health.Without minimizing the danger of man-made products, consider the annual im-pact of bacteriotoxins alone on humanmorbidity and mortality. Secretorydiarrhea, produced by toxins from bacte-ria (e.g., Vibrio, Salmonella, Shigella, andEscherichid), is responsible for the deathof 5,000,000 persons worldwide eachyear—most of whom are children—whodie from complications associated withdehydration and electrolyte imbalance.

This chapter synthesizes information onexposure, toxicokinetics, toxicodynamics,target organ toxicity, teratogenesis, mu-tagenesis, and carcinogenesis relating tospecific categories of environmentaltoxicants. Examples are presented for eachcategory, as well as pertinent informationon their routes of absorption, modes ofaction, toxicokinetics, and clinical symp-toms associated with toxicity. This is by

no means an exhaustive listing; rather, itis intended to illustrate the principles oftoxicology and explain by example theeffects of environmentally hazardous sub-stances on human health.

Pesticides

Pesticides are agents that destroy or repelunwanted organisms (i.e., pests; L. pes-tis, destruction, death, pestilence; -cide,kill). They are typically classified as to theorganisms they destroy, as the terms fun-gicides (fungi), herbicides (plants), insec-ticides (insects), or rodenticides (rodents)imply. Animal pesticides are effective ineliminating unwanted animals because thepesticides use, to the extreme, the sametoxicodynamics that produce toxicity inhumans. Only differences in dose, expo-sure, and toxicokinetics—but nottoxicodynamics—are usually evident.Neuronal transmission in cockroaches in-volves similar physiological phenomenaas in humans.

Insecticides

Most insecticides are neurotoxicants thatdisrupt the transmission of a nerve im-pulse either as it passes along the axon orat the synapse. Insects exposed toneurotoxicants respond with twitching,weakness, and paralysis, which leads todeath. Similar symptoms are also seen inhumans.

Organophosphates. Parathion,diazinon, and malathion all inhibitcholinesterases (particularly acetylcho-linesterase), the enzymes responsible for thedegradation of the neurotransmit ter ace-


tylcholine. Failure to degrade acetylcholinereleased into neuronal synapses of the CNSand at myoneural junctions results in con-tinued, repeated synaptic transmission thatmay lead to paralysis. In humans, absorp-tion is through percutaneous, respiratorysystem, or digestive system routes. On dis-tribution, organophosphates cross theblood-brain barrier to elicit CNS toxicity.Toxicants undergo phase I and phase IIbiotransformations in the liver and aresubsequently eliminated. As neurotoxins,organophosphates involve most organs,including the gastrointestinal tract (nau-sea, vomiting), the respiratory system (ex-cessive bronchial secretions), the cardio-vascular system (decrease/increase in heartrate or blood pressure), skeletal muscles(weakness, paralysis), and the CNS (men-tal confusion, fatigue).

Carbamates. Like organophosphates,carbamates (aldicarb, carbaryl, propoxur)inhibit the enzymatic action ofcholinesterases. The toxicant enters thebody through the percutaneous, respira-tory system, and digestive system routes.In humans, oral doses of as little as 3 mg/kg can result in toxicity, and fatalities dueto carbamate toxicity have been reported.Biotransformation reactions rapidly breakup (hydrolyze) the carbamate cholineste-rase molecule, thereby reactivatingcholinesterase. This explains the unusu-ally short duration of carbamate-inducedneurotoxicity. CNS and neuromuscularjunction symptoms include nausea, vom-iting, sweating, muscle weakness, and—in severe cases—convulsions.

Organochlorines. Dichlorodiphenylt-richloroethane (DDT) and other chlorin-ated organic insecticides act to stimulateor depress the CNS. The neurotoxicity of

DDT is thought to result from membrane-altering processes that act to diminish therate of repolarization, such as impairedtransport of Na+ and K+ in the axon, andCa2+ as it signals the release ofneurotransmitters in the region of thesynapse. Neurons that are not fullyrepolarized require less of a stimulus toinitiate signal transmission; thus, affectedneurons have increased sensitivity thatleads to repetitive signaling. Primaryroutes of absorption differ for representa-tive organochlorines. Case reports showthere is less toxicity associated with cuta-neous exposure, probably due to poorabsorption through the skin. The morecommon route of absorption leading totoxicity involves the ingestion of DDT.

The toxicokinetic properties oforganochlorines that make them goodinsecticides also are properties responsi-ble for the banning of DDT. For exam-ple, as a result of high chemical stability,DDT and its breakdown products/metabolites persist in the environment.DDT’s lipid solubility, coupled with a slowbiotransformation rate, promotes accu-mulation in individual organisms, as wellas in organisms farther up the food chain(i.e., biological magnification). Organo-chlorines are readily stored in fat and areslowly eliminated at a rate of about 1%per day. Complex biotransformationpathways include both phase I (dechlo-rination, demethylation) and phase II (glu-tathione conjugation) reactions, followedby elimination. Clinical symptoms asso-ciated with acute CNS toxicity includeheadaches, dizziness, tremors, and convul-sions. Symptoms of chronic toxicity areloss of memory, personality changes, anda reduction in sperm count in males.


Herbicides

Herbicides act to eliminate unwantedplants (e.g., weeds) by interfering withhormonal systems that regulate growth orby promoting water loss (i.e., desiccation).Although effective in eliminating plants,most herbicides are weakly toxic to hu-mans, most likely due to inherent differ-ences in plant and animal cell structure(e.g., cell membrane) and function (e.g.,biochemical pathways).

Bipyridyls. As herbicides, bipyridyls(e.g., diquat, paraquat) act to desiccateplants. Although these toxicants may beabsorbed through percutaneous, respira-tory, or digestive system routes, the capac-ity for paraquat to preferentially becomeconcentrated in the lung tissues is welldocumented. On distribution to the lung,paraquat decreases gas exchange by dam-aging pneumocytes. This in turn decreasesthe transport of O2 and CO2 across alveo-lar cell membranes. Biotransformation ofbipyridyls is poorly understood; however,elimination is via urinary and fecal routes.Clinical symptoms of paraquat toxicityinclude anoxia and coma, as well as dam-age to the lungs, liver, and kidneys. Theingestion of concentrated paraquat almostalways leads to death.

Chlorophenoxy Compounds. 2, 4-Dichlorophenoxyacetic acid (2, 4-D) and2, 4, 5-trichlorophenoxyacetic acid (2, 4,5-T) are well-known chlorophenoxy com-pounds. Agent Orange, a 50:50 mixtureof 2, 4-D and 2, 4, 5-T, was extensivelyused during the Vietnam conflict. The her-bicidal properties of these compoundspromote uncontrolled plant growth thatrapidly leads to plant death.Chlorophenoxy compounds are weaklytoxic to humans, but a trace impurity (2,

3, 7, 8-tetrachloro-dibenzo-p-dioxin orTCDD) that results from the manufactur-ing of 2, 4, 5-T is a powerful toxicant.Animal studies indicate that TCDD (ordioxin as it is commonly referred to inthe popular media) is a potent toxicantrecognized for its dermatotoxicity (chlo-racne) and teratogenic and carcinogenicproperties. Chlorophenoxy compoundsare absorbed through percutaneous or res-piratory and digestive system routes. Littleis known about their toxicodynamics ortoxicokinetics, especially as related tobiotransformation and routes of elimina-tion. Symptoms associated with toxicityinclude sweating, scanty urine production(oliguria), peripheral neuropathies, muscleweakness, dizziness, headaches, vomiting,and fatigue. The LD50 for 2, 4-D is about300 mg/kg, with threshold TDs rangingfrom 50 to 60 mg/kg.

Dinitrophenol. In addition to its herbi-cidal use, 2, 4-dinitrophenol (DNP) wasonce marketed as an over-the-counter an-tiobesity agent. Aside from pharmaceuti-cal dosing, exposure may occur throughpercutaneous or respiratory and digestivesystem routes. Toxicity is due to DNP’s ca-pacity to inhibit ATP synthesis, which maylead to acute symptoms of tachypnea (rapidbreathing), tachycardia (rapid heart rate),sweating and coma, and chronic symptomsof fatigue, anxiety, and weight loss. Of ad-ditional interest are the cataractogenicproperties of DNP. Cataracts (loss of trans-parency of the lens of the eye) developedin more than 100 persons who used DNPas an antiobesity agent from 1935 to 1937.As is typical for many aromatic nitro andamino compounds, DNP is also a carcino-gen. There is a paucity of information ontoxicokinetic properties related to biotrans-formation and elimination of DNP.


Fungicides

Although many fungicides are no longerin use, a review of their toxic propertiesprovides an important reminder of the gen-eral safety as related to the approximate100 million pounds of fungicides used eachyear in the United States. Fungi, the in-tended victims of fungicides, are themselvesresponsible for producing some of thedeadliest toxins (i.e., fungitoxins). For ex-ample, Amanita phalloides, the “death capmushroom,” contains potentially lethalphallotoxins, and aflatoxins (BI) from As-pergillus flavus are well documented fortheir hepatocarcinogenic activity.

Hexachlorobenzene. Prior to 1960,hexachlorobenzene (HCB) was used totreat seed grain. This prevented fungalinfestation in seeds before they wereplanted. In the late 1950s, about 4,000Turkish citizens became seriously ill whenthey mistakenly ingested HCB-treatedseed grain for food grain. Toxic responsesinclude skin blisters, hepatomegaly (en-larged liver), and thyroidomegaly (en-larged thyroid), as well as arthritis, os-teomyelitis (inflammation within bones),and osteoporosis (loss of bone density) inthe hands. Oral ingestion is the obviousroute of absorption. Little is known aboutthe toxicokinetics of HCB; however, likeorganochlorines (e.g., DDT), HCB persistsin the environment, is biomagnified, andhas a comparatively long biological T1/2

due to its slow rate of biotransformation.Organomercurials. Mercurial com-

pounds (e.g., methylmercury) were usedto treat seed grains as recently as the1970s. In two incidents, separated bythousands of miles, organomercurialswere implicated in epidemic poisonings.

In Iraq the direct ingestion of treated grainwas responsible for toxicity, whereas inNew Mexico, treated grain was fed tohogs that were subsequently slaughtered,and persons consuming these hogs becameill. Again the digestive system was theroute of absorption. Acute toxicity in-volves the gastrointestinal and renal sys-tems; particularly evident is proximal tu-bule damage within the nephron.

Phthalimides. These potent fungicides(Captafol, Folpet) have chemical structuressimilar to thalidomide—a similarity thathas caused controversy about their use (Fig-ure 9–1). In spite of the oral LD50 valuesof 10,000 mg/kg observed in rats, uncer-tainty over mixed results from teratogenicand mutagenic studies in hamsters, mice,and rats has halted their use. Thetoxicodynamics of phthalimides are un-clear; however, there is indication that theymay interfere with enzymes. As dusts,emulsions, and sprays, these chemicals areprimarily absorbed through respiratorysystem and percutaneous routes. Animalstudies indicate phthalimides are rapidlybiotransformed and eliminated in the urineand feces. Symptoms of acute toxicity in-clude irritant and allergic contact derma-titis, and chronic toxicity, mutagenicity,carcinogenicity, and teratogenicity as evi-denced from nonhuman animal studies.

Dithiocarbamates. Besides their use asfungicides (e.g., ethylene-bidithiocar-bamate, or EBDC), some dithiocarbam-ate derivatives are used as insecticides(acetylcholinesterase inhibitors) and in thechemical and rubber industries. As evi-denced in the chemical brand namesFerbam, Maneb, Nabam, and Zineb,dithiocarbamates contain metal ions, suchas Fe3+, Mn2+, Na+, and Zn2+, respectively.


Dithiocarbamates enter the body throughpercutaneous or respiratory and digestivesystem routes, and are rapidly distributed,biotransformed, and eliminated. In gen-eral, dithiocarbamates are of relativelylow toxicity to humans. Depending on theassociated metal ion, toxicity may includeboth irritant and allergic contact derma-titis and CNS depression. Animal studiesgive indication of dithiocarbamate’s mu-tagenic and carcinogenic potential.

Rodenticides

Numerous vertebrate organisms (such asbats, coyotes, rabbits, skunks, and wolves)have at one time or another been consid-ered pests. Rodenticides (agents lethal torodents) are of particular importance to

human health, since rodents (e.g., rats andmice) often serve as vectors for the trans-mission of disease. Most recently, theemergence and spread of the hantavirushas been linked to a sudden increase indeer mice, which are carriers of the virus.Use of rodenticides in the deer mice popu-lation is just one of many solutions tocontrol the spread of hantavirus. It is notsurprising that, due to similar cellular pro-cesses in the two orders, Rodentia andPrimates (both in the class Mammalia andphylum Vertebrata), there would be simi-lar manifestations of toxicity, the severityof which is dose related. Examples of ro-denticides include anticoagulants, inhibi-tors of cellular respiration, vasoconstric-tors, and diabetogenics. Since most roden-ticides are topically applied to palatable

Figure 9–1. Similarity between the chemical structure of thalidomide (a teratogen) andphthalimide (a fungicide). “R” represents a variety of functional groups, each of whichdistinguishes a specific phthalimide fungicide.


baits (e.g., seeds), the anticipated route ofabsorption is the gastrointestinal system.Aside from accidental percutaneous andrespiratory system exposures in the manu-facturing process, human toxicity often re-sults from the ingestion of rodenticides,particularly the accidental ingestion bychildren.

Anticoagulants. Medically importantanticoagulants (heparin, warfarin, aspirin)are routinely used in the treatment ofthrombi (stationary blood clots) and em-boli (transient blood clots). The ability ofthese drugs to prevent the aggregation ofthrombocytes (platelets) effectively“thins” blood—an action that is exploitedin their use as rodenticides. A commonrodenticide, warfarin (coumadin), inhib-its the synthesis of vitamin K. In the ab-sence of vitamin K, liver cells are unableto produce prothrombin, a molecule vitalto the completion of a series of cascadingreactions that lead to clot formation. Inhumans, repeated exposure to warfarin isusually required to cause blood to thin,which may result in internalhemorrhaging, as seen in bruises,gastrointestinal bleeding, cerebrovascularaccidents (strokes), and nosebleeds. Dif-ferences in anticoagulant toxicokineticsare apparent in plasma protein bindingcomparisons—warfarin (97%), aspirin(50–70%), heparin (trace %)—and bio-logical T1/2 comparisons: warfarin (40hours), aspirin (3 hours), and heparin (1hour).

Inhibitors of Cellular Respiration. De-rivatives of fluoroacetic acid (sodium fluo-roacetate, fluoroacetamide) are extremelytoxic rodenticides. These chemicals blockone of many enzymes involved in theKrebs cycle, impeding a major pathway

responsible for ATP production. In theabsence of the high-energy molecule ATP,cellular functions come to a halt. Clinicalsymptoms include nausea, vomiting, ab-dominal pain, increased heart rate, kid-ney failure, coma, and death (with thresh-old LDs of �10 mg/kg for sodium fluo-roacetate).

Vasoconstrictors. The toxicodynamicsof norbormide involve contraction of thesmooth muscles surrounding peripheralblood vessels. Under normal conditionsthe contraction and relaxation of this mus-culature surrounding blood vessels is animportant mechanism for regulatingblood pressure. However, in rats, 5–15mg/kg of norbormide elicit irreversiblevasoconstriction, leading to ischemia (re-duced blood flow to tissues) and necrosis(cell death), followed by death of the or-ganism. These toxic responses are not seenin other vertebrate species, including evenacute exposures of up to 300 mg in hu-mans.

Diabetogenics. As exemplified bypyrinimil, diabetogenics interfere with glu-cose metabolism by exerting cytotoxiceffects on ß islet cells of the pancreas. Thehormone insulin, produced by ß cells, isnecessary to facilitate the passage of glu-cose across the cell membranes of all bodycells. The absence of glucose in the cellleaves the cell without the ready energysource needed to produce ATP via glyco-lytic pathways. On exposure to pyrinimil,there is an initial period of hyperglyce-mia (abnormally elevated blood glucoselevels) due to the inability of glucose tomove from circulating blood into the sur-rounding tissue cells. Hypoglycemia (ab-normally decreased blood glucose levels)usually follows, with symptoms of light-


headedness and urinary retention. Neu-rological disorders, both sensory andmotor, may also be present. may also bepresent.

Plastics

Polymers that can be shaped by pressureor heat to the form of a cavity or moldare termed plastics (G. plastikos, fit formolding). Two forms of plastics are gen-erally recognized: thermoplastics and ther-mosetting plastics. Thermoplastics, whichcomprise over 80% of all plastics, can beremelted and remolded (e.g., polyethylene,polypropylene, polyvinylchloride, poly-styrene), and as such are of interest to nu-merous recycling efforts. Plastics that oncemolded cannot be remelted and remoldedare termed thermosetting plastics. Con-cern about plastics as environmental toxi-cants is twofold. First, many plastics—thermo-plastics in particular—are nonde-gradable. They resist biological degrada-tion (nonbiodegradable) and degradationfrom ultraviolet radiation (nonpho-todegradable). The possibility is that plas-tics could persist in landfills or as road-side pollutants for hundreds of years. Al-though more than a fourth of all alumi-num and paper are recycled in the UnitedStates each year, only about 1% of plas-tics are recycled. Second, attempts to in-cinerate some plastics (e.g.,polyvinylchloride or PVC), to reduce theircontribution to landfills, result in the pro-duction of toxic chemicals. One such toxicchemical, dioxane, may be absorbed viathe respiratory system. In animal studiesdioxanes are demonstrated carcinogens,most probably involving an epigeneticmechanism.

Metals

Although many of the 80 known metalsare vital to normal physiological processesin humans (e.g., Fe, Mg, Zn); other met-als, such as Pb, Hg, and Cd, are amongthe oldest toxicants known to humans.Metals are unique as toxicants—they areneither created nor destroyed by organ-isms, plants, or animals, because as chemi-cal elements they cannot be degraded be-yond their elemental states. In fact, insomewhat parallel roles (Mg in chloro-phyll and Fe in hemoglobin), metals of-ten accomplish their functional role byswitching between valences (e.g.,Fe++↔Fe+++) in their interaction with othermolecules. They may be in the form ofelemental metals (e.g., Hg), nonorganicsalts (e.g., HgCl2), or organic metal com-pounds (e.g., Hg(CH3)2).

Metals enter the body through digestiveand respiratory routes. Urine is the mostcommon route of elimination andnephrons are often the site of toxicity, asevidenced in the tubules. Metals may bedirectly excreted through the intestinalmucosa into the lumen of the digestivetract. The enterohepatic circulation ofsome organic metal compounds (e.g.,methylmercury) serves to increase their T1/

2. Other mechanisms contribute to theelimination of metals, such as loss throughbreast milk, hair, nails, and exfoliating skin.

Many metals are considered essentialto normal cellular activity. However, inexcess they may cause toxic responses(Table 9–1). Metals are also used in medi-cal diagnostics and treatments (Table 9–2). Pharmaceuticals are available to as-sist in the removal of toxic metals fromthe body. These chelating agents facilitate


the elimination of toxic metals by form-ing a “metal—ion” complex that is morereactive and hence more readily eliminated(Table 9–3). Most chelating agents arenonspecific. Monitoring essential metalions (e.g., Ca2+) during the administrationof chelating agents is necessary, since areduction in blood levels may lead topathologies (e.g., muscle and nervous sys-tem dysfunction).

Arsenic

The “toxic and tonic” value of arsenicalsis controversial. Toxic properties of arsenichave been recognized for hundreds ofyears, and the medicinal use of arsenic ton-ics was common as recently as 100 yearsago. Their use in insecticides, weed killers,and wood preservatives is of concern, sincearsenic is toxic to a variety of organisms.

Table 9–2. Medically important metals

Table 9–1. Essential metals


On ingestion arsenic iseffectively absorbed by the gastrointesti-nal system and also may enter the body viathe respiratory system. The kidneys are theprimary pathway for the elimination ofarsenic; however, it also is lost from thebody as a result of desquamation (loss ofstratified squamous epithelium—skin), insweat, hair, and in the fingernails and toe-nails. Signs of chronic arsenic toxicity areevidenced in the nails by the presence ofhorizontal white bands (i.e., Mee’s lines).Acute arsenic toxicity includes anorexia,hepatomegaly, possible cardiovascular fail-ure, and death (threshold LD of >70mg).Chronic toxicity results in both CNS andPNS pathologies, including muscle weak-ness and loss of sensory perception.

Beryllium

This metal is released by the combustionof coal and in the manufacture of alloys,mainly those associated with aero-spaceindustries. Beryllium is absorbed via therespiratory system, a route that, underchronic exposure, presents the lungs withsufficient concentrations of the metal toproduce berylliosis, a disease in which thelungs decrease in size, become fibrotic,

and may develop cysts (or honeycomblung). The most common toxic effect as-sociated with dermal exposure is allergiccontact dermatitis. Epidemiological stud-ies indicate that beryllium is a human car-cinogen.

Cadmium

Used in manufacturing processes and inmany household products, cadmiumreadily enters the body through the respi-ratory system. Once in the blood, it bindsto large proteins (e.g., albumin) for dis-tribution to tissues, primarily the kidneys.Cadmium has a long biological T1/2, pos-sibly 30 years. Toxicity associated withacute respiratory exposure may includepulmonary edema (accumulation of fluidin the lungs), whereas ingested cadmiummay result in nausea, vomiting, and ab-dominal pain. Chronic exposures arelinked to nephrotoxicity, most often af-fecting the tubules rather than the glom-eruli. Recent epidemiological studies ex-amining “Ni-Cad” (nickel—cadmium)battery workers in Britain and Swedensuggest a link between cadmium exposureand increased risk for developing cancerof the prostate and lung.

Table 9–3. Chelating agents


Chromium

The main sources of chromium exposurecome from chromite ore mining and sub-sequent uses in the production of stain-less steel, paint pigments, and wood pre-servatives, and in leather tanning. Of themany chromium valences (Cr2+ to Cr6+),only the trivalent and hexavalent arethought to be of biological importance;however, toxicity is usually only associ-ated with hexavalent chromium. The res-piratory system is the major route of chro-mium absorption. A common renal symp-tom associated with the ingestion of chro-mium is acute tubular necrosis (ATN).Chromium is recognized as a contact al-lergen and carcinogen (cancer of the lung).

Lead

In 1815, Orfila recognized “poisoning bylead as the most important to be knownof all those that have been treated of, upto the present time.” Lead in ceramics,paints, and automobile exhausts is still rec-ognized for its toxicity, particularly as itaffects children. Absorbed through respi-ratory or digestive system routes, lead pref-erentially binds to RBCs for distributionto the tissues. Of interest is the storage oflead in bone, where its T1/2 may exceed 20years. Although slow, the elimination oflead occurs through the kidneys. At about100 µg/dl (micrograms per deciliter ofblood), toxicity symptoms includehematopathies, neuropathies, and nephro-pathies. Carcinogenesis—demonstrated inanimals but not in humans—is also sus-pected. In children, lead encephalopathy(disease of the brain) may result in loss ofappetite, ataxia, coma, and death.

Mercury

Much of the mercury in the environmentoriginates from natural geological pro-cesses, such as the degassing of the earth’scrust. As with other metals, toxicity mayresult from elemental mercury, nonorganicmercury salts, and organic mercury com-pounds. There are differences in the ab-sorption of the various forms of mercury,with higher gastrointestinal absorption fororganic mercury. Also, different effects ontarget organs for each form of mercuryare evident, with nonorganic salts concen-trating in the kidneys and organic mer-cury showing a preference for the brain.Elimination, again form dependent, isthrough urine or feces. Animal studiesshow that all forms of mercury cross theplacental barrier, and it is most likely thatsince organic mercury crosses the blood-brain barrier in humans, it also movesacross the human placenta. The tragicconsequences of mercury toxicity werewell illustrated in Minamata Bay, Japan,during the 1950s, where mercury releasedfrom a chemical factory entered the bayand contaminated the food supply. Tox-icities resulting from exposure to meth-ylmercury include neuropathies, nephro-pathies, teratogenesis, and mutagenesis.

Nickel

Used in alloys, batteries, coins, electron-ics, and food processing, nickel is ab-sorbed primarily through the respiratorysystem, with less efficient absorption oc-curring in the digestive system. It readilybinds to plasma proteins and is rapidlyeliminated by the kidneys. In sensitizedindividuals, dermal exposures that occur


when handling coins or wearing costumejewelry may result in aller gic contact der-matitis—commonly called “nickel itch.”Epidemiological studies have shown astrong association between occupationalexposure to nickel and increased risk ofdeveloping lung cancer and cancer of thenasal cavities.

Organic Solvents

The toxicological parameters for allknown carbon-based solvents cannot befully described—there are just too many.However, representative categories (Fig-ure 9–2), with relevant examples, illustrate

the toxic effects of organic solvents onhuman health. In general, two toxic re-sponses are observed to result from ex-posure to organic solvents: (1) depressionof the CNS and (2) irritation of tissuesand membranes. The latter is expected,as lipids in the cell membrane are vulner-able to the solvent characteristics of or-ganic molecules—organic solvents “de-fat” the membrane!

The ability of organic solvents to depressthe CNS is exploited with intravenous andinhalational anesthetics. Since the blood-brain barrier (as well as the brain itself) islargely composed of lipids, effectiveanesthetics must have high lipid solubilities.In fact, the higher the lipid solubility of an

Figure 9–2. Representative organic chemicals.


anesthetic, the higher its potency. Once inthe CNS, organic solvents—like anesthet-ics—exert their depressant actions by in-terfering with neuronal signaling.

Although toxicity is often associatedwith occupational exposure to organicsolvents, their ingestion by children is aserious concern—one-fourth of allpediatric poisonings involve organic sol-vents, usually from household productsthat contain hydrocarbons. Solvents arealso potent nephrotoxins, hepatotoxins,and cardiotoxins.

Aliphatic Alcohols

Aside from water, ethyl alcohol (ethanol)is the most common solvent. Present inalcoholic beverages, ethyl alcohol entersthe body by ingestion. Other exposuresmay result from contact with “gasohol,”or its use as a chemical feedstock or inmanufacturing. Ethyl alcohol crosses theblood-brain barrier and depresses theCNS. Ethyl alcohol’s role as an occupa-tional toxicant pales next to its role as anabused “drink”—each year thousands ofinnocent victims are killed by drivers un-der the influence of alcohol’s CNS depres-sant effects. It is also a well-known ter-atogen (FAS) and carcinogen linked tooral, pharyngeal, laryngeal, esophageal,and hepatic cancers. Another aliphaticalcohol, methyl alcohol (methanol orwood alcohol), is easily absorbed throughpercutaneous or respiratory and digestivesystem routes. Although less inebriatingthan ethyl alcohol, methyl alcohol targetsthe neural cells in the retina of the eye toproduce intra-axonal swelling. This maylead to visual system pathologies, includ-ing permanent blindness.

Chlorinated Aliphatics

Used as an anesthetic (controlled neuro-toxicity) up to the early 1900s, chloro-form (CHCl3) is now recognized as anephrotoxin, hepatotoxin, andcardiotoxin. Absorbed through the respi-ratory system, chloroform is readily me-tabolized to reactive metabolites that aretoxic to the kidney, liver, and heart. Car-bon tetrachloride (CCl4), used as a sol-vent in dry cleaning, is highly hepatotoxic.One suggestion is that the toxic effects inthe liver result from CCl4 reactive metabo-lites that bind to and inactivate cyto-chrome P-450. Once inactivated, cyto-chrome P-450 is no longer able to facili-tate detoxication, leaving hepatocytes vul-nerable to other xenobiotics as well asendogenous molecules.

Carbon Disulfide

Carbon disulfide (CS2)is used in the pro-duction of cellophane and semiconductorsand, less frequently, as a pesticide. Inha-lation is the route of absorption, andbiotrans-formation produces sulfur-con-taining metabolites that are eliminated inthe urine. Distributed to the brain, CS2

produces severe CNS and PNS toxicities,including organic brain damage, sleep dis-turbances, memory loss, Parkinson’s dis-ease-like symptoms, and ocular and au-ditory disorders.

Glycols

Of the glycols (ethylene, diethylene, andpropylene), ethylene glycol is commonlyknown as automobile “antifreeze.” Be-cause of its sweet taste, ethylene glycol


is often ingested by cats, dogs, and otherdomestic animals. Ethylene glycol un-dergoes a series of biotransformation re-actions in the liver to form oxalic acid.In the kidney, oxalic acid accumulates incells that form the tubules of the neph-ron and as crystalline precipitateswithin the tubules. Tubule cytotoxicityand the presence of crystals within thetubule are thought to contribute to renalfailure. Propylene glycol antifreeze of-fers a less toxic alternative to traditionalethylene glycol, since it isbiotransformed to lactic acid, pyruvicacid, and eventually CO2 and water as aresult of the Krebs cycle.

Aromatic Hydrocarbons

Named for their “aromatic” scent, thesehydrocarbons each contain a six-carbonring: benzene, a single ring, andalkylbenzene, a ring with a side chain (ali-phatic). Used as a solvent and chemicalfeedstock, benzene has now replaced alkyllead compounds as an “antiknocking”agent in fuels. Benzene is absorbed per-cutaneously or via the respiratory system.Toxicity likely results from benzene me-tabolites rather than directly from ben-zene. Hematotoxicity may lead to pancy-topenia, as well as leukemia. Toluene, xy-lene, and ethylbenzene are examples ofalkylbenzenes. Inhalation is the primaryroute of entrance, and toxicity predomi-nately involves CNS depression.

Other Environmental Toxicants

Ionizing radiation in the form of alphaparticles, beta particles, gamma rays, and

x rays are usually not readily recognizedas “toxicants.” However, the energy con-tained in these agents is sufficient to dam-age cells and produce toxicity. During theearly 1900s thousands of people wereexposed to radium, either as participantsin faddish medical therapies or as radiumdial painters who “sharpened” the tips ofsmall paint brushes by twirling the bristleson their tongue. Epidemiological studiesof these two populations show an in-creased incidence of osteogenic carcinoma(bone cancer), probably the result of thesubstitution of radioactive 226Ra for Caduring biomineralization.

Epidemiological studies of persons ex-posed to radioactive 224Ra as a treatmentfor tuberculosis, x rays as a treatmentfor ringworm (both early to mid 1900s),and atomic bomb survivors (1945), alsoreveal these forms of ionizing radiationto be powerful genotoxic carcinogens.Dramatic increases in childhood thyroidcancer rates in countries most contami-nated by the 1986 nuclear accident atChernobyl have been linked to radiationexposure—most likely radioactive iodinereleased during the catastrophe(Figure 9–3).

Several epidemiological studies havelinked exposure to electromagneticfields (EMFs), associated with electricaltransmission lines and household elec-trical wires/devices, with an increasedrisk of developing cancer. Two types of“fields,” magnetic and electric, are ofpotential concern. However, researchershave been unable to determine if EMFsare causative or associative—no mecha-nism has been identified to explain thelink between EMFs and suspected bio-logical effects.


Figure 9–3. Increase in thyroid cancer cases per year following the nuclear accident in1986 at Chernobyl. Source: WHO, 1995.

1. Historical data suggest which category to be the greatest contributor to toxicity?

A. Household productsB. Industrial chemicalsC. Man-made productsD. Naturally occurring substancesE. Pesticides

2. Which of the following chemical categories are insecticides?

A. BipyridylsB. CarbamatesC. OrganochlorinesD. A and BE. B and C

3. On distribution to the lung, this toxicant affects gas exchange by damagingpneumocytes.

A. AldicarbB. CarbarylC. DDTD. DNPE. Paraquat

4. Which are true statements about Agent Orange?

A. It is a 50:50 mixture of 2, 4-D and 2, 4, 5-T.

Review Questions


B. It contains a trace impurity called dioxin or TCDD.C. It was once marketed as an antiobesity agent.D. A and BE. A, B, and C

5. This toxicant has cataractogenic properties:

A. Ethylene-bisdithiocarbamate (EBDC)B. 2, 4-Dinitrophenol (DNP)C. Fluoroacetic acidD. Hexachlorobenzene (HCB)E. Warfarin

6. Toxicity mechanisms of rodenticides include all of the following except:

A. AnticoagulantsB. DiabetogenicsC. Inhibitors of cellular respirationD. Proximal tubule damageE. Vasoconstrictors

7. Which are true statements about metals?

A. Chelating agents facilitate the removal of toxic metals from the body.B. Some metals are essential to normal cellular functioning.C. Metals are often destroyed by the human body.D. A and BE. A, B, and C

8. Sign(s) of chronic arsenic toxicity include:

A. Acute tubular necrosisB. Cancer of the nasal cavitiesC. Mee’s linesD. A and BE. A, B, and C

9. In general, which toxic responses result from exposure to organic solvents?

A. Depression of the CNSB. Irritation of tissues and membranesC. OsteoporosisD. A and BE. A, B, and C

10. Discuss epidemiological studies that link ionizing radiation to toxicity.

143

Chapter

10Risk Assessment


� Define risk and safety

� Describe the use of the termsprobability and incidence as related torisk

� Identify factors that contribute todifferences in risk perception

� List the processes of risk assessment

� Summarize the parameters needed toestimate risk

� Recognize the importance of riskmanagement

� Discuss the Safe Human Dose formula

� Explain the contributions ofenvironmental toxicology to thesurvival of all organisms

exposure evaluationincidencelifetime average daily dose

(LADD)maximum daily dose (MDD)probabilityriskrisk assessmentrisk estimationrisk managementrisk perceptionSafe Human Dose (SHD)safetySafety Factor (SF)toxicant evaluationtoxicant identification


Introduction to Risk

Risk is defined as the possibility of loss orinjury. Within the context of environmen-tal toxicology this broad definition canbe restated as the possibility of an unde-sirable biological response (toxicity) thatresults from exposure to a toxicant. Riskmay be expressed as a probability (e.g.,P=0.00001) or incidence (e.g., 1 in100,000) of a particular response for agiven exposure. Risk (probability or inci-dence) is based on statistical estimatesfrom sample populations studied duringtoxicity testing and on other observations,such as those from epidemiological stud-ies. Risk statements, when properly inter-preted, provide a valuable means for com-paring the relative, but not necessarilyabsolute, possibility of a response occur-ring on exposure to a toxicant.

Each individual interprets risk in aunique way. Differences in risk perceptionmay be attributed to how much a personknows about the toxicant, sources of ex-posure, and resulting toxicity. Two groupsthat frequently have knowledge differ-ences at the core of their disagreements in

risk perception are experts—often scien-tists—and lay persons (Table 10–1). Sci-entists have a social responsibility to edu-cate the nonscientific community aboutthe methods, results, and interpretationsof toxicity tests and epidemiological stud-ies regarding risks of environmentaltoxicants.

Also important to the perception of riskby individuals is their choice or controlover exposure to a toxicant, their view ofits catastrophic potential, and their con-cept of equanimity in the distribution ofbenefits and risks of the toxicant. Signifi-cant global differences, often delineatedby geopolitical boundaries, are most likelyinfluenced by cultural, social, and politi-cal factors.

Risk Assessment

Risk assessment is the process of examin-ing toxicological and epidemiological datafor a suspected toxicant and then, if war-ranted, estimating permissible exposures.Four steps characterize the process: (1)toxicant identification, (2) toxicant evalu-ation, (3) exposure evaluation, and (4) risk

Table 10–1. Differences in risk perception betweenexperts and lay persons

Note. Environment, vol. 21, p. 14, 1979. Adapted with permission of theHelen Dwight Reid Educational Foundation. Published by Heldref Publica-tions, 1319 Eighteenth St., NW, Washington, DC 20036–1802. Copyright ©1979.

Risk Assessment 145

estimation. These four steps may appearto have readily quantifiable, empiricallyderived answers—but not so! This is be-cause science as a methodology is verygood at proposing and answering ques-tions about the natural world (Step 1), butscience does not tell an environmentaltoxicologist how to interpret conclusionsor what to do with answers in Steps 2–4.This presents a serious challenge to thoseinvolved in risk assessment, as human livesmay ultimately be jeopardized.

Toxicant Identification

A review of existing literature, which mayinclude toxicity tests and epidemiology,may be used to identify a toxicant. In theabsence of relevant literature, descriptivetoxicity testing must be done. Remember,the dose-response conclusions of descrip-tive toxicology are sufficient for toxicantidentification—it is not necessary to un-derstand the mode of action of a toxicant(mechanistic toxicology). The bottom lineof toxicant identification is: Does theagent cause the adverse effect?

Toxicant Evaluation

Since toxicity tests are carried out in non-human species, the applicability of thosetests to humans must be evaluated. Anawareness of interspecific (between spe-cies) and intraspecific (within species) vari-ability in toxicity testing is necessary whenevaluating nonhuman tests. Careful atten-tion to variability in age, sex, diet, circa-dian rhythms, hormonal status, andbiotransformation capacity is required.

Questions to be asked may include:What type of test was performed—should

a chronic toxicity test have been used in-stead of an acute test, or was a test per-formed to determine teratogenic activity?What responses were measured—what isthe possibility of observing these sameresponses in humans? Are the results sci-entifically valid—can the results be repro-duced in another laboratory using thesame methods of toxicity testing? Werethere problems with the test methods,choice of test organisms, doses tested, orin the stated results or interpretation ofresults?

Exposure Evaluation

With good experimental design, exposureto a suspected toxicant is precisely regu-lated during toxicity testing. Descriptivetoxicologists predetermine the exposureparameters, including the organisms ex-posed, route of entry, dose, frequency(how often), and duration (how long) ofdosing. Under “field” conditions the regu-lation of these parameters can present anillusive element to the risk assessor—theinability to control the test organisms(young, old, male, female, or pregnantfemale), route of entry (percutaneous, res-piratory or digestive system), dose (?mg/kg), frequency of dose (?mg/kg/?min, day,week, year, lifetime), or duration of dose(seconds, minutes, hours, days, years).

Often overlooked, possibly because ofits complexity, exposure assessment needsto be pursued with the same diligence astoxicant identification and evaluation.The assessment should examine exposurescurrently experienced as well as thoseanticipated under different conditions.Exposure for noncarcinogenic toxicantsis expressed as maximum daily dose


(MDD) (mg/kg/day), whereas exposure tocarcinogens is stated as lifetime averagedaily dose (LADD) (mg/kg/ day/lifetime).

Risk Estimation

Risk is the probability of an undesirablebiological response resulting from expo-sure to a toxicant. Estimating risk requiresan integration of the toxicity conclusions(toxicant identification and dose-responseevaluation) and exposure assessment(MDD or LADD).

Risk is approximated by the equation:

where risk=R, toxicity=T, and exposure=E. However, since dose-response rela-tionships are not linear (as in the charac-teristic sigmoidal graph lines), exposureis more accurately expressed as a func-tion (f) as seen in the following equation:

This equation is the source of risk state-ments such as 1 in 1,000 individuals willdevelop toxicity (a specific disease) if ex-posed to a specific dose (MDD or LADD)of a toxicant for a certain period of time.

Risk Management

Risk is often presented as a declarativestatement, devoid of interpretation. To beof value to humanity, risk—once charac-terized—must be implemented into regu-latory policies that benefit society. Theintent of risk management is to examinerisk assessment data and, where needed,develop regulatory options that addresspublic health and social and economicconcerns. Federal agencies charged with

overseeing risk management often mustovercome the influence of negative pub-lic perceptions and legislative mandatesto arrive at responsible decisions.

A “healthy” approach to determiningacceptable risk is to answer the followingquestions: Is the substance really needed?Could alternate, less toxic substitutes beused? What is the realistic amount of pub-lic exposure? What are the risks versusbenefits for continued use of the agent?What is the environmental impact of thesubstance? Does the procurement of theagent deplete an environmental resource?Does existing technology permit the “fi-nal” disposal of the substance? If used,do we have the technology to ensure the“safe” use of the substance?

Safety

Safety is defined as the possibility that anundesirable biological response (toxicity)will not result from exposure to a toxi-cant—it is the inverse of the probabilityof risk (i.e., 1÷P). For example, when therisk of toxicity from a given exposure isP=0.00001, safety equals 1 divided by0.00001 (or 1/0.00001). It can be con-cluded that for every 100,000 exposuresonly 1 of those exposures will result in anadverse response.

Risk and safety are numerical estimatesthat result from consideration of toxicityand exposure—only two of the many fac-tors influencing the capacity of a toxicagent to cause disease and death. Recallthat toxicokinetic processes include ab-sorption, distribution, storage, biotrans-formation, and elimination—all capableof altering the fate of a toxicant in the

Risk Assessment 147

body. Also, since toxicity studies dependon nonhuman test organisms, some meansof extrapolating results to humans is re-quired. One calculation that permits morerealistic extrapolations of toxicokineticdata from test organisms to humans is theSafe Human Dose (SHD) formula:

Unlike the “measured” expressions inthe numerator of the SHD equation, the

safety factor (SF) in the denominator de-pends on the reliability of data used forextrapolation—a subjective judgmentbased on previous experience. The SF mayrange from 10 to 1,000, with lower SFsbeing used when valid human data isavailable and higher SFs indicative of alack of human data. The goal of the SHDis to establish doses at which risk equalsor approaches zero (P=0.0).

Conclusions

The role of environmental toxicology is notonly to identify environmental toxicants andtheir mode of action, but to assist in theevaluation and determination of issues con-cerning acceptable risk and safety—safedoses for humans and other species. More-over, environmental toxicology data shouldprompt us to restrict or prohibit the use ofagents toxic to plants and animals in ourecosystem. The fate of all species, includ-ing our own, depends on our ability to rec-ognize and effectively control the “ripples”generated by environmental toxicants.

whereED0.0 = threshold dose of toxicant

(NOEL)At/h = ratio of absorption of toxicant

in test organism and humanT1/2t/h = ratio of half-life of toxicant in

test organism and humanWt = weight of exposed individualDt/h = ratio of toxicant test dosages

in animals to exposure dos-ages in humans

SF = safety factor

1. All of the following are true statements about risk, except:

A. It is defined as the possibility of loss or injury.B. It may be expressed as probability or incidence.C. It is based on statistical estimates from sample populations.D. It is so absolute that there is only one interpretation.E. It provides a valuable means of comparing the relative possibility of a response for

two or more toxicants.

2. Risk perception is influenced by an individual’s:

A. Knowledge of the toxicantB. Ability to control exposure to the toxicant

Review Questions


C. View of the catastrophic potential of the toxicantD. A and BE. A, B, and C

3. Risk assessment includes all of the following, except:

A. Toxicant identificationB. Identification of toxicodynamicsC. Evaluation of toxicantD. Exposure evaluationE. Risk estimation

4. Science, as a methodology, is very good at telling environmental toxicologists whatto do with the answers resulting from risk assessment.

A. TrueB. False

5. Exposure assessment:

A. May include realistic assessment of “field” conditions as related to exposure.B. Should examine exposures currently experienced as well as those anticipated under

different circumstances.C. Needs to be pursued with the same diligence as toxicant identification and

evaluation.D. A and BE. A, B, and C

6. Which equation most accurately approximates risk?

A. R=T/EB. R=T×EC. R=T?f(E)D. R=1/PE. R=T×f(E)

7. What questions need to be answered in order to pursue a healthy approach todetermining acceptable risk?

8. Write out the Safe Human Dose equation and then discuss the objectivity of eachterm.

9. Discuss toxicant evaluation questions that you would ask as part of your approachto risk assessment.

10. What is the role of an environmental toxicologist?

149

Appendix

Books

Burrell , R. 1992. Toxicology of the Immune System. New York: Van Nostrand Reinhold.Cockerham, L.G., & Shane, B.S. (Eds.). 1993. Basic Environmental Toxicology. Boca

Raton, FL: CRC Press.Davey, B., & Halliday, T. (Eds.). 1994. Human Biology and Health: An Evolutionary

Approach. Buckingham, United Kingdom: Open University Press.DiPiro, J.T., et al. (Eds.). 1993. Pharmacotherapy: A Pathophysiologic Approach (2nd

ed.). Norwalk, CT: Appleton & Lange.Emsley, J. 1994. The Consumer’s Good Chemical Guide: A Jargon-Free Guide to

Controversial Chemicals. New York: W.H.Freeman.Francis, B. 1994. Toxic Substances in the Environment. New York: Wiley-Interscience.Guthrie, F., & Perry, J. (Eds.). 1980. Introduction to Environmental Toxicology. New

York: Elsevier.Halstead, B.W. 1965. Poisonous and Venomous Marine Animals of the World (Vol. 1–

3). Washington, DC: U.S. Government Printing Office.Hayes, A.W. (Ed.). 1994. Principles and Methods of Toxicology. New York: Raven

Press.Hodgson, E., & Levi, P.E. 1994. Introduction to Biochemical Toxicology (2nd ed.).

Norwalk, CT: Appleton & Lange.Katzung, B.G. (Ed.). 1992. Basic & Clinical Pharmacology (5th ed.). Norwalk, CT:

Appleton & Lange.Klaassen, C.D., Amdur, M.O., & Doull, J. (Eds.). 1996. Casarett and Doull’s Toxicology:

The Basic Science of Poisons (5th ed.). New York: McGraw Hill.Kupchella, C.E., & Hyland, M.C. 1989. Environmental Science: Living Within the System

of Nature (2nd ed.). Boston, MA: Allyn and Bacon.Landis, W.G. 1995. Environmental Toxicology. Boca Raton, FL: Lewis.Lu, F.C. 1996. Basic Toxicology: Fundamentals, Target Organs, and Risk Assessment

(3rd ed.). Washington, DC: Taylor & Francis.

Resources inEnvironmental

Toxicology


O’Flaherty, E.J. 1981. Toxicants and Drugs: Kinetics and Dynamics. New York: Wiley.Rubin, E., & Farber, J.L. (Eds.). 1994. Pathology (2nd ed.). Philadelphia, PA: J.B.

Lippincott.Smith, R.P. 1992. A Primer of Environmental Toxicology. Malvern, PA: Lea & Febiger.Stacey, H.H. 1993. Occupational Toxicology. London, United Kingdom: Taylor &

Francis.Timbrell, J.A. 1991. Principles of Biochemical Toxicology (2nd ed.). London, United

Kingdom: Taylor & Francis.Timbrell, J.A. 1995. Introduction to Toxicology (2nd ed.). London, United Kingdom:

Taylor & Francis.Williams, P.L., & Burson, J.L. (Eds.). 1985. Industrial Toxicology: Safety and Health

Applications in the Workplace. New York: Van Nostrand Reinhold.Zakrzewski, S.F. 1991. Principles of Environmental Toxicology. Washington, DC:

American Chemical Society.

Journals

Annual Review of Pharmacology and Toxicology (annually). Palo Alto, CA: AnnualReviews.

Chemical Research in Toxicology (8 per annum). Washington, DC: American ChemicalSociety.

Critical Reviews in Toxicology (bimonthly). Boca Raton, FL: CRC Press.Drug and Chemical Toxicology (quarterly). New York: Marcel Dekker.Drug Safety (bimonthly). Langhorne, PA: ADIS International.Environmental Toxicology and Water Quality (quarterly). New York: Wiley.Food and Chemical Toxicology (monthly). Oxford, United Kingdom: Pergamon Press.Fundamental and Applied Toxicology (10 per annum). Orlando, FL: Academic Press.Human and Experimental Toxicology (bimonthly). Hampshire, United Kingdom:

Macmillan Magazines Ltd.Inhalation Toxicology (9 per annum). Washington, DC: Taylor & Francis.Journal of Analytical Toxicology (7 per annum). Niles, IL: Preston.Journal of Applied Toxicology (bimonthly). West Sussex, United Kingdom: Wiley.Journal of Toxicology and Environmental Health (18 per annum). Washington, DC:

Taylor & Francis.Journal of Toxicology—Clinical Toxicology (bimonthly). New York: Marcel Dekker.Pharmacology & Toxicology (12 per annum). Copenhagen, Denmark: Munksgaard

International.The Toxicologist: An Official Publication of the Society of Toxicology (24 per annum).

Reston, VA: Society of Toxicology.Toxic Substance Mechanisms (quarterly). Washington, DC: Taylor & Francis.Toxicology & Ecotoxicology News (bimonthly). London, United Kingdom: Taylor &

Francis.


Toxicology and Applied Pharmacology (monthly). Orlando, FL: Academic Press.Toxicology and Industrial Health (6 per annum). Princeton, NJ: Princeton Scientific.Toxicology Letters (18 per annum). Ireland: Elsevier Science Ireland Ltd.Xenobiotica (monthly). London, United Kingdom: Taylor & Francis.

Electronic InformationResources

Numerous toxicology-related databases are available on disk or CD-ROM and online.Hundreds of Internet resources may be accessed by a using a variety of software tools,called clients, including the following: mail (join a list), FTP (file transfer protocol),News (Usenet news groups), Gopher (for access to universities and governmental agen-cies), and the WWW (World Wide Web).

153

absorption The taking in of substances by cellsor tissues.

active transport The movement of moleculesagainst a concentration gradient, from aless concentrated region to a more concen-trated region.

acute toxicity The sudden onset of adversehealth effects that are of a short duration;results in cellular changes that are revers-ible.

adenosine triphosphate (ATP) Energy-storingcompound found in all cells.

adipose tissue A connective tissue composedprimarily of adipocytes; functions in thestorage of fat.

agenesis Absence or imperfect developmentof any body part.

Agent Orange An herbicide composed of a50:50 mixture of 2, 4-D and 2, 4, 5-T.

agranulocytopenia An acute condition char-acterized by a reduction in the number ofmonocytes and lymphocytes (i.e., agranularleukocytes).

albumin A simple protein distributed through-out the tissues and fluids of plants andanimals.

aliphatic alcohols A class of organic com-pounds characterized by a straight- orbranched-chain structure with an attachedhydroxyl group.

alkylbenzenes A class of organic compoundscontaining a single benzene ring with oneor more aliphatic side chains.

allergic contact dermatitis A delayed hyper-sensitivity reaction affecting the skin thatresults from exposure to a chemical.

alveolar region The area of the respiratorysystem that contains respiratory bronchi-oles and their associated alveoli.

Ames assay A bacterial test system used todetermine mutagenicity.

anabolism Synthesis reactions in whichsmaller molecules are bonded together toform larger molecules; the reactions requireenergy and are catalyzed by enzymes.

anemia A deficiency of red blood cells or he-moglobin.

aneuploidy State of having an abnormal num-ber of chromosomes not an exact multipleof the haploid number; presence of onemore or one less chromosome.

anthracosilicosis The accumulation of carbonand silica in the lungs from inhaled coaldust that produces fibrous nodules.

anticoagulants Agents that prevent the aggre-gation of platelets; commonly used in ro-denticides.

anticodon A triplet of bases in tRNA thatmatches a codon in mRNA.

antidotes Agents that neutralize or counter-act the effects of a poison.

anuria The absence of urine formation.aromatic hydrocarbons A class of unsaturated

cyclic hydrocarbons containing one ormore rings.

arterial blood gas (ABG) The concentrationof oxygen and carbon dioxide in arteries.

arterial vessels Blood vessels that take bloodaway from the heart toward the capillaries.

asbestosis A lung disease resulting from inha-lation of asbestos particles; sometimes com-plicated by mesothelioma.

Glossary

A

154 Glossary

associative relationship An epidemiologicalfinding that establishes a link between twoor more variables and a disease state.

atmosphere The region of the earth that con-tains gases (air).

atresia Absence or closure of a normal open-ing or normally continuous lumen.

base analogues Molecules whose chemicalstructure closely resembles pyrimidines andpurines.

base substitution The exchange of a base (ad-enine, guanine, cytosine, thymine, or uracil)for a normally occurring base in the ge-netic code.

benzene A toxic hydrocarbon used as the ba-sic structure in aromatic compounds, as asolvent, and as a chemical feedstock.

berylliosis A lung disease characterized by fi-brosis that results from the inhalation ofberyllium.

bioactivation A sequence of chemical reac-tions that produce intermediate or finalmetabolites that are more toxic or reactivethan the original parent chemical; same astoxication.

biological half-life (T1/2) The time required toreduce by half the quantity of a toxicantpresent in the body.

biological magnification The accumulation oftoxicants or other chemicals by successiveorganisms in the food chain.

biosphere The region of the earth where lifeexists, including parts of the lithosphere,hydrosphere, and atmosphere.

biotransformation The process by which en-dogenous or exogenous substances arechanged from hydrophobic to hydrophilicmolecules to facilitate elimination from thebody.

bipyridyls A class of chemical compoundscharacterized by two nitrogen-containingrings; examples include the herbicides di-quat and paraquat.

blood dyscrasias Blood disorders that resultfrom abnormal cellular components, such

as too many of one blood cell type or toofew of another.

blood flow/mass ratio A ratio of the volumeof blood flowing through an organ to thesize or mass of the organ.

blood plasma The yellowish, noncellular fluidportion of whole blood.

blood urea nitrogen (BUN) A test used toevaluate kidney function in which the con-centration of urea in the blood is measured.

blood-brain barrier The barrier between thecirculating blood and brain tissue formedby astrocytes and capillaries; preventsharmful substances in the blood from dam-aging neurons.

bronchoscopy The process of inserting a bron-choscope (small fiber-optic instrument)into the tracheobronchial region to visu-ally examine the bronchi.

cancer The uncontrolled proliferation of cells.capillaries Blood vessels that take blood from

small arteries to small veins.carbamates A class of compounds containing

carbamic acid; as insecticides they are cho-linesterase inhibitors.

carbon disulfide A toxic liquid used as an or-ganic solvent and in the manufacture ofrayon, cellophane, carbon tetrachloride,and rubber accelerators.

carbon tetrachloride A colorless toxic liquidused as an organic solvent; once widelyused as a dry cleaning agent.

carcinogenesis The formation of cancer, in-cluding carcinomas and other malignantneoplasms.

carcinogens Cancer-producing substances.cardiac output The volume of blood pumped

per heart beat (stroke volume) times theheart rate (beats per minute); the restingaverage is 5 to 6 L/min.

catabolism Degradation reactions in whichlarger molecules are broken down to formsmaller molecules; these reactions often re-lease energy and are catalyzed by enzymes.

cataractogenic Cataract producing.

B

C

Glossary 155

causal relationships A direct cause-and-effectlinkage involving a single variable that isthe basis for establishing a dose-responserelationship.

ceiling effect The region (right side) on a cu-mulative dose-response graph where theline becomes almost horizontal indicatinglittle or no change in response with in-creased doses.

cell membrane The membrane composed ofphospholipids, proteins, and cholesterolthat forms the outer boundary of a cell andregulates the movement of substances intoand out of the cell.

cells The basic units of structure and func-tion in a living organism; complex assem-blages of atoms, molecules, and complexmolecules.

cellular division The reproduction of somaticand germ cells.

central nervous system (CNS) The part of thenervous system that consists of the brainand spinal cord.

centromere The region on a chromosomewhere chromatids join together.

chelating agents Substances that facilitate theelimination of toxic metals by forming ametal—ion complex that is more reactiveand more readily eliminated from the body.

chlorinated aliphatics A class of organic com-pounds characterized by a straight- orbranched-chain structure with an attachedchlorine group.

chloroform Trichloromethane, used as a sol-vent and to produce general anesthesia.

chlorophenoxy compounds A class of chemi-cals characterized by a phenol with at-tached chlorines; included are the herbi-cides 2, 4-D and 2, 4, 5-T.

chromosome Structure made of DNA andprotein found in the nucleus of a cell; hu-man somatic cells contain 46 chromo-somes.

chronic toxicity Adverse health effects thatare of a long and continuous duration; dueto irreversible cellular changes in the or-ganism.

clinical toxicology The branch of toxicologythat examines the effects of toxicants on

an individual and the efficacy of treatmentfor symptoms related to toxication.

codon A sequence of three bases in DNA ormRNA that codes for one amino acid; alsocalled a triplet code.

concentration gradient The relative amountsof a substance on either side of a mem-brane; diffusion occurs from the region ofhigh concentration to the region of lowconcentration.

conjugate A metabolite that results from thejoining of a phase II molecule with a toxi-cant (or its intermediate metabolite) thatis more water-soluble than the originaltoxicant (or its intermediate metabolite).

conjugation reactions Phase II biotransforma-tions in which a molecule provided by thebody is added to a toxicant (or phase Imetabolite).

creatinine A waste product produced whencreatine phosphate is used for energy; ex-creted by the kidneys in urine, and oftenmeasured as an indicator of kidney func-tion.

cumulative dose-response graph The cumu-lative sum of responses from lower tohigher doses; the line on this graph appearssigmoidal.

cytochrome P-450 An iron—protein complexwith a maximum absorbance of visible lightat 450 nm that functions as a nonspecificenzyme system during phase I biotransfor-mation reactions.

cytosolic enzymes Enzymes that are non-mem-brane-bound and occur free within the cy-toplasm; catalyze phase II biotransforma-tion reactions.

delayed toxicity The development of diseasestates or symptoms many months or yearsafter exposure to a toxicant.

deoxyribonucleic acid (DNA) A nucleic acidwith the shape of a double helix that ispresent in chromosomes; the repository ofhereditary characteristics (genetic code).

dermatotoxicity The adverse effects producedby toxicants in the skin.

D

156 Glossary

descriptive toxicology The branch of toxicol-ogy concerned with the identification oftoxic substances.

detoxication The process by which toxicantsare chemically converted to metabolitesthat are more readily eliminated by theurinary and biliary systems; same as detoxi-fication.

developmental syndromes Multiple, but re-lated, tissue or organ anomalies that mayresult from a teratogen.

diabetogenic Causing diabetes.digestive system The organ system responsible

for changing food into simple moleculesthat can be absorbed by the blood andlymph, and used by cells; made up of thedigestive tract and related accessory organs(liver and pancreas).

dioxin A trace impurity (TCDD) associatedwith 2, 4, 5-T; a powerful toxicant.

diploid The normal number of chromosomesfound in a somatic cell; in humans 2n=46.

distribution A toxicokinetic process that oc-curs after absorption when toxicants enterthe lymph or blood supply for transportto other regions of the body.

dithiocarbamates Chemical agents used asfungicides and insecticides.

division failures These result when, under theinfluence of teratogens, fused structures failto separate (e.g., syndactyly).

dose-response relationship Exists when a con-sistent mathematical relationship describesthe proportion of test organisms respond-ing to a specific dose for a given exposureperiod.

dysraphic anomalies These result when, un-der the influence of teratogens, apposedstructures fail to fuse (e.g., spina bifida).

Ebers papyrus One of eight Egyptian papyri,which dates from 1500 B.C., containingdirections for the collection, preparation,and administration of more than 800 me-dicinal and poisonous recipes.

ecosystem A self-regulating community ofanimals and plants interacting with one

another and with their nonliving environ-ment.

ectopia This results when, under the possibleinfluence of teratogens, organs or parts ofthe body are formed outside their normallocation.

ED50 The dose at which 50% of the test or-ganisms are observed to exhibit an effec-tive response.

effective dose (ED) A dose at which the pre-determined response is observed.

efficacy The range of doses over which a toxi-cant produces a response; a toxicant is saidto have a higher efficacy when the dose-response relationship continues to bepresent over a greater range of doses.

electromagnetic fields Fields of force that con-sist of associated electric and magneticcomponents; possess a specific amount ofelectromagnetic energy.

elimination The toxicokinetic processes re-sponsible for the removal of toxicants ortheir metabolites from the body.

embryogenesis The formation of the embryo.embryolethality Death of the fertilized ovum

or embryo during the first 8 weeks (i.e.,embryonic stage).

end effect A response that is observed andrecorded during toxicity testing.

endocytosis The process, including pinocyto-sis and phagocytosis, whereby substancesare taken into a cell by invagination of thecell membrane.

environmental toxicants Agents in our sur-roundings that are harmful to humanhealth.

environmental toxicology The study of thepoisons around us; the hazardous effectsof poisons on human health.

epidemiology The study of the prevalence andspread of disease and death in a population.

epigenetic A carcinogenic mechanism thatdoes not act to directly affect DNA, termednon-DNA reactive.

epithelium One of the four main types of tis-sues; forms glands, lines cavities, and cov-ers body surfaces.

erythrocytes Red blood cells.ethyl alcohol Ethanol or beverage alcohol.

E

Glossary 157

ethylene glycol A thick, sweet, colorless liq-uid used as antifreeze, coolant, and hydrau-lic fluid.

etymology The study of word origins.exocytosis The cellular secretion of macromol-

ecules by the fusion of vesicles with the cellmembrane; the process of transportingcellularly derived substances across the cellmembrane.

exposure evaluation The process of determin-ing the validity of exposures used in toxic-ity testing as compared to the anticipatedor known exposures encountered underfield conditions.

facilitated diffusion The spontaneous passageof molecules and ions that are bound tospecific carrier proteins across the cellmembrane; dependent on the concentra-tion gradient.

fecal excretion The process by which toxicantsor their metabolites enter bile for transportto the duodenum and subsequent elimina-tion.

fetal alcohol syndrome (FAS) A collection ofsigns and symptoms (e.g., fetal malforma-tion, intrauterine growth retardation, cran-iofacial anomalies, and CNS dysfunction)found in offspring of mothers who arechronic alcoholics.

fluoroacetic acid Two derivatives, sodiumfluoroacetate and fluoroacetamide, arerodenticides that act to block the Krebscycle, impeding a major pathway for ATPproduction.

forced vital capacity (FVC) A pulmonary func-tion test that measures the time it takes toexhale the total volume of air contained inthe lungs (i.e., inhalatory reserve capacity,tidal volume, and exhalatory reserve capac-ity).

forensic toxicology The branch of toxicologyconcerned with medical and legal questionsrelating to the harmful effects of knownor suspected toxicants.

frameshift mutation A mutation occurringwhen the number of nucleotides deleted or

inserted is not a multiple of three; producesan improper grouping of codons.

frequency dose-response graph A graph thatplots the percentage of organisms (Y axis)responding to a given dose (X axis); usuallyrecognized by its bell-shaped appearance.

fungicides Agents that destroy or repel fungi.

gene Hereditary unit; portion of the DNA ona chromosome that represents a sequenceof bases that contains the “blueprint” fora protein.

genetic code The sequence of three nucleotides(codon) that signifies a specific amino acid;there are four nucleotides that in differentcombinations of three result in 64 possiblecodons.

genotoxic A carcinogenic mechanism that actsdirectly to affect DNA, termed DNA reac-tive.

germ cells Egg or sperm cells; ova or sperma-tozoa.

glial cells Support cells (nonconducting) in thenervous system; include astrocytes, micro-glia, and oligodendrocytes.

glomerular filtration The first process in urineformation; results when blood enters thevascularized glomerulus where water andsmall molecules are forced by hydrostaticpressure across the glomerular filter andinto Bowman’s capsule.

glucuronidation The process of adding glu-curonide to a toxicant or phase I metabo-lite during phase II biotransformation.

glycols Compounds containing adjacent al-cohol groups; ethylene glycol is the sim-plest glycol.

glycosuria The presence of sugar in the urine.

haploid Denoting the number of chromo-somes in sperm or ova; in humans, n=23.

hazardous waste Waste that, because of itsbiological, chemical, or physical character-istics, or quantity or concentration, mayproduce disease.

F

G

H

158 Glossary

hematotoxicity The presence of disease in theblood as produced by a toxicant.

hematotoxins Agents that produce toxicsymptoms (i.e., disease) in the blood.

hematuria The presence of blood cells in theurine.

hemolytic anemias A reduction in the oxy-gen-carrying capacity of blood resultingfrom the destruction of erythrocytes.

hepatotoxicity The presence of disease in theliver as produced by a toxicant.

hepatotoxins Agents that produce disease inthe liver.

herbicides Agents that destroy or inhibit plantgrowth.

hexachlorobenzene A chemical once used toprevent fungal infestation in seeds beforeplanting.

histogenesis The formation of tissues in thebody.

hydrolysis The chemical process whereby acompound is split into two or more simplercompounds with the uptake of H and OHparts of a water molecule on either side ofthe chemical bond at the splitting site.

hydrophilic Attracted to water.hydrophobic Repelled by water.hydrosphere That portion of the earth com-

posed of water.hypoplasia Underdevelopment of a tissue or

an organ that results from a decrease inthe number of cells.

hypoxia A decrease in the normal level ofoxygen in the blood; inadequate supply ofoxygen to the tissues.

immediate toxicity The rapid occurrence ofsymptoms following exposure to a toxi-cant.

in vitro In an artificial environment such as atest tube or tissue culture medium; not in aliving organism.

in vivo Processes or reactions occurring withina living organism.

incidence A statistical estimate of the risk ofa particular response for a given exposure(e.g., 1 in 100,000).

industrial toxicology The branch of toxicol-ogy concerned with the study of toxicsymptoms (i.e., diseases) found in individu-als who have been exposed to toxicants intheir place of work.

infinite dilution The disposal of “small”quantities of wastes in “large” reposito-ries such as the hydrosphere, lithosphere,and atmosphere where they are thought topose little harm because the repositoriesare so vast.

initiation The subtle alteration of DNA orproteins within target cells by carcinogensthat renders the cell capable of becomingcancerous.

insecticides Agents that destroy or repel in-sects.

integumentary system Composed of skin andrelated structures such as hair, glands, andnails; largest organ system in the humanbody.

interstitial fluid Fluid found between cells;intercellular.

intestinal excretion Process by which toxicantsor their metabolites are transported fromthe blood in the submucosal region intothe intestinal lumen.

intracellular fluid Fluid found within a cell;cytoplasm.

inulin A fructose polysaccharide used by in-travenous injection to determine the rateof glomerular filtration.

ionization Occurs when atoms or moleculesdissociate into electrically charged atomsor molecules; to be ionized.

ionizing radiation Energy radiated in the formof waves or particles, such as alpha particles,beta particles, gamma rays, and x rays.

irritant contact dermatitis Inflammatory re-sponse that occurs in skin on exposure toa toxicant.

karyotype A systematized array of chromo-somes arranged in pairs in descending or-der of size and position of the cen-tromere.

I

K

Glossary 159

LD50 The dose at which 50% of the test or-ganisms are observed to exhibit a lethalresponse.

lethal dose (LD) The dose that results in thedeath of the test organism.

leukemia Blood disease characterized by anincrease in white blood cells.

leukocytes White blood cells.lifetime average daily dose (LADD) Allowable

exposure to carcinogens; expressed as mg/kg/day/lifetime.

linear dose sequence A series of test doses thatincrease by adding a constant; for example1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 mg/kg wherethe constant is 1.

lipid soluble Substances that are dissolved bynonpolar solvents or by the lipid region ofthe phospholipid bilayer that forms the cellmembrane.

lipophilic Attractive to lipid-soluble sub-stances.

lithiosphere The region of the earth’s surfacecomposed of rock.

local toxicity Occurs when symptoms are re-stricted to the site of initial exposure to atoxicant.

logarithmic dose sequence A series of testdoses that increase by multiplying eachprevious dose by 10; for example, 0.1, 1.0,10, 100, 1,000 and 10,000 mg/kg.

lymph Watery fluid in the lymphaticsystem.

lymph capillaries Microscopic vessels thattransport lymph.

lymph node A rounded mass of lymphoid tis-sue surrounded by a capsule of connectivetissue; lymph vessels enter and exit lymphnodes.

lymphatic system Part of the circulatory sys-tem that drains excess fluid from the tis-sues; includes lymph capillaries, lymph ves-sels, lymph nodes, aggregations of lym-phoid tissue such as the tonsils, spleen, andthymus, and circulating lymphocytes.

lymphocyte One of the five different types ofwhite blood cells; agranular white bloodcells that function in immunity.

lymphoid tissue Present in the tonsils, spleen,and thymus.

macromolecules Large molecules.margin of safety Expresses the magnitude of

the range of doses between a noneffectiveor minimally effective dose and a lethal dose;expressed as a ratio such as LD50/ED50.

maximum daily dose (MDD) Allowable ex-posure to noncarcinogenic toxicants; ex-pressed as mg/kg/day.

mechanistic toxicology The branch of toxi-cology concerned with determining the bio-chemical processes by which identifiedtoxic substances have an impact on theorganism.

Mee’s lines Horizontal white bands seen inthe nails of persons experiencing chronicarsenic toxicity.

meiosis A special process of cell division thathalves the chromosome number in the for-mation of gametes.

metabolism The sum of biochemical changesoccurring in an organism; includes anabo-lism, those reactions that convert smallmolecules into large, and catabolism, thosereactions that convert large molecules intosmall.

metaphase A stage of mitosis (or meiosis) inwhich the chromosomes are aligned on theequatorial plate of the cell with the cen-tromeres mutually repelling each other.

methyl alcohol Wood alcohol or methanol.microcytic hypochromic anemia Reduced

oxygen-carrying capacity that results whenred blood cells are unusually small and lackthe normal quantity of hemoglobin.

microsomal enzymes Enzymes associated withphase I biotransformation reactions; al-though normally bound to the endoplas-mic reticulum, upon disruption of cells,small spherical vesicles (microsomes) de-rived from the endoplasmic reticulum con-tain the enzymes.

minute volume respiration (MVR)The amount of air breathed in on each

L

M

160 Glossary

respiratory cycle multiplied by the num-ber of res pirations per minute; expressedas liters per minute (L/min).

mitosis A type of cell division that results inthe production of two daughter cells iden-tical in chromosome number as comparedto the parent cell.

mixed toxicity This results when one toxicantis not consistently more potent over therange of doses tested as compared to an-other toxicant; evident when two lines oncumulative dose-response graphs cross oneanother; same as reversed toxicity.

molecule Particle formed by chemical bond-ing of two or more atoms; smallest sub-unit of a compound.

monosomy Occurs when a cell loses one mem-ber of a pair of chromosomes; in humans,the presence of 45 chromosomes or 2n–1.

morbidity A diseased state.morphogenesis The differentiation of cells and

tissues in the embryo which results in es-tablishing the form of organs and parts ofthe body.

mortality Death.mucociliary escalator Responsible for the

movement of mucus from the respiratorysystem; involves the coordinated sweepingaction of cilia found on the surface of co-lumnar epithelial cells that line the trachea,bronchi, and bronchioles.

mutagenesis The production of a mutation orchange in the genetic code.

mutagens Agents that cause the productionof mutations.

myelin Coiled fatty membrane that covers andinsulates the axons of some neurons.

nasopharyngeal region Region of the respira-tory system composed of the nares, nasalcavity, and adjacent pharynx.

nephritic syndrome Clinical symptoms char-acterized by hematuria or blood cells in theurine.

nephron The functional unit of the kidney;composed of the Bowman’s capsule, proxi-mal tubule, and distal tubule.

nephrotic syndrome Clinical symptoms char-acterized by proteinuria or protein in theurine.

nephrotoxicity The adverse effects producedby toxicants in the kidney.

nerve Collection of individual motor or sen-sory neurons.

neurons Nerve cells; composed of the cellbody, dendrites, and axon.

neurotoxicity The adverse effects produced bytoxicants in the nervous system.

neurotransmitter A chemical released from thedistal axonal region of a presynaptic neu-ron that enables a nerve impulse to cross asynaptic junction to stimulate or inhibit apostsynaptic neuron; acetylcholine.

nickel itch Allergic contact dermatitis thatresults from exposure to nickel in sensitizedindividuals.

No Observable Effect Level (NOEL) A sub-threshold dose; ED0.0.

nonbiodegradable Resistant to biological deg-radation.

nonphotodegradable Resistant to degradationfrom ultraviolet radiation.

nonpolar molecules Chemicals that possess nopositive or negative molecular charge; lipid-soluble.

normal distribution Represented by a bell-shaped line on a frequency dose-responsegraph or a sigmoidal line on a cumulativedose-response graph; the word “normal”should not be interpreted as “usual” or asthe opposite of “abnormal.”

nucleic acids Biological molecules (DNA andRNA) that permit organisms to reproduce;composed of purines or pyrimidines, asugar, and a phosphate; a sequence ofnucleotides.

nucleotide The building block of nucleic ac-ids (i.e., DNA and RNA), consisting of afive-carbon sugar bonded to a nitrogen-con-taining base (adenine, cytosine, guanine,thymine, or uracil) and a phosphate group.

obstructive uropathies Renal diseases thatresult when the flow of urine is blocked byintra- or extratubular pathologies.

N

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Glossary 161

occluding cell junctions The tight junctionsformed between adjacent cells; prevent theentrance of substances by blocking theirpassage along cell—cell junctions.

oliguria Scanty or small amount of urine pro-duction.

oogenesis The process in the ovary that re-sults in the production of female gametes.

organ system Two or more organs that to-gether perform a special function.

organelle One of many subcellular units thatperforms a specific function in the cell.

organochlorines A category of insecticidescomposed of chlorinated hydrocarbons;DDT, dieldrin, and Kepone.

organogenesis The formation and develop-ment of organs in the body.

organomercurials A category of fungicidesthat includes methylmercury.

organophosphates A category of insecticidesthat includes parathion, diazinon, andmalathion.

organs A group of several tissue types thatunite to form structures that perform aspecial function.

Overtone’s Rules Permeability is directly pro-portional to the lipid solubility of the toxi-cant and inversely proportional to themolecular size of the toxicant.

oxidation The loss of hydrogen or electronsby a compound or element; the reverse ofreduction.

PAH clearance A kidney function test thatexamines the capacity of nephrons to re-move p-aminohippuric acid from bloodplasma; used to estimate the rate of plasmaflow through the kidneys.

pancytopenia A reduction in the number ofall blood cell types.

partition coefficient The ratio of a toxicant’ssolubility in a nonpolar solvent to its solu-bility in water.

percutaneous absorption The passage of sub-stances through unbroken skin.

peripheral nervous system (PNS) The sensory

and motor neurons that connect to the cen-tral nervous system; all the nerves and ner-vous tissue outside the central nervous sys-tem.

pesticides Agents that act to destroy or repelpests.

phagocytosis The process by which large par-ticles are taken into a cell; cellular eating.

phase I biotransformation A chemical reac-tion that exposes or adds a small polargroup to a toxicant; enhances the watersolubility of a toxicant.

phase II biotransformation A chemical reac-tion that adds a large molecule to a reac-tive site on a toxicant or its metabolite toenhance water solubility.

phospholipid Molecule containing phosphatesand lipids found in the cell membrane; thephosphate head is a region that is hydro-philic, whereas the lipid tail is a region thatis hydrophobic.

phospholipid bilayer Term used to describethe “sandwich” appearance of the cellmembrane; two layers of phospholipidmolecules in which the phosphate headsare found on the inner and outer surfacesand the lipid tails are directed inward.

phototoxicity A form of dermatotoxicity thatresults when skin is overexposed to ultra-violet light or from the combination of spe-cific wavelengths of light and a phototoxicsubstance.

phthalimides A category of potent fungicidesthat includes Captafol and Folpet; similarin structure to thalidomide.

phytotoxin A poison produced by a plant.pinocytosis An active transport mechanism

used to transfer fluids or dissolved sub-stances into cells; cellular drinking.

placental barrier A barrier that limits the dif-fusion of water-soluble toxicants frommaternal to fetal circulation.

plasma protein A simple protein distributedin the fluids of organisms; albumin is themost abundant circulating plasma proteinto which toxicants are bound.

platelets The smallest cellular components ofblood; thrombocytes.

pneumoconioses Abnormal conditions

P

162 Glossary

resulting from mineral dust in the lung;commonly leads to pulmonary fibrosis.

pneumocytes Special cells that line the alveoliof the lung.

poietins Stimulating factors that regulate thefate of stem cells in the production of bloodcomponents; erythropoietins.

point mutation A change in the chromosomeat a single nucleotide within a gene.

poisons Substances that in relatively smalldoses act to destroy life or seriously im-pair cellular function.

polar molecules Chemicals that possess a posi-tive or negative molecular charge; water-soluble.

polyploidy A chromosomal aberration inwhich there are more than two completechromosome sets; more than 2n, such as3n, 4n, or 5n.

portal vein The major vein entering the liver.potency The toxicological activity of a sub-

stance; a relative concept for comparingtoxicants.

potent A toxicant’s capacity to effect a re-sponse; when comparing two toxicants, theone with the smaller ED50, TD50, or LD50 isthe more potent.

probability A statistical estimate of the riskof a particular response for a given expo-sure to a toxicant (e.g., P=0.00001).

procarcinogen A nonreactive substance thatby itself cannot lead to the formation ofcancer, but once activated can become acarcinogen.

promotion Occurs when initiated cells areacted upon by promoting agents to giverise to cancer.

proteinuria The presence of protein in theurine.

pulmonary fibrosis The abnormal conditionof fibrous tissue as a reparative or reactiveprocess in the lung.

pulmonotoxicity The adverse effects producedby toxicants in the respiratory system.

purines Nucleotides formed from two car-bon—nitrogen rings; adenine and guanine.

pyrimidines Nucleotides formed from a singlecarbon—nitrogen ring; cytosine, thymine,and uracil.

Pyrinimil A rodenticide that interferes withglucose metabolism; diabetogenic.

reabsorption The second process in urine for-mation; results in the return to the bloodof water, glucose, potassium, and aminoacids lost during filtration.

reduction The gain of hydrogen or electronsby a compound or element; the reverse ofoxidation.

regulatory toxicology The branch of toxicol-ogy concerned with assessing the data fromdescriptive and mechanistic toxicology todetermine the legal uses of specific chemi-cals and the risks posed to the ecosystemby their marketing.

replication The process of duplicating a cell’sDNA so that on cellular division (mitosis)the genetic content of each daughter cellwill be identical to the parent cell.

respiratory system The organ system thatfunctions as an air distributor and gas ex-changer; made up of the nares, pharynx,larynx, trachea, bronchi, and lungs.

reversed toxicity Results when one toxicantis not consistently more potent over therange of doses tested as compared to an-other toxicant; evident when two lines oncumulative dose-response graphs cross oneanother; same as mixed toxicity.

ribonucleic acid (RNA) A single-strandednucleic acid involved in protein synthesis,the structure of which is determined byDNA; mRNA is the end result of transcrip-tion, and rRNA, tRNA, and mRNA areall involved in translation.

risk The possibility of loss or injury.risk assessment The process of examining

toxicological and epidemiological data andestimating permissible exposures.

risk estimation The integration of toxicantidentification and exposure evaluation toestimate risk.

risk management The use of risk assessmentconclusions in the development and imple-mentation of regulatory options that

R

Glossary 163

address public health, social, and economicconcerns.

risk perception How each individual interpretsthe possibility of loss or injury.

rodenticides Agents that destroy or repel ro-dents.

Safe Human Dose (SHD) A formula used toextrapolate toxicokinetic data from testorganisms to humans; expressed as mg/day.

safety The possibility that an undesirable bio-logical response (toxicity) will not resultfrom exposure to a toxicant; the inverse ofthe probability of risk.

Safety Factor (SF) A subjective value in thedenominator in the Safe Human Dose for-mula that reflects the uncertainties inherentin extrapolating toxicokinetic data from testorganisms to humans; a small value (e.g.,10) indicates that valid human data is avail-able, and a large value (e.g., 1,000) indicatesa lack of relevant human data.

secretion The third process in urine formationthat occurs in the distal convoluted tubules;involves the active transport of moleculesout of the blood and into the urine.

semipermeable membrane A characteristic ofcell membranes; permits the passage ofsome molecules but not others.

simple diffusion The spontaneous movementof a substance down its concentration gra-dient from a more concentrated to a lessconcentrated region.

slow vital capacity (SVC) A measurement ofthe total volume of air contained in thelungs.

somatic cell Any cell in a multicellular organ-ism other than a germ cell (sperm or egg).

spermatogenesis The process of meiosis in thetestes to produce sperm cells.

storage The accumulation of toxicants or theirmetabolites in specific tissues or as boundto circulating plasma proteins.

subthreshold doses Doses at which no re-sponse is observed as a result of toxicitytesting; NOEL.

sulfate conjugation A phase II biotransforma-tion reaction in which phosphoadenosylphosphosulfate (PAPS) reacts with a toxi-cant or phase I metabolite to produce apolar sulfate conjugate.

systemic toxicity Occurs when toxicants areabsorbed at one site and distributed to adistant body region where they produceadverse effects.

target organ toxicity The adverse effects ordisease states manifested in specific organsof the body.

TD50 The dose at which 50% of the test or-ganisms are observed to exhibit a toxic re-sponse.

teratogen An agent that alters normal cellu-lar differentiation or growth processes,which results in a malformed fetus.

teratogenesis The origin or production of amalformed fetus.

teratology The branch of science concernedwith the production, development, andclassification of malformed fetuses; studyof developmental anomalies in fetuses.

thermoplastics Polymers that can be shapedby pressure and heat to the form of a mold;they can also be remelted and remolded.

thermosetting plastics Polymers that can beshaped by pressure and heat to the formof a mold; they cannot be remelted andremolded.

Threshold dose The dose at which the firstresponse is observed as a result of toxicitytesting; below this dose no responses areobserved.

Threshold Limit Value (TLV) Doses workerscan be exposed to for 8 hours per day, 5days per week for a working lifetimewithout exhibiting adverse healtheffects.

thrombocytopenia A condition in which thereis an abnormally small number of plate-lets in the blood.

tissue A collection of cells that together per-form a similar function.

S

T

164 Glossary

toxic dose (TD) The dose at which test or-ganisms are observed to exhibit toxicity.

toxicant An agent capable of producing symp-toms of intoxication or poisoning.

toxicant evaluation Process of evaluating theapplicability of results from toxicity test-ing performed on nonhuman organisms tohumans.

toxicant identification Process whereby exist-ing literature is reviewed to identify a toxi-cant.

toxication A sequence of chemical reactionsproducing intermediate or final metabolitesthat are more toxic or reactive than the origi-nal parent chemical: same as bioactivation.

toxicity The state of being poisonous; theadverse effects or symptoms produced bytoxicants or poisons in organisms.

toxicity testing The scientific methodologyused to establish a dose and response rela-tionship.

toxicodynamics Study of the mechanisms bywhich toxicants produce their unique ef-fects in an organism.

toxicokinetics Study of time-dependent pro-cesses related to toxicants as they interactwith living organisms; includes study ofabsorption, distribution, storage, biotrans-formation, and elimination.

toxins Poisonous substances produced byplants, animals, or bacteria.

tracheobronchial region Region of the respi-ratory system composed of the trachea,bronchi, and bronchioles.

transcription Process of transferring geneticinformation from a DNA molecule into anRNA molecule.

translation Process of transferring informa-tion from an RNA molecule into a pro-tein.

trisomy Occurs when a cell gains one mem-ber of a pair of chromosomes; in humans,the presence of 47 chromosomes or 2n+1.

vasoconstrictors Agents that initiate the con-traction of the smooth muscles surround-ing peripheral blood vessels.

venoms Poisonous substances secreted by cer-tain animals, such as bees, spiders, andsnakes.

venous vessels Blood vessels that take bloodaway from the capillaries toward theheart.

volume of distribution (VD) The volume ofbody fluids into which a toxicant is dis-tributed; plasma, interstitial fluid, and cel-lular fluid.

warfarin An anticoagulant that prevents theaggregation of thrombocytes; used as arodenticide.

xenobiotics Substances not naturally pro-duced within an organism; substances for-eign to an organism.

V

W

X

165

Abiotic interactions, 11Abortion, spontaneous, 112, 119Absorption, 21

active transport, 47ATP and active transport, 47concentration gradient, 47endocytosis, 48exocytosis, 48facilitated diffusion, 47passive transport, 47percutaneous, 50–51phagocytosis, 48pinocytosis, 48routes, 49–57simple diffusion, 47of toxicants, 21

Absorption routes, 49–57digestive system, 54–57, (illus.) 55-56percutaneous, 50, (illus.) 51

(See also Skin)respiratory system, 51–54, (illus.) 52

Acetaminophen, biotransformation of, 73Acetylcholine, 95, 128Acetylcholinesterase, 27–128Acne vulgaris, 97Acridines, (table) 119Acrocentric chromosomes, 108Active transport, 47Acute exposure, 21Acute toxicity, 20Acute tubular necrosis (ATN), 93, 136Adenine, 108Adenosine triphosphate (ATP), 47Adipose tissue, 67

blood flow to, 64toxicant storage, 67

Adult respiratory distress syndrome (ARDS), 100

Aflatoxins, 130Agenesis, 114Agent Orange, 129Agranulocytopenia, 89Albumin, 65Alcohol consumption, 116Alcohols, (illus.) 137Aldehydes, (illus.) 137Aldicarb, 128Aliphatic alcohols, (illus.) 137, 138Alkylating agents, (table) 119Alkylbenzenes, 139Allergic contact dermatitis, 96, 135, 136–137Aluminum, 133, (table) 134Alveolar region, 53Alveoli, 99Amanita phalloides, 130Amelia, 114Ames assay, 120, 122Amides, (illus.) 137Aminoaciduria, 93Aminohippuric acid, 93Ammonia, 100Anabolism, 72Anemia, 88Anesthetics, 97, 137–138Aneuploidy, 119Anorexia, 135Anoxia, 89, 129Anthracosilicosis, 101Antibiotics, 93, 97Anticoagulants, 131, 132Anticodons, 111Antidotes, 6Antifreeze, 138–139Antihistamines, 97Antiobesity agent, 129

Index

A

166 Index

Anuria, 93Aromatic:

amino compounds, 129nitro compounds, 129

Aromatic hydrocarbons, 139Arsenic, 82, 97, 101, 134–135Arterial blood gases (ABGs), 89, 101Arterial vessels, 60, (illus.) 62Arthritis, 130Asbestos, 101Asbestosis, 101Aspergillus flavus, 130Aspirin, 100, 132Associative relationships, 30, 111Asthma, 100Astrocytes, 65, 94Ataxia, 136Atmosphere, (illus.) 11, 12Atomic bomb survivors,

treatment, 139Atoms, 13, (illus.) 15Atresia, 114Avicenna, 6Ayurveda, 6

Bacteriotoxin, 3, 95, 127Barriers to toxicant entrance, 65Base analogues, 117Base pairing, 108Base substitution, 117, (illus.) 118Batrachotoxin, 95Benzene, 89, 139Berylliosis, 101, 135Beryllium, 101, 135Bile, 80, 90Bile salts, 90Biliary excretion, 80Bilirubin, 90Bioactivation, 73

and carcinogenesis, (illus.) 121Biological degradation, 133Biological half-life (T1/2), 73, 93, 130Biological magnification, 128Biomineralization, 139Biosphere, 10, (illus.) 11Biotic interactions, 11Biotransformation, 21, 60, 72–78

in aged, 75changes that facilitate elimination, 72circadian rhythms, 75endogenous chemicals, 74factors affecting, 75in fetuses and neonates, 75“first pass”, 74gender differences, 75in liver, 60

location of reactions, 73microbes and, 75microsomal enzymes, 74–75nutritional status, 75phase I reactions, 73–76, (illus.) 74, (illus.) 77phase II reactions, 73–78, (illus.) 74, (illus.) 78reactions, 73schematic relationships, (illus.) 74of toxicants, 21, 72xenobiotics, 74

Bipyridyls, 129Bismuth, (table) 134Blood, 60

arterial blood gases (ABGs), 89, 101clot formation, 132complete count, 89differential count, 89dyscrasias, 89erythrocytes, 60hematotoxicity, 87–89leukocytes, 60, 87plasma, 60platelets, 60tests, 89thrombocytes, 60

Blood flow, 64to body regions, (table) 64

Blood flow/mass ratio, (table) 64Blood urea nitrogen (BUN), 94Blood-brain barrier, 65, 136Bone matrix, 67Botulin toxin, 95British anti-Lewisite (BAL), (table) 135Bromouracil, (table) 119Bronchoscope, 101Bronchoscopy, 101Busulfan, (table) 102

Cadmium, 81, 82, 93, 135Calcium oxalate, 93Cancer, 120

bladder, 121lung, 120, 139nasal cavities, 137scrotum, 120thyroid, (illus.) 140vaginal and cervical, 21

Capillaries, 60Captafol, 130Carbamates, 128Carbaryl, 128Carbon disulfide, (table) 102, 138Carbon monoxide, 88, (table) 102Carbon tetrachloride, 20, 90, 138Carboxylic acid, (illus.) 137Carcinogenesis, 107, 120–122

B

C

Index 167

bioactivation, (illus.) 121initiation, (illus.) 121, 122

Carcinogens, 107, (illus.) 121, 122cocarcinogens, 122DNA reactive, 122epigenetic, 122genotoxic, 122, 139non-DNA reactive, 122

Carcinoma of the lung, 100, 101Cardiac output, (table) 64

to adipose tissue, 64to bone, 64to kidneys, 64to liver, 64

Cardiovascular system, 60Catabolism, 72Cataractogenic, 129Causal relationships, 30Cause-and-effect relationships, 13Ceiling effect, 34Cell membrane, 45

hydrophilic region, 45hydrophobic region, 45lipophilic, 45phospholipid bilayer, 45phospholipid, 45semipermeable, 45structure of, 45, (illus.) 47

Cellophane, 138Cells, 13, 15, (illus.) 46, 87, (illus.) 88

daughter, 107, 108germ, 107interaction of toxicants with, 45major structures and functions, (table) 49membrane structure, 45–47somatic, 107toxicity differences, 87typical cell, (illus.) 46

Cellular absorption processes, 47Cellular damage, 20

irreversible, 20reversible, 20

Cellular division, 107Cellular respiration, inhibitors, 132Cellular uptake of toxicants, 48–49Cement, 97Central nervous system (CNS), 94Centromere, 107Cerebrospinal fluid (CSF), 96Chelating agents, 133–135

British anti-Lewisite (BAL), (table) 135Edetate calcium disodium, (table) 135Penicillamine, (table) 135Succimer (DMSA), (table) 135

Chemicals:man-made, 13synthetic, 13

Chernobyl, 139, (illus.) 140Chloracne, 97, 98, (illus.) 99 129

Chlordane, 89Chlorinated aliphatics, 138Chlorinated organic insecticides, 128Chlorine, 100Chloroform, 138Chlorophenoxy compounds, 129Cholestatic agents, 90Cholinesterase, 95, 127–128Chromates, 97Chromatids, 107Chrome, 97Chromium, 101, 136Chromosome, 107

aberrations, 119–120acrocentric, 108aneuploidy, 119metacentric, 108monosomy, 119, (table) 120mutagenic effects on, (table) 120polyploidy, 119ring, 119trisomy, 119, (table) 120

Chronic exposure, 21Chronic toxicity, 20Chyme, 57Cigarette smoke, 100, 101Cilia, 53Circulatory system:

cardiovascular, 60diagram of, (illus.) 62lymphatic, 60portal vein, 60

Cirrhosis, 90Clara cells, 100Classification of toxicants, 21, (table) 22Clear-cell adenocarcinoma, 115Clinical toxicology, 10Clot formation, 132Coal dust, 101Coal tar compounds, 97Cobalt, (table) 102, (table) 134Cocarcinogens, 122Codon, 108Coke oven emissions, 101Collagen, 67Coma, 129, 136Communities, (illus.) 15Complete blood count (CBC), 89Computerized axial tomography (CAT), 96, 101Concentration gradient, 47, 63Congenital anomalies, 111Conium maculatum, (illus.) 5Conjugate, 76Conjugate reactions, 76Connective tissue, 16Copper, 82, (table) 134Cosmetics, 97Coumadin, 132Creatinine, 94

168 Index

Cumulative dose-response graphs, 32Curare, 95Cyanide, 89Cytochrome P-450, 138Cytosine, 108Cytosolic enzymes, 75Cytotoxic hypoxia, 89

Delayed functional maturation, 112Delayed toxicity, 20Deoxyribonucleic acid (DNA), 107Depolarization, 94Dermatitis:

allergic contact, 96, 135, 136–137irritant contact, 96

Dermatotoxicity, 96–98, (illus.) 99dioxin, 129evaluating, 98examples of, 97–98, (illus.) 99toxicity mechanisms, 96–97

Dermatotoxins, 97Dermis, 50Descriptive toxicology, 9Detergents, 97Detoxication, 60, 73Detoxification (see Detoxication)Developmental syndromes, 114Diabetogenics, 131, 132Diarrhea, secretory, 127Diazinon, 95, 127–128Dibenzofurans, 98Dichlorodiphenyltrichloroethane (DDT), 89, 90, 95,128Dichlorophenoxyacetic acid (2, 4-D), 129Diethylene glycol, 138Diethylstilbestrol (DES), 20, 115–116Diffuse alveolar damage (DAD), 100Digestive system, 54–57, (illus.) 55–56

absorption of toxicants, 54–57absorptive surface area, 54accessory organs, 54composed of, 54diagram of, (illus.) 55large intestine, 54, (illus.) 55lymphatic transport, 57microvilli, 54, (illus.) 56mucosa, 54muscularis, 54peristaltic contractions, 54serosa, 54small intestine, 54, (illus.) 55–56stomach, 54, (illus.) 55submucosa, 54villi, 54, (illus.) 56

Dinitrophenol (DNP), 129

Dioxane, 133Dioxin (Tetrachloro-dibenzo-p-dioxin or TCDD), 98,129Diploid, 107, 119Diquat, 129Disease, 13, (illus.) 14Distribution, 21, 60, 63

apparent volume of, 63of blood, 60of blood plasma, 63concentration gradients, 63factors affecting, 63interstitial fluid, 63intracellular fluid, 63of lymph, 60processes affecting, 60of toxicants, 60volume of, 63

Dithiocarbamates, 130Division failures, 114Dose, 23

subthreshold, 34test, 23therapeutic, 36threshold, 34

Dose-response, 30–38assumptions, 30ceiling effect, 34effective dose, 36efficacy, 36, (illus.) 37Gaussian distribution, 32interpreting data, 36–38lethal dose (LD), 36margin of safety, (illus.) 37, 38mean or average response, 34, (illus.) 35No Observable Effects Level (NOEL), 34normal distribution, 32number of test organisms, 32potency, 36relationship, 30relative toxicity, 36sampling distribution, 32standard deviation, 34statistical considerations, 32subthreshold dose, 34therapeutic dose, 36threshold dose, 34toxic dose (TD), 36

Dose-response graphs, 30, 32–38cumulative, 32, (illus.) 33, 35frequency, 32, (illus.) 33, 35horizontal axis on, 32labeling of, 32mixed toxicity, 36, (illus.) 37reversed toxicity, 36, (illus.) 37statistical considerations, 32vertical axis on, 32visual characterization of, 34

D

Index 169

Dyes, 97and bladder cancer, 121

Dysraphic anomalies, 114

Ebers papyrus, (illus.) 4Ecosphere, (illus.), 15Ecosystem, 11

impact of human population, 12infinite dilution, 12positive and negative feedback, 11stability of, 12

Ectoderm, 111, (illus.) 112Ectopia, 114ED, 36ED50, 36Edema, 96Edetate calcium disodium, (table) 135Effective dose (ED), 36Efficacy, 36, (illus.) 37Egyptian papyri, 4Electroencephalography (EEG), 96Electromagnetic fields, 139Electromyography (EMG), 96Elimination of toxicants, 21, 78–82

bile, 80biliary excretion, 80cerebrospinal fluid, 82enterohepatic loop, 81fecal excretion, 80–81glomerular filtration, 78hair, 82hepatocytes, 80intestinal excretion, 80kidney and nephron anatomy, (illus.) 79lactation, 82milk, 82nails, 82reabsorption, 79respiratory elimination, 81saliva, 81secretion, 79skin, 82sweat, 81urinary, 78–80urinary filtration, 78

Embryogenesis, 111, (illus.) 113Embryolethality, 112Encephalopathy, 136End effect, 23

and species specificity, 26Endocytosis, 48Endoderm, 111, (illus.) 112Endoplasmic reticulum, 75Endothelium, 65Enterohepatic loop, 81Environmental Protection Agency (EPA), 13

Environmental toxicants, 127–140electromagnetic fields, 139fungicides, 130–131herbicides, 129insecticides, 127–128ionizing radiation, 139, (illus.) 140metals, 133–137organic solvents, (illus.) 137, 137–139pesticides, 127–133plastics, 133rodenticides, 131–133

Environmental toxicology, 3, 10relevance of, 12, 13and risk, 144role of, 147terminology, 3

Enzyme:cystolic, 75microsomal, 74–75

EPA, 13Epidemic poisonings, 130Epidemiological studies:

beryllium, 135death certificates, 30, (illus.) 31electromagnetic fields, 139radium dial painters, 139retrospective, 30sample populations, 144

Epidermis, 50Epigenetic carcinogens, 122Epithelial:

hyperplasia, 98tissue, 15–16

Epithelial cells, 45ciliated columnar, 99, 100pulmonary types of, 99stratified squamous, 99structure of, 45

Epithelium, 45Erythema, 96Erythrocytes, 60, 87Erythropoietin, 87Escherichia, 127Esters, (illus.) 137Ethanol, 90, 95

elimination of, 81Ethers, (illus.) 137Ethyl alcohol, 138Ethylbenzene, 139Ethylene glycol, 93, 138–139Ethylene-bidithiocarbamate (EBDC), 130Etymology, 3Excretion:

biliary, 80fecal, 80intestinal, 80

Exocytosis, 48Exposure:

E

170 Index

acute, 21chronic, 21

Exposure evaluation, 145

Facilitated diffusion, 47Fatty liver disease, 90Fecal excretion, 80Ferbam, 130Fetal alcohol syndrome (FAS), 114, 116, 138Fibroblasts, 101Filtration, 92Fluoroacetic acid, 132Fluorocarbons, (table) 102Folpet, 130Food additives, 127Food and Drug Administration (FDA), 114Forced vital capacity (FVC), 101Forensic toxicology, 10Frameshift, 117, (illus.) 118Freon, (table) 102Frequency dose-response graphs, 32Fungicides, 130Fungitoxin, 100, 127, 130

Gallium, (table) 134Gene, 108Genetic code, 108, (illus.) 109Genotoxic carcinogens, 122, 139Gentamycin 92Germ cells, 107Glial cells, 94Glomerular filtration, 92Glomerular filtration rate (GFR), 93Glucuronidation, 77, (illus.) 78Glycols, (illus.) 137, 138–139Glycosaminoglycans, 67Glycosuria, 93Gold, (table) 134Graphs, dose-response, 30, 32, (illus.) 33Greases, 97Guanine, 108

Halogenated aromatic compounds, 97–98Hantavirus, 131Haploid, 107, 108, 119Hazardous waste, 13Health hazards, 13Heart, 60, (illus.) 61

target organ toxicity, (illus.) 102Heavy metals, 93

Hematocrit, 89Hematotoxicity, 87–89

evaluating, 89examples of, 88–89toxicity mechanisms, 87–88

Hematotoxins, 87–88Hematuria, 92Hemlock, 5Hemoglobin, 88, 89, 117

biotransformation of, 72Hemolytic anemias, 89Heparin, 132Hepatocytes, 67, 90Hepatomegaly, 130, 135Hepatotoxic, 73Hepatotoxicity, 89–92

cholestatic, 90cytotoxic, 90evaluating, 90, 92examples of, 90toxicity mechanisms, 90

Herbicides, 129Heroin, 92Hexachlorobenzene (HCB), 130Hexachlorophene, 95Histogenesis, 111, (illus.) 112History of toxicology, 4–7Honeycomb lung, 135Household products, 127Hydrogen sulfide, 89Hydrophilic, 45

metabolites, 72Hydrophobic, 45Hydrosphere, (illus.) 11, 12Hydroxyapatite, 67Hyperemia, 96Hyperglycemia, 132Hyperpigmentation, 96, 97Hypodermis, 50Hypoglycemia, 132Hypopigmentation, 96Hypoplasia, 114Hypoxia, 89

Immediate toxicity, 20In vitro, toxicity testing, 23In vivo, toxicity testing, 23Incidence, 144Industrial:

chemicals, 127toxicology, 10

Infinite dilution, 12Inhalational anesthetics, 137–138Initiation, (illus.) 121, 122Inorganic mercury, 93Insecticides, 127

F

G

H

I

Index 171

Insulin, 132Integumentary system, 50Interstitial alveolar fibrosis, 101Interstitial fluid, 63Intestinal excretion, 80Intracellular fluid, 63Intrauterine growth retardation (IUGR), 116Inulin, 93Iodine, radioactive, 139Ionized molecules, 73Ionizing radiation, 139Ipomeanol, 100Iron, 82, (table) 134Irritant contact dermatitis, 96Ischemia, 132

Kanamycin, 92Karyogram, 108Karyotype, 108Ketones, (illus.) 137Kidney, (illus.) 79

failure, 93Krebs cycle, 132, 139

LD, 36LD50, 36Lead, 67, 82, 92, 93, 95, (table) 102, 136

and bone, 136encephalopathy, 136and loss of appetite, 136

Leather tanning, 136Lethal dose (LD), 36Leukemia, 88Leukocytes, 60, 87Levels of structural and functional organization, (illus.)15Lifetime average daily dose (LADD), 145–146Linear dose sequence, 25Lipid soluble, 48, (illus.) 74Lipophilic molecules, 72Lithium, (table) 134Lithosphere, (illus.) 11, 12Liver, 89–92

lobule, 89–90, (illus.) 91scan, 90structure of lobule, (illus.) 91

Local toxicity, 20Logarithmic dose sequence, 25Long-term planning, 11Lung:

cancer, 120carcinoma of, 100and cigarette smoking, 120

honeycomb, 135neoplasms, 101tissue compliance, 101

Lung cancer, and exposure to:beryllium, 135cadmium, 135chromium, 136nickel, 137

Lymph, 60capillaries, 60nodes, 60trunks, 60

Lymphatic system, 57, 60, (illus.) 61common thoracic duct, 57diagram of, (illus.) 61lymph ducts, 57right lymphatic duct, 57transport, 57

Lymphocytes, 60Lymphoid tissue, 60

Macromolecules, 13Macrophages, 101Magnetic resonance imaging (MRI), 96Malathion, 95, 127–128Maneb, 130Manganese, (table) 134Margin of safety, 38Marmosets, 114, (illus.) 116Maximum daily dose (MDD), 145Mechanistic toxicology, 9Mee’s lines, 135Meiosis, 108Melanin, 97, (illus.) 98Melanocytes, 50, 96Melanotoxicant, 97Mercury, 93, 97, 136Mercury toxicity, 93, 136

Minamata Bay, Japan, 136Mesoderm, 111, (illus.) 112Mesothelioma, 101Messenger RNA (mRNA), (illus.) 110Metabolism, 72Metabolite, reactive, 73Metacentric chromosomes, 108Metal compounds, 97Metals, 93, 97, 133–137

chelating agents, 133–134, (table) 135enterohepatic circulation of, 133essential, 133, (table) 134heavy, 93, 97in medical diagnostics, 133, (table) 134valences of, 133

Metaphase, 108Methanol, (table) 102, 138Methoxypsoralen, 97

K

L

M

172 Index

Methyl alcohol, 138Methylation, (illus.) 78Methylmercury, 130Microbes, and biotransformation, 75Microcephaly, 116Microcytic hypochromic anemia, 88Microglia, 94Micrognathia, 116Microsomal enzymes, 74–75Minamata Bay, Japan, 136Minute volume respiration (MVR), 53–54Mitosis, 107Mixed toxicity, 36, (illus.) 37Molecules, 13, (illus.) 15

complex, (illus.) 15ionized, 73

Molybdenum, (table) 134Monosomy, 119, (table) 120Morbidity, 13, (illus.) 14Morphogenesis, 112Mortality, 13Mucociliary escalator, 53, 100Mucus, 53Muscle tissue, 16Mutagenesis, 117–120Mutagens, 107, 117, (table) 119Mutation:

base analogues, 117, (table) 119carcinogenesis, (illus.) 121deletion, 117, (illus.) 118frameshift, 117, (illus.) 118insertion, 117, (illus.) 118missense, 117, (illus.) 118nonsense, 117, (illus.) 118point, 117–118substitution, 117, (illus.) 118

Myelin, 94

Nabam, 130Naphthalene, 89, 98a—Naphthylisocyanate, 90Nasopharyngeal region, 53Necrosis, 132Neoplasms, 112Nephritic syndrome, 92Nephron, 67, 78, (illus.) 79Nephrotic syndrome, 92Nephrotoxicity, 92–94

evaluating, 93–94examples of, 92–93obstructive uropathies, 93toxicity mechanisms, 92

Nerve, 94Nerve tissue, 16Nettles, 97Neurons, 94

Neurotoxicity, 94–96evaluating, 95–96examples of, 95toxicity mechanisms, 94–95

Neurotoxins, 94Neurotransmitter, 95Ni-Cad battery workers, 135Nickel, 82, 101, 136–137Nickel itch, 137Nitrites, (illus.) 137Nitrogen dioxide, 100No Observable Effects Level (NOEL), 34Nonpolar, 48Norbormide, 132Normal distribution, dose-response, 32Nucleic acids, 108Nucleotides, 108

Obstructive uropathies, 93Occluding cell junctions, 45Occupational:

asthma, 100exposure to Cadmium in battery workers, 135toxicology, 10

Oils, 97Oligodendrocytes, 94Oliguria, 93, 129Oogenesis, 108Orfila, 7, (illus.) 8, 9, 136Organ systems, (illus.) 15, 16Organelles, (illus.) 15Organic chemicals, 93, 137–139, (illus.) 137Organisms, (illus.) 15

complex, (illus.) 15single-cell, (illus.) 15

Organochlorines, 128Organogenesis, 111, (illus.) 112, 113Organomercurials, 130Organophosphates, 127Organs, (illus.) 15, 16Osteoblasts, 67Osteoclasts, 67Osteogenic carcinoma, 139Osteomyelitis, 130Osteoporosis, 130Overton’s Rules, 48Oxidation, 76, (illus.) 77Ozone, 100

PAH clearance, 93Paint pigments, 136Pancytopenia, 89Paracelsus, 6, (illus.) 7, 23

N

P

O

Index 173

Paralysis, 128Paraquat, 100–101, 129Parathion, 127–128Partition coefficient, 48Passive transport, 47Patch test, 98Penicillamine, (table) 135Penicillin, 97Percutaneous, 49

absorption of toxicants, 50–51Perfumes, 97Peripheral nerve conduction velocity, 96Peripheral nervous system (PNS), 94Pesticides, 127–133Petroleum oils, 97Phagocytosis, 48Phallotoxins, 130Pharmaceuticals, 100Phase I biotransformation, 73–76, (illus.) 74, (illus.) 77

carbonyl reductions, (illus.) 77cytochrome P-450, 76dehydrogenation, (illus.) 77desulfuration, (illus.) 77hydrolysis, 76, (illus.) 77ionized metabolites, 76N-oxidations, (illus.) 77oxidation, 76reduction, 76representative reactions, (illus.) 77S-oxidations, (illus.) 77

Phase II biotransformation, 73–78, (illus.) 74, (illus.) 78conjugate, 76conjugation reactions, 76, (illus.) 78glucuronic acid, 76glucuronidation, 77, (illus.) 78methylation, 78, (illus.) 78representative reactions, (illus.) 78sulfate conjugation, 77, (illus.) 78

Phenols, (illus.) 137Phocomelia, 114, 115, (illus.) 116Phospholipid bilayer, 45Phospholipids, 45Phototoxicants, 97Phototoxicity, 96Phthalimides, 130, (illus.) 131Phytotoxin, 3, 95, 127Pinocytosis, 48Placental barrier, 65, 136Plants, 97Plasma proteins, 63, 65–67, 132Plastics:

incineration of, 133thermoplastics, 133thermosetting, 133

Platelets, 60, 87Platinum, (table) 134Pneumoconioses, 100Pneumocytes, 53, 54, 100, 129

Clara cells, 100

type I, 99type II, 99

Poietins, 87Point mutation, 117, (illus.) 118Poiseuille’s Law, 53Poison ivy, 97Poisonings, epidemic, 130Poisons, 3Polar molecules, 48Polycyclic aromatic hydrocarbons, 97, (table) 102Polyhalogenated biphenyls, 98Polyploidy, 119Polyvinylchloride (PVC), 133Populations, (illus.) 15Portal vein, 60, 74Potency, 36, (illus.) 37Potent, 36, (illus.) 37Primate species, 114, (illus.) 116Probability, 144Procarcinogen, (illus.) 121Promotion, (illus.) 121, 122Propoxur, 128Propylene glycol, 138–139Prostate cancer, exposure to cadmium, 135Protein synthesis, (illus.) 110, 111Proteinuria, 92Proteoglycans, 67Proximal tubule, 92Psoralens, 97Puffer fish, 95Pulmonary fibrosis, 100Pulmonotoxicity, 99–101

evaluating, 101examples of, 100–101toxicity mechanisms, 99–100

Purines, 108Puromycin, 92PUVA, 97Pyrimidines, 108Pyrinimil, 132

Radiation exposure, childhood thyroid cancer and, 139Radioactive iodine, 139Radiopharmaceutical, 90Radium, 139Ramazzini, 6Reabsorption, 79Reactions:

Phase I biotransformation, 73–76, (illus.) 74, (illus.)77

Phase II biotransformation, 73–78, (illus.) 74,(illus.) 78

replacement, 66Reactive metabolite, 73, 121Red blood cells, 60, 87Reduction, 76, (illus.) 77

R

174 Index

Regulatory toxicology, 10Relative toxicity, 36Replacement reactions, 66Replication, 107–108Respiratory system, 49, 51–54, (illus.) 52

absorption of toxicants, 51–54alveolar region, 53ciliated columnar epithelium, 52ciliated cuboidal epithelium, 52diagram of, (illus.) 52elimination of toxicants, 81minute volume respiration (MVR), 53–54mucociliary escalator, 53nasopharyngeal region, 53pneumocytes, 53respirations per minute, 54respiratory passageways, 53terminal bronchioles, 53tidal volume, 53tracheobronchial region, 53

Response (see End effect)Reversed toxicity, 36, (illus.) 37Ribonucleic acid (RNA), 107, 110, 111Ribosomal RNA, 111Ringworm, and x rays, 139Risk, 144–147

determining acceptable, 146equation to approximate, 146expressed as incidence, 144expressed as probability, 144management, 146perception, differences in, (table) 144and safety, 146–147sample populations, 144

Risk assessment, 144–146exposure evaluation, 144, 145risk estimation, 144, 146toxicant evaluation, 144, 145toxicant identification, 144, 145

Risk estimation, 144, 146Risk management, 146Risk perception, (illus.) 144Rodenticides, 131, 132Routes of absorption (see Absorption routes)Rubber antioxidants, 97

Safe Human Dose (SHD), 147Safety, 146–147

and risk, 146–147safe human dose (SHD), 147

Safety factor (SF), 147Salmonella, 127Salmonella typhimurium, 120Sample populations, 144Scientific terms, defining, 3Secretion, 79, 80

Selenium, (table) 134Semiconductors, 138Semipermeable membrane, 45Shigella, 127Sickle-cell disease, 117Silica, 101Silicon dioxide, 101Silicosis, 101Simple diffusion, 47Skin:

absorption of toxicants, 50–51composition of, 50cutaneous adsorption of a toxicant, 50diagram of human, (illus.) 51enhanced epidermal permeability, 51factors affecting diffusion, 50fatty region, 50hair follicle, 50, (illus.) 51keratinized, 50premature aging, 96sweat gland, 50, (illus.) 51

Slow vital capacity (SVC), 101Sodium fluoroacetate, 132Solder fumes, (table) 102Solubility of toxicants, 48Somatic cells, 107Species specificity, 26Spermatogenesis, 108Spina bifida, 114Spirometer, 101Stainless steel, 136Steroids, 90Storage of toxicants, 21, 65–68

in bone, 67in fat, 67in kidneys, 67in liver, 67in neutral fat, 67in plasma proteins, 65–67in triglycerides, 67

Stratum corneum, 50Stratum germinativum, 50Subatomic particles, 13, (illus.) 15Subthreshold dose, 34Succimer (DMSA), (table) 135Sulfate conjugations, 77–78, (illus.) 78Sulfur dioxide, 100Sulfuric acid, 100, 127Sunlight, 97Synaptic transmission, 95, 128Syndactyly, 114Syndrome:

Cri-du-chat, (table) 120Down’s, (table) 120Edward’s, (table) 120Jacob’s, (table) 120Klinefelter’s, (table) 120metafemales, (table) 120Patau’s, (table) 120

S

Index 175

Turner’s, (table) 120Systemic toxicity, 20

T1/2, 73, 93, 130Tachycardia, 129Tachypnea, 129Tanning agents, 97Target organ toxicity, 87–102, (table) 102

dermatotoxicity, 96–98, (illus.) 99eye, (table) 102heart, (table) 102hematotoxicity, 87–89hepatotoxicity, 89–92nephrotoxicity, 92–94neurotoxicity, 94–96ovary, (table) 102pulmonotoxicity, 99–101testes, (table) 102uterus, (table) 102

TD, 36TD50, 36Teratogen, delayed, 115–116Teratogenesis, 107, 111–116Teratogens, 107Teratology, 111Tertiary butyl phenol, 97, (illus.) 98Test organism, 23Tetrachloro-dibenzo-p-dioxin (TCDD), 129Tetracycline, 97Tetrodotoxin, 95Thalidomide, 26, 114–116, 130, (illus.) 131

effects in child, (illus.) 115effects in fetal marmosets, (illus.) 116and phocomelia, 114, (illus.) 115–116and phthalimides, 130, (illus.) 131

Thermoplastics, 133Thermosetting plastics, 133Threshold dose, 34Threshold Limit Value (TLV), 34Thrombocytes, 60, 87Thrombocytopenia, 88Thymine, 108Thyroid cancer, (illus.) 140Thyroidomegaly, 130Tissue, (illus.) 15, 16

connective, 16epithelial, 15mass, (table) 64muscle, 16nerve, 16sensitivity to teratogens, (illus.) 113

Toluene, 139Toluene di-isocyanate, 100Toxic:

substances, 13waste, disposal of, 12

Toxic dose (TD), 36Toxicant evaluation, 144, 145Toxicant identification, 144, 145Toxicants, 3

affinity for specific tissues, 64binding to albumin, 65binding to plasma proteins, 63biotransformation of, 72–78cellular uptake of, 48classification of, 21, (table) 22dilution by fluid volume, 63distribution of, 60effects on the cell, 16and lactation, 82lipid soluble, 48nonpolar, 48polar, 48produce toxic effects, 16properties affecting distribution to tissues, 63relative solubility, 48storage, 65, (illus.) 66, 67storage in bone, 67storage in fat, 67storage in the kidneys, 67storage in the liver, 67storage in neutral fat, 67storage in plasma proteins, 65storage sites, 64, (illus.) 66storage in triglycerides, 67structural barriers to entrance, 65

Toxication, 73Toxicity, 3

acute, 20chronic, 20definition of, 20delayed, 20determination of, 23immediate, 20local, 20mechanisms, 87–88, 94, 96mixed, 36, (illus.) 37, 38reversed, 36, (illus.) 37systemic, 20testing, 23variables affecting, 25, 26

Toxicity testing, 23–26comparison of weight, dosage, and dose, (table)

25determination of test doses, 25doses to be tested, 23duration of test, 23estimated costs, (table) 24exposure period, 23in vitro, 23in vivo, 23linear dose sequence, 25logarithmic dose sequence, 25

T

176 Index

responses observed, (illus.) 24species differences in, 26types of, (table) 24

Toxicodynamics, 21–22cellular effects, 21schematic representation, (illus.) 22

Toxicokinetics, 21–22schematic representation, (illus.) 22time-dependent processes, 21

Toxicology, 4–10clinical, 10descriptive, 9disciplines that contribute to, (table) 10environmental, 10forensic, 10history of, 4–7industrial, 10mechanistic, 9occupational, 10regulatory, 10subdisciplines of, 7

Toxins, 3Tracheobronchial region, 53Transcription, 108–111, (illus.) 110Transfer RNA, 111Translation, 108–111, (illus.) 110Trichloroethylene, 90Trichlorophenoxyacetic acid (2, 4, 5-T), 129Trisomy, 119, (table) 120Tuberculosis, and radium, 139Tubular secretion, 92Tyrosine, 97, (illus.) 98

Ultraviolet light, 96Ultraviolet radiation, 97, (table) 119Uracil, 108Urinary elimination, 78–80

VD, 63Valences, 133Vasoconstrictors, 131, 132Venom, 3Venous vessels, 60, (illus.) 62Vibrio, 127Vitiligo, 97Volume of distribution (VD), 63

Warfarin, 132Waste:

and disease, 13, (illus.) 14domestic solid, 13generated in the U.S., 13hazardous, 13health hazards of, 13

White blood cells (WBCs), 60Wood alcohol, 138Wood preservatives, 136World Health Organization (WHO), 13, 120, 122

X rays, 96, 101, (table) 119, 139Xenobiotics, 3, 73, 74, 91Xylene, 139

Zinc, 82Zineb, 130Zootoxins, 3Zygote, 111

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Documents

Essentials of Environmental Toxicology