Toxic Metals and Antioxidants: Part II. The Role of

Page 106 Alternative Medicine Review ◆ Volume 8, Number 2 ◆ 2003

Toxic Metals Part II Review

Copyright©2003 Thorne Research, Inc. All Rights Reserved. No Reprint Without Written Permission

Lyn Patrick, ND – 1984 graduate, Bastyr University;associate editor, Alternative Medicine Review; privatepractice, Tucson, Arizona, 1984-2002.Correspondence address: 21415 Hwy 140, Hesperus, CO81326Email: [email protected]

Toxic Metals and Antioxidants: Part II.The Role of Antioxidants in Arsenic

and Cadmium Toxicity

AbstractExposure to toxic metals has become anincreasingly recognized source of illnessworldwide. Both cadmium and arsenic areubiquitous in the environment, and exposurethrough food and water as well as occupationalsources can contribute to a well-definedspectrum of disease. The symptom picture ofarsenic toxicity is characterized by dermallesions, anemia, and an increased risk forcardiovascular disease, diabetes, and liverdamage. Cadmium has a significant effect onrenal function, and as a result alters bonemetabolism, leading to osteoporosis andosteomalacia. Cadmium-induced genotoxicityalso increases risk for several cancers. Themechanisms of arsenic- and cadmium-induceddamage include the production of free radicalsthat alter mitochondrial activity and geneticinformation. The metabolism and excretion ofthese heavy metals depend on the presenceof antioxidants and thiols that aid arsenicmethylation and both arsenic and cadmiummetallothionein-binding. S-adenosyl-methionine, lipoic acid, glutathione, selenium,zinc, N-acetylcysteine (NAC), methionine,cysteine, alpha-tocopherol, and ascorbic acidhave specific roles in the mitigation of heavymetal toxicity. Several antioxidants includingNAC, zinc, methionine, and cysteine, whenused in conjunction with standard chelatingagents, can improve the mobilization andexcretion of arsenic and cadmium.(Altern Med Rev 2003;8(2):106-128)

Lyn Patrick, ND

IntroductionHeavy metals are found in increasingly

hazardous concentrations in air, food, and water.The Agency for Toxic Substances and DiseaseRegistry (ATSDR) lists arsenic and cadmiumamong the top seven of the 275 most hazardoussubstances in the environment. This listing is basedon the toxicity of the substance and the potentialfor exposure from air, water, or soil contamina-tion at any one of the 1,560 National PrioritiesList Cleanup or “Superfund” sites.1 Arsenic andcadmium, in addition to mercury and lead, havebeen identified as the most probable causes ofheavy metal-related disease observed in primarycare medicine.2

As the prevalence of heavy metal expo-sure is increasingly recognized and identified inindividuals seen in private practice clinics, theneed for effective prevention and treatment willincrease. In this article, the clinically relevant as-pects of arsenic and cadmium exposure are re-viewed, as are current exposure data. The role ofantioxidants in mitigating the damage of heavymetal toxicity and assisting the process of chela-tion has been explored in in vitro and animal stud-ies; however, clinical trials in humans have beenlimited. The relationship of oxidant stress to thetoxic effects of arsenic and cadmium is summa-rized, in addition to the potential role of antioxi-dants as adjunctive treatment in heavy metal ex-posure.


Alternative Medicine Review ◆ Volume 8, Number 2 ◆ 2003 Page 107

Review Toxic Metals Part II

ArsenicSources of Exposure and Symptomsof Toxicity

It is estimated that several million peopleworldwide suffer the effects of chronic arsenicexposure resulting from environmental release ofarsenic.3 Arsenic was identified as a hazardouswaste at 1,014 of the 1,598 National Priorities List“Superfund” sites in the United States in 2000.4

The National Institute of Occupational Safety andHealth (NIOSH) estimates that 55,000 workers inthe United States were exposed to high levels ofarsenic in the early 1980s.5 These estimates areconsidered low because they exclude mining andagriculture, two occupational sources of arsenicexposure. Groundwater contamination providesthe majority of worldwide arsenic exposure. In theUnited States, groundwater concentrations exceedthe U.S. Environmental Protection Agency (EPA)limit of 50 ppb.6 Levels as high as 166 ppb havebeen measured in public water systems in Utah7

and a private well in Nevada was found to contain1,312 ppb.8 Levels of 20 ppb or more are found inthe water supply of at least 725,000 people in theUnited States.9

Inorganic arsenic, the form found in soil,water, and crops, is classified by the EPA as aGroup A human carcinogen, meaning that suffi-cient knowledge exists to substantiate a causalrelationship between human exposure and canceroccurrence.10 The current EPA water standard,known as the Maximum Contaminant Level(MCL) of 50 ppb (50µg/L), has been criticized bythe scientific community, specifically the NationalAcademy of Sciences. Based on their 1999 RiskEstimates, the lifetime risk of contracting bladderor lung cancer from arsenic at a 50-ppb concen-tration is 1 in 100. Even at the EPA-proposed MCLof 10 ppb, rejected by the federal government in2001, the lifetime risk is still 1 in 500. This riskestimate is significantly higher than the EPA’scurrent acceptable cancer-risk definition for wa-ter contaminants: 1 in 10,000 risk of fatal cancer.10

Inorganic arsenic is also found in environmentaltobacco smoke and arsenic-treated wood, used inover 90 percent of the outdoor wooden structuresin the United States.

Of growing concern is the presence ofhigh levels of heavy metals in industrial wasteprocessed for use as fertilizer. The presence of highlevels of arsenic in agricultural fertilizer (whichcan be legally sold as organic fertilizer) has beenshown to exceed EPA limits for arsenic inbiosolids.11 A minimal risk level (MRL) of 0.8 µgof arsenic/kg/day (approximately 5.6 µg for anadult) has been established for chronic arsenicexposure as a result of studies showing that ap-proximately twice that dose resulted in hyperpig-mentation and keratosis, both symptoms of chronicarsenic exposure-induced skin damage.12

High levels in soil used for agriculturalpurposes in Denmark are considered over 20 µgper gram.13 Soils in Butte, Montana, collected nearmine tailings (a source of zinc sulfate for fertil-izer), contained as much as 13,800 µg arsenic pergram of soil, and soil collected from a previoushazardous waste site in Jersey City, New Jersey,contained 1,120 µg per gram.14 Adults in Denmark,eating 376 grams (about 4 servings) of vegetablesper day grown in soil containing 30 µg of arsenicper gram of soil, consumed 5.3 µg of arsenicdaily.13 Adults living near point sources of arsenicexposure may have a constant daily total inorganicarsenic intake as high as 12 µg/kg body weight/day.15 Acute symptoms of arsenic poisoning – nau-sea, diarrhea, abdominal cramping, hyperesthesiain extremities, abnormal patellar reflexes, andabnormal electrocardiograms – have been esti-mated to occur at levels of exposure equal to 50µg/kg body weight/day.16

Arsenic exposure has been linked to car-diovascular disease and diabetes. In epidemiologi-cal studies in Bangladesh, where arsenic toxicityis endemic as a result of tube-well contamination,arsenic has been linked to the prevalence of hy-pertension.17 Exposure to 20 ppb or more in drink-ing water has been associated with increased mor-tality from cardiovascular diseases.18 Cumulativearsenic exposure has also been positively associ-ated with the incidence of type 2 diabetes in Tai-wan, another area where arsenic contamination ofwater is common.19




Chronic exposure is associatedwith anemia, peripheral neuropathy, liverand kidney damage, and irritation of theskin and mucous membranes. Peripheralvascular disease has also been seen inchronically exposed individuals.20

Chronic inhalation of inorganic arsenichas been shown to be strongly associatedwith the risk of human lung cancer.2 Be-cause inorganic arsenic binds to sulfhy-dryl proteins, specifically keratin, depos-its are left in skin, hair, and nails. Expo-sure to inorganic arsenic has been linkedto arsenical keratoses, squamous cellcarcinoma in situ of the skin, and basalcell carcinoma. Arsenic exposure hasalso been linked to hepatocellular carci-noma, angiosarcoma, cirrhosis, andhepatoportal sclerosis.21 While animalstudies have shown inorganic arsenic tobe fetotoxic and teratogenic, few studieshave looked at arsenic toxicity in preg-nant females. Ingested arsenic can crossthe placenta and result in cord blood con-centrations that resemble maternal bloodconcentrations.22 Arsenic, however, has not beendetected in measurable amounts in the breast milkof arsenic-exposed women.23

Chronic arsenic intoxication may presentas diffuse symptoms: headache, fatigue, confusion,polyneuritis with distal weakness, exfoliative der-matitis, hyperkeratosis (especially on the soles ofthe feet), hyperpigmentation, and Mees’ lines(transverse white striae of the fingernails). Ane-mia, leucopenia, slight proteinuria, and liver en-zyme abnormalities may also develop.2

Arsenic Methylation andDetoxification

Arsenic exists in both inorganic and or-ganic states. The organic forms that accumulatein fish and shellfish, arsenobetaine andarsenocholine, have been found to be essentiallynontoxic.24 The inorganic forms, airborne arsenictrioxide and arsenate/arsenite (found in soil, wa-ter, and food), are the forms of concern to humanhealth. Arsenic is well absorbed, 40-60 percent if

inhaled25 and approximately 95 percent if in-gested.26 Arsenic is distributed and stored in alltissues of the body and is metabolized for elimi-nation by two sequential processes (Figure 1). Thefirst are oxidation/reduction reactions thatinterconvert arsenate to arsenite. Glutathione hasbeen shown to form complexes with arsenic andmediate the reduction of arsenate to arsenite. Theseglutathione complexes can be eliminated in thebile and a positive correlation has been found be-tween glutathione and arsenic levels in bile.27 Se-lenium is also able to complex with glutathioneand arsenic to form a compound that is also ex-creted through the bile. Binding to unidentifiedproteins is another possible mechanism for arsenicdetoxification. These proteins appear to be pri-mary, along with glutathione and possibly sele-nium, in the removal of arsenic.28

The second step, methylation, whichoccurs mainly in the liver, requires s-adenosylmethionine (SAMe) and possibly othermethyl donors (choline, cysteine, glutathione,reduced lipoic acid) to produce monomethylarsinic

Figure 1. Proposed Detoxification Pathway ofInorganic Arsenic

Arsenate

GSH

GSSG

Arsenite

MMA

DMA

Urine

Bile GSH/AsGSH/As/Se

Possible methylationof arsenite to DMAvia methylcobalamin +GSH + Se

Methylation of arsenite toDMA via SAM andmethyltransferases

Key GSH = GlutathioneGSSG = Oxidized glutathioneSe = Selenium

SAM = S-AdenosylmethionineMMA = Monomethylarsinic acidDMA = Dimethylarsinic acid




acid (MMA) and dimethylarsinic acid (DMA).29

Several studies have shown that SAMe is actuallyessential for the methylation of arsenic and lowmethionine intakes can inhibit arsenic methylationin animals.30 Both MMA and DMA have beenfound in human urine and are considered end-products of arsenic metabolism. Because DMA iscleared from cells more rapidly than MMA orinorganic arsenic, and methylation reduces theamount of arsenic retained in tissues by increasingthe water solubility of arsenite, methylation isconsidered by some researchers to be adetoxification mechanism.31 Other researchersdisagree because MMA may be the most toxicintracellular form of arsenic due to its ability toinduce enzyme inhibition, oxidative stress, andDNA damage.32 Therefore, methylation maysimply be a way of biotransforming arsenic ratherthan detoxifying it.28

Dermal and pulmonary tissues are un-able to convert MMA to DMA as efficiently asother tissues, and both are the sites of specificarsenic-induced cancers. DMA is not a benignmetabolite either, and has been shown to pro-duce free radicals that may contribute to themechanisms of arsenic-related cancers.33 MMAand DMA have also been shown to complexwith glutathione and other sulfhydryl proteins,resulting in sulfhydryl-related enzyme inhibi-tion and cellular damage.31

The methylation of arsenic is a topic ofsignificant debate and interest in the toxicologyfield because the ability to methylate and elimi-nate arsenic is influenced by nutrition, gender,lifestyle, and individual genetic polymor-phisms.34 There appears to be significant indi-vidual variation in the ability to methylate ar-senic. Malnourished individuals exposed to highlevels of arsenic are less able to methylate itand are more at risk for arsenic toxicity symp-toms and diseases than well-nourished individu-als.35 In a study of an arsenic-exposed popula-tion, smoking more than 10 cigarettes daily hada stronger inhibitory effect on the ability to com-pletely methylate arsenic than gender, age, orethnicity.36 However, these factors only ac-counted for 20 percent of the variation in me-

thylation capacity. The amount of exposure ap-pears to be the most important factor affectingarsenic methylation; the higher the chronic expo-sure the lower the individual’s ability to methy-late MMA to DMA. The ability to transform MMAto DMA is significant; for example, the dermalsigns of arsenic exposure (including skin cancer)are related to a buildup in the body of MMA.31

Selenium and Arsenic ToxicityThe presence of selenium also affects ar-

senic toxicity. Animal research has established abidirectional effect of selenium and arsenic witheach metal preventing a toxic effect of the other.37

As mentioned, selenium is believed to bind to ar-senic to form an insoluble complex in the liver.28

Animal studies with injected sodium selenite (0.5mg/kg) increased the excretion of arsenite-sele-nium compounds in the bile and reduced hepatic

Figure 2. DMPS and/or Sodium Selenite-Stimulated Nonenzymatic Methylation ofArsenite by Methylcobalamin

500

400

300

200

100

0

600

Na Selenite(µg)

DMPS (mM)

0

0

0

5.1

4.9

0

4.9

5.1

MM

A F

orm

ed, n

g

Adapted from: Zakharyan RA, Aposhian HV. Arsenite methylation by methylvitamin B12 and glutathione does not require an enzyme. Toxicol Appl Pharmacol 1999;154:287-291.




arsenite concentrations.38 Rats given selenium-sufficient diets (0.2 ppm) and toxic doses of ar-senic were able to eliminate arsenate, arsenite, andDMA more quickly than rats on a selenium-defi-cient diet (0.02 ppm).39 The methylation of arseniccan also occur in vitro in the presence ofmethylcobalamin (methylated vitamin B12) andglutathione.40 This methylation reaction was in-creased with the addition of selenium (in the formof sodium selenite) or the chelating agent 2,3-dimercaptopropane sulfonate (DMPS). When bothDMPS and selenium were used, the amount ofMMA produced from inorganic arsenic was ap-proximately doubled (Figure 2). The authors ofthe paper suggest the vitamin B12, selenium, andmethionine (the essential component of SAMe)content of the diets of those exposed to inorganicarsenic be carefully considered as factors that as-sist in the elimination of arsenic.

Oxidative Stress in Arsenic ToxicityThe exact cellular mechanisms of

arsenic’s carcinogenicity are not completely un-derstood; however, it is believed to be a co-car-cinogen and tumor promoter rather than a tumorinitiator.31 The lack of direct evidence is, at leastin part, because there are no animal models forarsenic-induced carcinogenesis. Arsenic may bethe only agent that has been determined to be adefinite human carcinogen even though there isnot enough evidence to prove it is a carcinogen inanimals. Therefore, it is both possible and discon-certing that humans are actually more sensitive tothe toxic effects of arsenic than experimental ani-mals.

The four main areas of research on thecellular mechanisms of arsenic toxicity are: (1)mutation inductions and chromosomal aberrations;(2) altered signal transduction, cell-cycle control,cellular differentiation and apoptosis; (3) directdamage from oxidative stress; and (4) alterationsin gene expression.41 None of these mechanismsare exclusive, and oxidative stress has been shownto influence all of them, directly or indirectly.33

Arsenic-induced oxidative stress has beenshown to cause DNA damage through the produc-tion of superoxide and hydrogen peroxide radi-cals.42 This particular form of genotoxicity has

been linked to arsenic-related skin cancers. Oxi-dant-induced damage was found significantlymore frequently in biopsies of individuals withknown arsenic exposure and measurable arsenicin skin biopsies than in those with squamous-cellcarcinoma who had no known arsenic exposureand no measurable arsenic in skin biopsies (78percent versus 9 percent, respectively).43 In vitrostudies have found superoxide dismutase, catalase,dimethyl sulfoxide (DMSO), glutathione, N-acetylcysteine (NAC), and vitamin E can effec-tively block DNA mutations, prevent the produc-tion of high levels of superoxide, and protect fi-broblasts from arsenic-induced chromosomaldamage.44 These studies indicate oxidative dam-age-related mechanisms are involved in arsenicgenotoxicity.

Arsenic also has a direct toxic effect oncellular respiration in mitochondria.45 This toxiceffect on cellular respiration occurs because ar-senic binds to lipoic acid in the mitochondria andinhibits pyruvate dehydrogenase. The resultinguncoupling of mitochondrial oxidative phospho-rylation leads to increased production of hydro-gen peroxide. The resulting oxidative damage mayplay an important role in altering gene-expressionpatterns, another mechanism for arsenic-inducedcarcinogenesis.46 The uncoupling of oxidativephosphorylation, decrease in cellular respiration,and resulting increase in free radical productionalso lead to hepatotoxicity and porphyrinuria.These symptoms of arsenic toxicity are seen morecommonly with acute exposure but also occur withlow-dose chronic exposure.47

Evidence of oxidative stress has also beenmeasured in humans with arsenic exposure. Stud-ies in those with very high arsenic exposure fromgroundwater contamination (a mean of 410 µg/Lor 400 ppb) had serum lipid peroxide levels sig-nificantly higher (24 percent) than a control groupwhose drinking water had much less arsenic (20µg/L).48 The high exposure group also had a 57-percent reduction in whole blood glutathione lev-els compared to the lower exposure group. On thewhole, individual glutathione levels were inverselyrelated to both whole blood inorganic arsenic con-centrations and the presence of methylated formsof arsenic (MMA and DMA).




Another human study of northeastern Tai-wan residents confirmed these findings.49 Thecoast of northeastern Taiwan is an area of endemicarsenic toxicity where well water concentrationsvary from 0 to over 3,000 µg/L (3,000 ppb). Ar-senic whole blood concentrations in individualsfrom this area with high arsenic exposure havebeen positively associated with plasma oxidantlevels and negatively correlated with plasma anti-oxidant capacity. A correlation was also foundbetween lower levels of plasma antioxidants anda lowered ability to methylate inorganic arsenic.Taiwan residents living in arsenic-hyperendemicareas diagnosed with arsenic-related ischemicheart disease had significant decreases in serumalpha- and beta carotene.50

Arsenic, Antioxidants, and ChelatingAgents

Both dimercaptosuccinic acid (DMSA)51

and DMPS52 can be used as chelating agents inarsenic toxicity. Studies looking at the effects ofantioxidants used in conjunction with chelatingagents have investigated their role as potential aidsto chelators. A study evaluating chronic arsenic

intoxication (100 ppm in water for 12 weeks) inrats evaluated the ability of NAC and a chelatingagent, DMSA, to preserve hepatic and brain glu-tathione levels and to normalize erythrocyte en-zyme levels.53 Dosages of therapeutic agents weregiven orally to approximate those used in humantreatment: NAC and DMSA each at a dose of 1mmol/kg for five days. The combination treatmentsignificantly elevated reduced glutathione levelsin the liver and decreased levels of oxidized glu-tathione (Table 1). The simultaneous use of bothcompounds was significantly stronger than eitherindividually. NAC treatment alone decreased lev-els of hepatic malondialdehyde (the result of ar-senic-induced oxidant activity), while the effectof DMSA by itself was insignificant. Effects inthe brain were less apparent, with neither treat-ment alone or in combination able to affect a sig-nificant shift in the reduced glutathione/oxidizedglutathione ratio. Brain levels of malondialdehydewere significantly reduced, however. The abilityof arsenic to alter heme synthesis was also evalu-ated. Arsenic toxicity is known to interrupt hemo-globin synthesis and changes in the erythrocyteenzyme delta aminolevulinic acid dehydratase(ALAD) levels were measured to reflect this.

Table 1. Effects of N-acetylcysteine, meso 2,3-Dimercaptosuccinic Acid andtheir Combination

I. Control

II. Arsenic

III. Arsenic + NAC

IV. Arsenic + DMSA

V. Arsenic + NAC + DMSA

42.3 ± 1.0

21.6 ± 0.9

30.1 ± 2.5

29.2 ± 3.2

37.2 ± 1.0

1.40 ± 0.07

4.45 ± 0.34

2.97 ± 0.21

3.30 ± 0.10

2.17 ± 0.21

30

5

10

9

17

GroupGSH

(nmol/mg protein)GSSG

(nmol/mg protein)GSH/GSSG

Ratio

NAC, N-acetylcysteine; DMSA, meso 2,3-dimercaptosuccinic acid; GSH, glutathione; GSSG, oxidized GSH

Adapted from: Flora SJ. Arsenic-induced oxidative stress and its reversibility following combined administration of N-acetylcysteine and meso 2,3-dimercaptosuccinic acid in rats. Clin Exp Pharmacol Physiol 1999;26:865-869.




Blood ALAD activity was reduced 62 percent byarsenic exposure. DMSA alone or with NAC wasable to restore ALAD levels to those of controls(not exposed to arsenic); only the combination ofboth was able to restore RBC glutathione levels.The level of acute arsenic exposure in this study(100,000 ppb) was significantly higher than lev-els of chronic human exposure, even in hyperen-demic areas of Taiwan and West Bengal wheretube well contamination reaches 1,500-3,000ppb.54,55

Arsenic has been detected in humanplacental tissue and human fetal tissue. Neonatalbrain tissue has shown significant oxidativedamage when exposed to arsenic, at levels as lowas 50 ppb.56 Four groups of female rats were fedarsenic-contaminated water for the length ofgestation at concentrations of 300 ppb. The studycompared arsenic alone to arsenic plus vitamin E,arsenic plus vitamin C, and arsenic with thechelating agent DMSA added for two days at theend of gestation. When vitamin C and vitamin Ewere added, significant improvements occurred inlevels of lipid peroxidation and glutathione content(Table 2). The levels ofantioxidants used were small –vitamin C at 2.5 mg/kg/day andvitamin E at 148µg/kg/day(0.148 IU/kg/day). Vitamin E atthat level, however, was able toalmost completely restoreglutathione levels in braintissue and increase catalaselevels 100-percent higher thanin the control group. Becausearsenic-induced neuronaldamage has been shown to bedirectly related to lipidperoxidation,55 the role ofantioxidants in protecting bothadult and fetal nervous tissue isof increasing importance.

CadmiumCadmium is considered one of the most

toxic substances in the environment due to its widerange of organ toxicity and long elimination half-life of 10-30 years.57 Cadmium was identified asa contaminant at 776 of the 1,467 EPA NationalPriorities List sites in 1998.58 It has been estimatedthat at least 512,000 U.S. employees each yearwork in an environment that potentially exposesthem to cadmium.59 Cadmium-contaminatedtopsoil, however, is considered the most likelymechanism for the greatest human exposurethrough uptake into edible plants and tobacco.60

The EPA estimates approximately 3.4 billionpounds of sewage sludge are transferred to soilannually in the United States, estimated to containup to 1,000 µg/g cadmium.61 Fertilizer rawmaterials are also contaminated with cadmium;1.3 million pounds of cadmium-contaminated zincsulphate containing up to 215,000 ppm cadmiumentered the United States in November 1999. It isnot known how much of this product has been soldand applied to agricultural lands.62 Fertilizers

Table 2. Effects of Vitamins C and E and DMSA asProtective Agents

Arsenic

Arsenic + vitamin C

Arsenic + vitamin E

Arsenic + DMSA

131 ± 7

81 ± 5

64 ± 11

38 ± 14

77 ± 9

82 ± 11

98 ± 8

88 ± 12

Lipid peroxidation(∆OD/mg protein)

GSH content(µg/mg protein)

*

*

*

***

**

***

* value lower than control

** value same as control

*** p < 0.05 compared to arsenic onlyAdapted from: Chattopadhyay S, Bhaumik S, Purkayastha M, et al. Apoptosis and necrosis in developing brain cells due to arsenic toxicity and protection with antioxidants. Toxicol Lett 2002;136:65-76.




continue to be contaminated with cadmium as aresult of the recycling of industrial waste sold aszinc sulphate or other raw materials for agriculturaland home use fertilizers. Assays of commonly solddry fertilizer and soil amendment in WashingtonState in 1998 revealed concentrations of cadmiumas high as 160 mg/kg dry weight.63

Cadmium ExposureThe uptake of cadmium from the soil

through produce results in elevated concentrationsin vegetables, fruits, and grains, with the highestlevels in leafy greens and potatoes. High levelsare also found in shellfish (up to 30 mg/kg) andorgan meats.64 The current federal minimal risklevel (MRL) for cadmium – a level at whichchronic exposure in humans is not likely to causecancer or adverse health effects – is 0.2 µg/kg/day (14.0 µg for the average adult). The averageAmerican diet in 1986 provided 0.4 µg/kg/day ofcadmium.65 The overall range of dietary cadmiumin Swedish diets in 1994-1996 was 2.0-175 µg/day and is estimated to be increasing at a rate oftwo percent per year.57 The World Health Organi-zation has shown that dietary cadmium exposurehas a very wide range: inhabitants of worldwidenonpolluted areas have a daily dietary intake ofapproximately 40-100 µg, while inhabitants ofpolluted areas may obtain 200 µg or more as anaverage daily intake.57

Cadmium AbsorptionBetween 10-50 percent of cadmium fumes

are absorbed through the respiratory tract and ap-proximately five percent of oral cadmium is ab-sorbed through the digestive tract. Smokers ab-sorb 1-2 µg cadmium per pack of cigarettes, ap-proximately doubling the average exposure of anonsmoker and doubling the average amount ofcadmium found stored in the kidneys.57 Althoughabsorption through the gastrointestinal tract is sig-nificantly lower, low dietary intakes of calcium,protein, zinc, iron, and copper may increase cad-mium absorption in the gut.66

Iron Deficiency as a Risk FactorIron deficiency creates a significant risk

for increased cadmium exposure by increasinggastrointestinal absorption from five percent to asmuch as 20 percent.67 Individuals with a serumferritin less than 12 µg/L are considered to be highrisk for cadmium-induced kidney lesions.57

A study of 57 nonsmoking Swedishwomen found those with a serum ferritin of lessthan 20 µg/L (indicating reduced body iron stores)had significantly higher blood cadmium levelsthan those with a serum ferritin above 30 µg/L.68

The authors concluded that, since 44 percent ofthe women in the study had depleted bone mar-row stores (serum ferritin less than 15 µg/L) and23 percent had reduced body iron stores (serumferritin less than 30 µg/L), iron deficiency appearsto create a significant high-risk category for cad-mium-induced renal damage in Swedish women.This is consistent with data on the Swedish fe-male population in which 10-40 percent of womenare reported to have depleted iron stores (serumferritin less than 12 µg/L)57 and is probably thereason why women, in general, tend to have higherblood cadmium levels than men.

Iron deficiency is an international healthproblem, with an estimated incidence of two bil-lion, particularly young children and women ofreproductive age.69 Vitamin C, which has beenshown to significantly increase iron uptake, mayplay a role in protecting against increased cad-mium absorption. In a study of women in theUnited States, where the prevalence of low ironstores in females ages 12-49 is 10-19 percent, thosewho took vitamin C supplements had half the riskof low iron stores.69

Cadmium Metabolism andMechanisms of Toxicity

When cadmium is absorbed it circulatesin erythrocytes or bound to albumin. In the liver itcan induce and bind to metallothionein, a cysteine-rich protein that can concentrate cadmium up to3,000-fold.70 The metallothionein/cadmium com-plex is slowly released over time from the liverand circulates to the kidneys where it can accu-mulate in renal tissue. Cadmium also accumulates




in the bone, pancreas, adrenals, and placenta. Themajority of accumulation, approximately 50 per-cent of total body stores, occurs in the liver andkidney.71 The main pathologies related to chroniccadmium toxicity, renal disease and bone loss, arereflective of cadmium concentration in the kid-ney and the alteration of renal function that ulti-mately causes osteoporosis and osteomalacia.57

Acute exposure (acute occupational exposure iscommon in jewelry braziers and sodering) canmanifest as dysuria, polyuria, dyspnea, chest pain,irritability, fatigue, headache, and dizziness. Lev-els of urinary alpha1-microglobin or beta2-microglobin are often elevated in early cadmium-induced renal damage. A review of the symptomsof acute cadmium toxicity has been summarizedby Wittman et al.59

The mechanisms of cadmium toxicity arenot completely understood, but some of the cellu-lar effects are known. Fifty to sixty percent ofexposed populations have been shown to havechromosomal damage.72 Cadmium is known tobind to the mitochondria of the cell and is capableof inhibiting both cellular respiration (by 75%) andoxidative phosphorylation (by 100%) at low con-centrations. This mitochondrial toxicity can com-pletely inhibit the hydroxylation of vitamin D inrenal tissue at concentrations of 0.025 mmol.67

Some of the specific changes that lead totissue damage and death in chronic exposure havebeen related to oxidative stress and thiol deple-tion.33 Cellular damage results from cadmiumbinding to sulfhydryl groups in tissue, the produc-tion of lipid peroxides, and the depletion of glu-tathione. Cadmium also has a very high affinityfor glutathione and can form a complex with glu-tathione that is eliminated in bile. Cadmium alsoinhibits the activity of antioxidant enzymes, in-cluding catalase, manganese-superoxidedismutase, and copper/zinc-superoxidedismutase.73 Cadmium-induced lipid peroxidationhas been seen in animal studies in liver, kidney,brain, lung, heart, and testes.33 Cadmium can alsosubstitute for zinc or selenium inmetalloenzymes.71 Lowered levels of selenium aswell as lowered activity of glutathione peroxidase(a selenium-dependent enzyme) have been seen

in cadmium-exposed workers.74 Cadmium’s abil-ity to generate free radicals also leads to the ex-pression of inflammatory chemokines andcytokines,75 the oxidation of nucleic acids, the al-teration of DNA repair mechanisms, eventual celldeath, and the mutagenic changes involved in cad-mium-induced cancers.72

MetallothioneinMetallothionein is a zinc-concentrating

protein that contains 33-percent cysteine. Prima-rily induced and stored in the liver, it forms a com-plex with cadmium, sequestering it from insidethe hepatic cytosol, thus reducing the amount ofcadmium available to injure hepatocytes and pre-venting cadmium from depleting glutathionestores. Metallothionein has also been shown toprevent acute cadmium-induced hepatotoxicityand cell death in animal studies.70 Mice with ge-netically-induced high levels of hepaticmetallothionein and newborn animals with natu-rally high levels of metallothionein are resistantto cadmium-induced hepatotoxicity.76

Metallothionein also has free-radical scavengingproperties and is known to function like glu-tathione.77 The ability of metallothionein to scav-enge hydroxyl and superoxide radicals and func-tion like superoxide dismutase in microorganismshas been demonstrated.78

Metallothionein, although it appears toassist in cadmium detoxification and prevent cad-mium-induced damage, can also contribute to cad-mium-induced renal damage. Cadmium bound tometallothionein can leak into plasma, leave stor-age sites in the liver, and be taken up by the kid-ney. The cadmium-metallothionein complex isdissolved and free cadmium is released in the kid-ney and reabsorbed in the proximal tubules. Thesefree cadmium ions can again be bound by newlysynthesized metallothionein. If production of kid-ney metallothionein and non-metallothionein de-fense and detoxification systems (glutathione) arenot sufficient, free cadmium can damage cellularmembranes in the renal tubules.70 Mice that aregenetically unable to produce metallothionein aremuch more susceptible to renal injury and hepa-totoxicity resulting from long-term cadmium tox-icity than metallothionein-producing mice.70




Renal Damage in Cadmium ToxicityAn extensive review by Jarup et al in-

cludes an investigation of cadmium and renal dam-age.57 The highest load of cadmium is found inthe renal cortex. Renal concentrations in secondtrimester fetuses and infants compared to autopsystudies in adults show renal cadmium concentra-tion increases about 5,000 times from birth toadulthood.79 Studies of cortex concentrations havefound that women have significantly higher con-centrations than men, in spite of a higher malesmoking rate.

The average cadmium exposure leads tokidney concentrations of 20 µg/g for nonsmokersand 40 µg/g for smokers.80 At an average total in-take of 30 µg/day, it is estimated that renal tubu-lar damage occurs in one percent of the popula-tion. At an intake of 70 µg/day (the World HealthOrganization provisional tolerable weekly intake),seven percent of the adult population and up to 17percent of high-risk groups would be expected todevelop kidney lesions.57 A Belgian study exam-ining kidney cadmium and renal damage estimatesthat 10 percent of the Belgian population maycurrently have kidney cadmium concentrations of50 µg/g, resulting in early signs of renal damage,proteinuria, and calcium loss.81 In Japan, wherecadmium exposure through environmental con-tamination of food and water has led to outbreaksof cadmium toxicity-related disease, cadmium-induced tubular lesions have been identified inmore than 20,000 people. In Swedish studies, earlysigns of renal damage have appeared in those withurine cadmium levels of 0.5-2.0 µg/g creatinine,corresponding to renal cortex concentrations of10-40 mg/kg, levels found in 50 percent of theadult Swedish population.57 Glomerular damageand kidney stones have been seen in those withoccupational exposure to cadmium. Studies ofworkers with cadmium-induced renal damage es-timate 40-80 percent increased annual mortalityrisk as a result of cadmium exposure and renaldamage. Once cadmium-induced nephropathy isinitiated, it is accepted that it is irreversible.2

Cadmium and BoneThe cadmium content of human bone in

North America has increased by a factor of 50 inthe last 600 years. The majority of that increase isbelieved to have occurred in the last 100 years.82

Classic cadmium poisoning (known at itai-itai dis-ease in Japan) has been characterized by multiplefractures, osteomalacia, bone pain, and osteoporo-sis that occurs along with renal disease.83 Animalstudies indicate postmenopausal women may beat greater risk for cadmium-related bone loss andthat cadmium may increase bone loss in womenwith pre-existing postmenopausal osteoporosis.57

Epidemiological studies have found apositive correlation with elevated urinary cad-mium levels and increased urinary calcium lossand elevated serum alkaline phosphatase levels.84

Studies have also found correlations between cad-mium-induced renal tubular damage and bone loss.A study of 1,021 men and women, who had eitherworked at a factory or lived in a community inSweden where nickel-cadmium or lead batterieswere produced, evaluated the relationship of cad-mium and lead exposure to kidney and bone dis-ease.85 Those who were environmentally exposedand had the highest blood cadmium levels had afour-fold risk of tubular proteinemia. Older indi-viduals (over 60 years) in that group had a three-fold risk of significant bone loss (Z-score < -1)compared to a same-age group with no knowncadmium exposure. The Z score results from acomparison to the average bone density scores ofa group of similar-aged individuals. A score of lessthan 0 indicates bone loss greater than the aver-age of that same group.

The mechanisms behind cadmium andbone loss are related to renal tubular cell damagethat results in elevated levels of urinary calciumand lowered levels of 1,25 dihydroxy-cholecal-ciferol, a consistent finding in women environ-mentally exposed to significant levels of cad-mium.86 Lower levels of activated vitamin D3 al-ter calcium homeostasis by decreasing absorptionof calcium in the gut and altering deposition inbone.




Cadmium and Cancer, HeartDisease, and Reproduction

Cadmium is classified as a group I hu-man carcinogen, meaning sufficient evidence forcarcinogenesis has been found in both animals andhumans. Occupational and environmental expo-sure has been shown to increase risk for lung can-cer with co-exposure to arsenic,87 and renal can-cer with cadmium exposure alone.88 While ani-mal studies support a role for cadmium-inducedprostate cancer, inconsistent findings exist forcadmium’s role in human prostate, breast, testicu-lar, and bladder cancers.57

Cadmium appears to be completely fil-tered by the placenta when adequate zinc and cop-per are available for the induction ofmetallothionein. Studies with newborn rats revealnewborns whose cadmium-exposed mothers hadbeen given adequate zinc and copper during preg-nancy were cadmium-free at birth, as opposed tonewborns whose cadmium-exposed mothers hada zinc and copper-deficient diet.89 Maternal hy-pertension and low birth weight have been asso-ciated with elevated cadmium levels in infants.90

Environmental exposure to lead, cadmium, andarsenic in pregnant women has been correlatedwith increased levels of lipid peroxides, and theincidence of threatened spontaneous abortion, tox-emia, and anemia.91 Only lead and cadmium ex-posure correlated with decreased levels of reducedglutathione.

Risk for hypertension and cardiovasculardisease in nonpregnant women and in men is notconclusively a result of cadmium exposure. Stud-ies have found both increased risk for cardiovas-cular mortality in one exposed group92 and no in-creased risk for ischemic heart disease or hyper-tension in another large study.93

Antioxidants in Cadmium ToxicityZinc and Metallothionein Induction as aProtective Mechanism

Metallothionein production is induced bythe presence of metals, including cadmium, mer-cury, copper, gold, bismuth, and most powerfully,zinc.94 Low level zinc treatments have been usedin animal studies to induce metallothionein andprotect against acute cadmium-induced hepatotox-icity.95 Similarly, hepatocyte cell lines treated withzinc became resistant to cadmium-induced celldeath as a result of metallothionein induction.96

In animals, both hepatic and intestinalmetallothionein have been induced using oral zinc,and metallothionein induction using nontoxic zincinjections has been successful in reducing cad-mium toxicity in animals.97 The induction of in-testinal metallothionein in humans, using zinc ac-etate, is the mechanism for the FDA-approvedtreatment of Wilson’s disease, an inherited condi-tion where accumulation of copper in the liver,brain, and other organs leads to copper toxicosis.98

The mechanisms of cadmium-inducedrenal damage result from the dissolution of thecadmium/metallothionein complex in the kidney,exposing renal tissue to unbound cadmium. Cad-mium/cell membrane binding, cellular apoptosisof renal proximal tubules, increased calcium lossin the urine, and increased protein excretion areseen in animals given long-term doses of cadmiumor repeated doses of cadmium/metallothioneincomplexes. Studies have also shown when the kid-ney is able to induce adequate de novo synthesisof metallothionein, no membrane damage occurs.70

Zinc has been used to induce renalmetallothionein in animal studies and protectsagainst cadmium/metallothionein-induced renalinjury.99 Rats pretreated with zinc or copper haveshown less sensitivity to cadmium toxicity, spe-cifically in renal proximal tubule cells. Proteinuriacaused by cadmium-metallothionein injectionswas more effectively reduced by pretreatment in-jections with zinc than with copper.100 Althoughthere have been no human clinical trials with zincor copper to assess metallothionein induction, zincacetate, used to stimulate intestinal




metallothionein in the treatment of Wilson’s dis-ease, is nontoxic in 150 mg daily doses and hasminimal side effects.98 In those without Wilson’sdisease, the possibility of inducing a copper defi-ciency with high doses of zinc is preventable withcopper supplementation.

Alpha-Lipoic AcidAlpha-lipoic acid (ALA), injected in cad-

mium-exposed murine hepatocytes, was shown toprotect cells from toxic effects of cadmium, in-cluding hepatocyte membrane damage, lipidperoxidation, and depletion of intracellular glu-tathione.101 These protective effects have also beenseen in rats who had experimentally-depleted glu-tathione stores prior to cadmium exposure.102 Al-though the acute toxicity induced in these studies(150 µM or about 17 mg cadmium) is vastly dif-ferent than low level chronic exposure in humans,oxidant stress and glutathione depletion are alsorecognized toxic mechanisms of low level expo-sure.103,104 The authors of the first study concludedthat dihydrolipoic acid (the reduced form of al-pha-lipoic acid) is an effective extra- and intra-cellular chelator of cadmium in hepatocytes as a

result of a significant decrease in in-tra- and extracellular levels of cad-mium after ALA was added to thecells.101 The authors measured bothintra- and extracellular cadmium/lipoate and cadmium/dihydrolipoatecomplexes to conclude that cadmiumwas actually being removed from thehepatocytes by lipoic acid com-pounds themselves and not glu-tathione generated by lipoic acid.They also noted, however, that theseeffects occurred only at low levels ofcadmium exposure and high levels oflipoic acid concentration.101

SeleniumThe theory that selenium and

cadmium can form complexes hasbeen substantiated by researchers inanimal studies with concomitant se-lenium and cadmium exposure.105,106

In a study with acute cadmium toxicity (8 mg/kgoral cadmium) and concomitant oral seleniumsupplementation (350 µg/kg sodium selenite), ratswho received both had a 25-percent reduction inkidney cadmium. The ability of selenium to de-crease the tissue burden of cadmium has been re-peated in other animal studies.105 Acute toxicitystudies have found that, as a result of dosing sele-nium and cadmium at the same time, organ tissuelevels of both metals increased and the toxic bur-den of cadmium decreased, possibly as a result ofthe inert nature of the cadmium/selenium com-plex.107 A low-level cadmium exposure study inmice (1 ng/L drinking water) with a varied sele-nium diet revealed significant differences in cad-mium retention.108 In mice that received a normalselenomethionine/sodium selenite diet (99.25 µgselenomethionine and 68 µg sodium selenite/kgfood) the whole body retention of cadmium wasless than half of the retention in mice on a lowselenium diet (31.25 µg selenomethionine/kgfood) (Table 3). For comparison, the averageAmerican gets approximately 65 µg of seleniumper day through diet and supplementation.

Table 3. Effects of Dietary Selenium Levels on WholeBody Retention of Cadmium Chloride

Whole-body retention

Liver

Kidneys

14.4

0.70

0.72

6.4*

0.48

0.52

Low selenium diet,cadmium in drinking water

Normal selenium diet, cadmium in drinking water

* p < 0.05 compared to low Se diet in the same experiment.

Adapted from: Andersen O, Nielsen JB. Effects of simultaneous low-level dietary supplementation with inorganic and organic selenium on whole-body, blood and organ levels of toxic metals in mice. Environ Health Perspect 1994;102:321-324.




Selenium supplementation has a knownantioxidant action in cadmium toxicity. Seleniumsupplementation in acute cadmium toxicity hasbeen shown to decrease lipid peroxidation in ratstudies109 and has also been shown to increase theproduction of glutathione S-transferase and glu-tathione peroxidase in rhesus monkeys.106 Thedosage of selenium in the primate study was farbeyond what would be used in human trials, 500µg/kg body weight, but the cadmium exposure wasalso elevated beyond possible environmental oroccupational human exposure to 5 mg/kg bodyweight/day. Similar results – elevations of glu-tathione peroxidase and decreased whole body andrenal burden of cadmium – were found in ratsgiven daily selenium supplementation of 350 µg/kg body weight.105 Selenium also appears to act inconjunction with other antioxidants. When sele-nium was given to rats simultaneously with vita-min E and glutathione, the cadmium uptake in liver

and kidneys was significantly inhibited.110 Sele-nium has also been shown to decrease lipidperoxidation in testicular tissue of rats,111 an ef-fect relevant to human health due to the correla-tion of blood cadmium levels in men with de-creases in sperm motility and alterations in spermmorphology.112

Plant TriterpenesTriterpenes are common plant compounds

shown to have antioxidant,113 hepatoprotective,114

anti-inflammatory,115 and antitumor116,117 proper-ties. Triterpenoids also induce metallothionein incadmium toxicity. Oleanolic acid, a triterpenoidpresent in many plants and one of the active con-stituents of Ligustrum lucidum, is used in Chinato treat hepatitis. It has also been shown to inducehepatic metallothionein in cadmium toxicity.118

Doses of 100 mg/kg of oleanolic acid were givento mice for three days prior to cadmium injections

Table 4. Effect of Chelator and/or Methionine on Tissue Cadmium Levels

Normal-control

Cd (control)

CaNa3DTPA

DMPS

Methionine

CaNa3DTPA+ Methionine

DMPS + Methionine

1.67 ± 0.19

43.47 ± 1.77a

31.81 ± 1.83c

24.60 ± 1.29c

19.77 ± 2.54c

15.20 ± 2.76c**

13.76 ± 2.60c*

0.55 ± 0.12

47.68 ± 3.72a

29.24 ± 4.13c

22.36 ± 1.20c

17.06 ± 2.68c

12.74 ± 3.22c***

7.79 ± 2.62c*

0.05 ± 0.02

2.59 ± 0.45a

1.54 ± 0.35c

1.75 ± 0.44c

1.42 ± 0.28c

1.47 ± 0.46c

1.04 ± 0.07c***

Treatment Liver(µg/g fresh tissue)

Kidney(µg/g fresh tissue)

Brain(µg/g fresh tissue)

Values are mean (± S.D.; n=6).

* p < 0.001; ** p < 0.01; *** p < 0.05 versus CaNa3 DTPA or methionine/DMPS or methionine at 5% level of significance (ANOVA).

The Cd removal in % control, has been given in parenthesis.a p < 0.001; b p < 0.05 versus normal-control (Student's t-test); c p < 0.01 versus Cd (control).

Adapted from: Tandon SK, Singh S, Prasad S. Influence of methionine administration during chelation of cadmium by CaNa3DTPA and DMPS in the rat. Environ Toxicol Pharmacol 1997;3:159-165.




Tabl

e 5.

Inf

luen

ce o

f Cys

tein

e or

N-a

cety

lcys

tein

e on

the

Effi

cacy

of D

MP

S

Cd

(con

trol

)

Cd

-cys

tein

e

Cd

-N-a

cety

lcys

tein

e

Cd

-DM

PS

Cd

-DM

PS

+ c

yste

ine

Cd

-DM

PS

+ N

-ace

tylc

yste

ine

Nor

mal

Ani

mal

80.2

4 ±

10.5

5

37.4

1 ±

3.43

***

55.3

6 ±

3.36

***

39.7

9 ±

5.82

***

42.8

5 ±

4.96

***

37.1

3 ±

3.86

***

ND

WT

72.4

6 ±

11.9

5

31.2

9 ±

6.85

***

49.2

8 ±

5.35

***

32.1

9 ±

5.95

***

37.6

8 ±

3.45

***

33.9

9 ±

3.28

***

ND

SC

F

6.83

± 1

.69

6.01

± 2.

03

5.95

± 0

.79

6.61

± 0

.54

4.60

± 0

.69*

*✝✝

2.16

± 0

.74*

**✝✝✝

ND

NM

F

57.9

3 ±

7.87

33.8

3 ±

3.90

***

31.8

2 ±

4.64

***

27.0

8 ±

1.52

***

48.9

0 ±

4.89

*✝✝✝

45.7

2 ±

5.01

***✝✝

✝

ND

WT

48.5

7 ±

9.02

24.1

3 ±

6.83

***

24.3

4 ±

5.46

***

19.6

2 ±

2.04

***

43.2

7 ±

4.50

***✝✝

✝

41.2

8 ±

5.85

***✝✝

✝

ND

SC

F

8.80

± 1

.87

9.38

± 1

.47

7.55

± 2

.16

5.52

± 0

.80*

**

5.38

± 1

.10*

**

5.21

± 0

.33*

**

ND

NM

F

Live

r (µ

g g-1

fres

h tis

sue)

Kid

ney

(µg

g-1 fr

esh

tissu

e)Tr

eatm

ent

a Va

lues

are

mea

ns ±

SD

(n=

6); W

T, w

hole

tis

sue;

SC

F, s

uper

nata

nt c

ytos

ol fr

actio

n; N

MF,

nuc

lear

mito

chon

dria

l fra

ctio

n; N

D, n

ot d

etec

ted

;

* p

< 0

.05,

***

p <

0.0

1 an

d *

** p

< 0

.001

vs

Cd

-con

trol

and

✝✝ p

< 0

.01

and

✝✝✝

p <

0.0

01 v

s C

d +

DM

PS

at

5% le

vel o

f sig

nific

ance

(AN

OVA

).

Ad

apte

d fr

om: T

and

on S

K, P

rasa

d S

, Sin

gh S

. Che

latio

n in

met

al in

toxi

catio

n: in

fluen

ce o

f cys

tein

e or

N-A

cety

l cys

tein

e on

the

effi

cacy

of 2

,3-d

imer

cap

top

rop

ane-

1-su

lpho

nate

in t

he t

reat

men

t of

cad

miu

m t

oxic

ity. J

Ap

pl T

oxic

ol 2

002;

22:6

7-71

.




in doses known to induce acute liver injury.Oleanolic acid resulted in a 30-fold increase inhepatic metallothionein and a significant in-crease in the mobilization of cadmium, prevent-ing cadmium-binding to intracellular proteins.Liver injury was also significantly reduced, asindicated by reductions in ALT and sorbitol de-hydrogenase levels.

Specific triterpenoids, includingoleanolic acid, betulinic acid, ursolic acid,soyasapogenol A and B (all present inGlycyrrhiza glabra and Betula alba L.), uvaol(present in Betula alba and Syzgium sp.), andglycyrrhizin have been found to be effectivein reducing the hepatotoxicity of cadmium.117

Betulin, present in high levels in white birchbark (Betula alba L.), was found to have thestrongest ability to reduce the cytotoxicity ofcadmium-poisoned hepatocytes and com-pletely prevented toxicity at doses as minimalas 0.1 µg/mL.119 The mechanism of betulinappears to be a gene-promoting effect in hepa-tocytes that eliminates the toxic effects of cad-mium.

GlycyrrhizinGlycyrrhizin, a triterpenoid saponin, is

known to act in hepatic tissues as an antioxidantby reducing lipid peroxidation.120 The Japanesedrug, Stronger Neo-Minophagen C (SNMC),which contains glycyrrhizin, glycine, and cysteine,has also been shown to protect against acutecadmium toxicity-related hepatic damage andrenal damage in animal studies.104,121 The dosageof the glycyrrhizin compound used in these studieswas small – 2 mg glycyrrhizin, 20 mg glycine,and 1 mg cysteine/kg – yet was able to reverse thenephrotoxicity brought on by 19 weeks of dailyhigh-level cadmium injections. As a result of theSNMC studies, researchers investigated the effectof glycine to differentiate therapeutic effects ofglycine from those of glycyrrhizin. Studies withglycine (12 mmol/L hepatic perfusion) alone foundit could prevent the decrease in bile flow causedby cadmium, but glycine was unable to reversethe 30-90 percent decrease in hepatic glutathionecaused by cadmium.122

MelatoninMelatonin, a known antioxidant, has been

studied as a preventive agent in cadmium-inducedlipid peroxidation. Pretreatment by single injec-tion of 15 mg/kg body weight in hamsters com-pletely prevented lipid peroxidation in the brainand the kidney induced by cadmium injection.123

Antioxidants and Chelating AgentsCadmium is known to bind tightly to

metallothionein in complexes stored intracellularlyin the liver and kidney.124 Because standard chelat-ing agents do not work intracellularly, manysources state there is no clinically effective treat-ment for cadmium poisoning.2,59 While DMSA iseffective for both lead and mercury toxicity, it isnot an intracellular chelator. DMPS has limitedability to enter the cell and chelate cadmium.124

Table 6. Influence of Zinc Supplementationduring Chelation Treatment on CadmiumConcentration in Liver and Kidneys

Cd (control)

Cd + DTPA

Cd + DMSA

Cd + Zn

Cd + Zn + DTPA

Cd + Zn + DMSA

Normal Animal

48.2 ± 4.14b

38.2 ± 3.59

42.7 ± 2.89

50.1 ± 3.08

32.7 ± 1.82c

43.4 ± 2.14

71.4 ± 3.94b

54.5 ± 2.30c

66.2 ± 3.94

78.4 ± 3.28

35.8 ± 1.76d

64.2 ± 3.37

Liver (µg g-1)

0.02 ± 0.004

Kidneys (µg g-1)

0.016 ± 0.004

a values are mean ± SEM (n=6)b p < 0.001 compared to normal animalsc p < 0.01 compared to Cd-exposed controld p < 0.001 compared to Cd-exposed control

Adapted from: Flora SJ, Gubrelay U, Kannan GM, Mathur R. Effects of zinc supplementation during chelating agent administration in cadmium intoxication in rats. J Appl Toxicol 1998;18:357-362.




Diethylenetriaminepentaacetate (DTPA), a chelat-ing agent used to chelate uranium isotopes, bindstightly to cadmium.125 Prior animal studies with

lead toxicity have shown me-thionine (a glutathione precur-sor), used in conjunction withchelating agents, increased leadelimination.126 Two studies havefollowed looking at the co-ad-ministration of DMPS andDTPA using the glutathione pre-cursors methionine, cysteine,and NAC in cadmium chela-tion.125,127 In the first study, ratspre-exposed to cadmium weregiven a combination of oral me-thionine and injections of eitherDMPS or DTPA as chelatingagents.125 After three days of me-thionine/chelation treatment thecadmium content of organ tissuewas compared to rats receivingeither chelating agent or me-thionine alone, or controls whohad received only cadmium andcontrols who had not receivedany treatment (Table 4). DMPSplus methionine was signifi-cantly more effective in remov-ing cadmium from the liver, kid-ney, and brain than DTPA plusmethionine or any of the treat-ments alone.

A second study evalu-ated a three-day oral dosing ofthe antioxidants cysteine andNAC with oral DMPS in acutecadmium-exposed rats.127 Be-cause DMPS is 60-percentorally bioavailable, it was ad-ministered by mouth. The com-bination treatments were moreeffective than any agent alone atmobilizing cadmium from bodystores and delivering it to the

kidney (Table 5), indicated by the elevated renalcadmium levels in the rats that received combina-tion treatment. The combinations were also sig-nificantly more effective at removing cadmium

Tabl

e 7.

Inf

luen

ce o

f Zin

c Su

pple

men

tatio

n du

ring

Che

latio

n Tr

eatm

ent o

n th

e L

evel

s of

Bio

chem

ical

Var

iabl

es in

Liv

er o

f Cad

miu

m-e

xpos

ed R

ats

Cd

(Con

trol

)

Cd

+ D

TPA

Cd

+ D

MS

A

Cd

+ Z

n

Cd

+ Z

n +

DTP

A

Cd

+ Z

n +

DM

SA

Nor

mal

Ani

mal

25.7

± 1

.98

32.8

± 0

.07b

26.2

± 1

.03*

**

39.2

± 2

.60*

*

29.5

± 0

.76

20.5

± 0

.81*

*

29.5

± 2

.50

AS

T

31.6

± 4

.25

44.8

± 1

.60c

29.1

± 2

.18*

**

47.3

± 6

.06

36.4

± 2

.50*

32.5

± 3

.12*

39.4

± 0

.59*

*

ALT

4.0

± 0.

85

8.5

± 0.

65b

4.8

± 0.

48**

7.7

± 0.

75

5.0

± 0.

47**

3.8

± 78

.0**

2.4

± 0.

08**

*

ALP

6.52

± 0

.32

8.29

± 0

.21d

6.49

± 1

.13

9.31

± 0

.45

8.17

± 1

.23

9.97

± 1

.25

7.75

± 0

.74

γ-G

T

26.3

± 1

.72

28.0

± 0

.44

29.0

± 2

.24

22.7

± 1

.61

33.5

± 0

.61

29.4

± 5

.45

29.0

8 ±

2.11

Ure

a

1.95

± 0

.52

2.41

± 0

.58

1.97

± 0

.41

1.83

± 0

.52

2.03

± 0

.87

1.92

± 0

.37

1.86

± 0

.26

Cre

atin

ine

a Va

lues

are

mea

ns ±

SE

M(n

=6)

; * p

< 0

.05

and

**

p <

0.0

1 an

d *

** p

< 0

.001

com

par

ed t

o C

d-e

xpos

ed c

ontr

ol.

b p

< 0

.001

com

par

ed t

o no

rmal

ani

mal

s.c

P <

0.0

5 co

mp

ared

to

norm

al a

nim

als.

Ad

apte

d fr

om: F

lora

SJ,

Gub

rela

y U

, Kan

nan

GM

, Mat

hur

R. E

ffect

s of

zin

c su

pp

lem

enta

tion

dur

ing

chel

atin

g ag

ent

adm

inis

trat

ion

in

cad

miu

m in

toxi

catio

n in

rat

s. J

Ap

pl T

oxic

ol 1

998;

18:3

57-3

62.




from the intracellular hepatic compartments.DMPS/NAC was significantly more effective (p<0.001) than the DMPS/cysteine (p<0.01) combi-nation. The authors credited the glutathione pre-cursor status of cysteine and NAC in their abilityto decrease cadmium levels in the nuclear frac-tions of hepatic tissue compared to chelation alone.They also theorized that both cysteine and NACwere able to induce metallothionein, explaining

the significant elevations in renal and hepaticmetallothionein in rats receiving combinationtreatment as opposed to those receiving DMPSalone.

Zinc has also been given in combinationwith either DMSA or DTPA to rats pre-exposedto low levels of cadmium (10 ppm/liter water).128

Zinc sulphate (20 mg/100 g body weight) was

Table 8. Summary of Nutrients and their Effects on Arsenic and Cadmium

Necessary for As methylation30

Forms insoluble complexes with As28,37-39

In vitro with GSH and methylcobalamin; supports As methylation40

Forms inert complexes with Cd and decreases toxicity105,106

� lipid peroxidation,109,110 � glutathione-recycling enzymes106

� tissue retention of Cd108

+ GSH and vitamin E results in � Cd uptake in liver110

+ DMSA � hepatic GSH and normalizes RBC GSH in As toxicity53

+ DMPS � hepatic metallothionein and � Cd chelation from intracellular hepatic stores127

Hepatic protection from Cd-induced damage101

Mobilizes Cd by forming ALA-Cd complex101

Increases metallothionein levels in liver, kidney, intestines93

+ DTPA � hepatic Cd stores128

� hepatic metallothionein and mobilizes Cd118

� hepatoxicity of Cd117

Prevents hepatic and renal damage from Cd toxicity104,121

Prevents lipid peroxidation in acute Cd toxicity123

+ DMPS improves ability of DMPS to chelate Cd from liver and kidneys125

+ DMPS � hepatic metallothionein and � Cd chelation from intracellular hepatic stores127

SAMe

Selenium

NAC

Lipoic acid

Zinc

Oleanolic acid

Betulin

Glycyrrhizin

Melatonin

Methionine

Cysteine




given with either chelating agent for two five-dayperiods, with a seven-day rest period in between(to prevent side effects from the chelating agents).Only DTPA and zinc had any significant lower-ing effect on liver concentrations of cadmium, andonly DTPA alone or DTPA with zinc had any ef-fect on renal cadmium (Table 6). Zinc supplemen-tation alone, however, was able to normalize se-rum AST and ALT levels, reflecting cadmium-in-duced hepatic damage (Table 7). And zinc, addedto DMSA, was able to significantly reduce serumAST and ALT levels compared to DMSA alone.Zinc alone resulted in a significant increase in bothhepatic and renal metallothionein levels. The au-thors suggest from the results of this study thatzinc-induced metallothionein is capable of bind-ing cadmium and reducing cadmium toxicity, andthat zinc aids in the mobilization of cadmium fromintracellular storage depots.

ConclusionArsenic and cadmium are ubiquitous and

dangerous environmental toxins. Arsenic ingroundwater and concentrated in soil and food isa Group A human carcinogen. Exposure can causea variety of cancers, most commonlynonmelanoma skin cancers, and chronic toxicitymay manifest as diffuse symptoms not easily rec-ognizable as chronic heavy metal toxicity. Arsenicis metabolized via a methylation sequence thatuses glutathione and SAMe, eventually beingeliminated through the intestines and kidneys.Recent research suggests the end products of me-thylation are also carcinogenic. Methylation isaided by methylcobalamin and possibly selenium,which has long been known to both aid in arsenicelimination and bind arsenic in a nontoxic sele-nium-arsenic complex.

Oxidant stress and lipid peroxidation arewell-defined mechanisms of arsenic toxicity re-lated to arsenic-induced skin cancers and the re-duction of whole-blood glutathione stores in hu-mans with environmental exposure.

In vivo studies with animals and fetal cellcultures exposed to arsenic have shown that anti-oxidants, particularly NAC, vitamin E, and vita-min C, given in conjunction with a chelating agent(DMSA), have been able to restore glutathione

levels and reduce damage secondary to oxidativestress.

Cadmium, through contaminated soil,food, and tobacco smoking, may have significantlyincreased toxicity in those with iron, zinc, or cal-cium deficiency due to increased gastrointestinalabsorption and increased calcium loss. Chroniccadmium toxicity has been linked to lung and kid-ney cancers, irreversible renal damage, osteoporo-sis, and osteomalacia. Cadmium is stored prima-rily in the kidneys and liver, tightly bound tometallothionein in an intracellular complex.Metallothionein has protective effects on cadmiumtoxicity, but may also facilitate renal damage if itis not produced in renal tissues in sufficient quan-tities. Zinc induces hepatic and renalmetallothionein, and has been shown to protectboth organs from cadmium-induced damage. Li-poic acid, selenium, naturally occurringtriterpenoid compounds, and melatonin have beenshown to inhibit oxidant production secondary tocadmium exposure and to mitigate cadmium tox-icity in animal studies. NAC, methionine, cysteine,and zinc improve the efficacy of the chelatingagents DMPS and DMSA by allowing removal ofcadmium from intracellular stores and raisingmetallothionein levels.

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