Xenobiotic-FKBiotransformation

8/10/2019 Xenobiotic-FKBiotransformation

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Biotransformasi

Metabolisme Xenobiotik


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Xenobiotic metabolism


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PRINSIP DASAR XENOBIOTIK DIBAGI :

1.Golongan OBAT-OBATAN:

anti biotik, anti piretik/analgetik, suplemen, obat

jantung, dll


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2. KARSINOGEN:

Zat pewarna makanan, penyedap,nitrosamin, alkohol, pemanis (sakarin), dll


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3. BAHAN KIMIA/ POLUTAN LINGKUNGAN :

Jenis pencemaran ini ada > dari 200.000 jenis


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Contoh bahan kimia yang terkandung di dalam rokok


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KANKER

OBATBAHAN KIMIA

DARI MANAMEREKA DATANG?


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Metabolisme Xenobiotik

Metabolisme Xenobiotik adalah satu set jalurmetabolisme yang memodifikasi struktur kimia dari

senyawa asing ( obat2-an & racun) menjadi senyawa

normal

Jalur ini dikenal juga dengan nama bio transformasi(biotransformation)

Proses reaksi ini sering menyebabkan detoksifikasi

senyawa racun, tetapi beberapa kasus hasil senyawaantara (the intermediates in xenobiotic metabolism) dapat

menyebabkan efek toksik.


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Absorpsi senyawa xenobiotik

Xenobiotik Darah


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Absorpsi

xenobiotikXenobiotik

dalam darah

Metabolit

Urina

Ekskresi

lain (cairan)

JaringanDistribusi

jaringan lain

Xenobiotik


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Nasib senyawa xenobiotik

Xenobiotik

Metabolit racun /

toksik

Metabolit aktif

Metabolit inaktif

Metabolik yang

reversibel

Racun/toksik

Aktivitas

berubah

Aktivitas

meningkat

Kehilangan

aktivitas

Aktivitas

memanjang


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Xenobiotic metabolism is divided into three phases.

In phase I, enzymes such as cytochrome P450oxidases introduce reactive or polar groups into

xenobiotics.

These modified compounds are then conjugated topolar compounds in phase II reactions. These

reactions are catalysed by transferase enzymes such

as glutathione S-transferases.

Finally, in phase III, the conjugated xenobiotics maybe further processed, before being recognised by

efflux transporters and pumped out of cells.


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Phase I reactions

OxidationHydroxylation (addition of -OH group)

N- and O- Dealkylation (removal of -CH side chains)

Deamination (removal of -NH side chains)

Epoxidation (formation of epoxides)

Oxygen addition (sulfoxidation, N-oxidation)

Hydrogen removal

Reduction

Hydrogen addition (unsaturated bonds to saturated)

Donor molecules include GSH, FAD, NAD(P)H

Oxygen removal

Hydrolysis

Splitting of C-N-C (amide) and C-O-C (ester) bonds

O

C C

OC

See also Chapter 6 of Casarett and Doulls Toxicology

Table 6.1

epoxide


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Phases of detoxification

The metabolism of xenobiotics is oftendivided into three phases:

modification, conjugation, and

excretion. These reactions act in

concert to detoxify xenobiotics and

remove them from cells.


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Sitokrom P450 mitokondriaflavoprotein NADPH-,

adrenodoxin reduktase,

zat besi nonheme-sulfur protein,

adrenodoxin


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Contoh lain senyawa yang berfungsi induksibel:

Obat-obatan : barbital, etanol, steroid dan nikotin

Kimia industri :alkohol, khlordan, DDT dan khloroform

Hidrokarbon poliaromatik (polyaromatic hydrocarbon)

:benzopyrenes,dibenzanthrazena


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Biotransformation

Potentially toxic xenobiotic

Inactive metabolite

Relatively harmless

Reactive intermediate

DetoxificationMetabolicactivation


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Converting lipophilic to water

soluble compounds

Xenobiotic

Reactive intermediate

Conjugate

Phase I - Activation

Phase II - Conjugation

Excretion

Lipophilic

(non-polar)

Water soluble

(polar)


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Biotransformation

Water soluble xenobiotics are easier toeliminate ( t1/2)

Urine, feces but not exhalation

If within barrier, no out Multiple enzymes (families)

Constitutively expressed

Inducible

Broad specificity Polymorphic (allelic variants)

Stereo-isomer specificity: 6-OH in hormones:

CYP2A1 6-OH

CYP3A 6-OH


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Cytochrome P450 (CYP)

Mixed Function Oxidases (MFO) Located in many tissues but highly in liver ER

Human: 16 gene families

CYP 1,2,3 perform drug metabolism

>48 genes sequenced

Key forms: CYP1A2, CYP2C9, CYP2C19, CYP2D6,CYP2E1, and CYP3A4

Highly inducible

Alcohol CYP2E1 Dioxin/PCBs CYP1A

Barbiturates CYP2B

CYP genes have multiple alleles (2D6 has 53, and 2E1 has13)


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Metabolic enzymes

1. Microsomal:1. CYP450 monooxygenases

2. Flavin monooxygenase

2. Non-microsomal1. Alcohol dehydrogenase

2. Aldehyde dehydrogenase

3. Monoamine and diamine oxidases

3. Both

1. Esterases and Amidases

2. Prostaglandin synthase

3. Peroxidases


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Glucuronidation of phenol


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Sulfation of phenol and toluene


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Glutathione

-glutamyl-cysteinyl-glycine

Active site of a GST:


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GSH conjugation of

acetaminophen


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Cooxidation of

acetaminophenby prostaglandin

endoperoxide

synthetase

Compare to fig. 6-2

Benzene trasformation to


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leukemia-causing metabolite


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Phase III - further modification

and excretionAfter phase II reactions, the xenobiotic conjugates may be further metabolised. A

common example is the processing of glutathione conjugates to acetylcysteine

(mercapturic acid) conjugates.[7] Here, the -glutamate and glycine residues in theglutathione molecule are removed by Gamma-glutamyl transpeptidase and dipeptidases.

In the final step, the cystine residue in the conjugate is acetylated.

Conjugates and their metabolites can be excreted from cells in phase III of their

metabolism, with the anionic groups acting as affinity tags for a variety of membrane

transporters of the multidrug resistance protein (MRP) family.[8] These proteins are

members of the family of ATP-binding cassette transporters and can catalyse the ATP-

dependent transport of a huge variety of hydrophobic anions,[9] and thus act to remove

phase II products to the extracellular medium, where they may be further metabolised

or excreted.[10]


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Toxication and Detoxication: Must consider both

parent and metabolites:

Metabolism may either decrease or increase toxicity.Toxicants can be

formed in a variety of ways:

A) Parent compound toxic. Metabolites non-toxic.

B) Parent compound non-toxic. Metabolites toxic.


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Toxic metabolites formed in situ

Toxic metabolites formed in liver and transported

e.g., Parathion (nerve poison):

ABSORPTION/ELIMINATION


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ABSORPTION/ELIMINATION:

CHARACTERISTICS OF XENOBIOTICS?Lipophilic

Penetrate membranes by diffusion

Transported by lipoproteins in blood

HOW ELIMINATE?

Diffusion (limitations?)

Excrete (limitations?)

MAKE IT WATER SOLUBLE!!!!!!


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Permeability barriers and

detoxification That the exact compounds an organism is exposed to will be largely

unpredictable, and may differ widely over time, is a major

characteristic of xenobiotic toxic stress.[1] The major challenge faced

by xenobiotic detoxification systems is that they must be able to

remove the almost-limitless number of xenobiotic compounds from thecomplex mixture of chemicals involved in normal metabolism. The

solution that has evolved to address this problem is an elegant

combination of physical barriers and low-specificity enzymatic

systems.


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All organisms use cell membranes as hydrophobic permeability

barriers to control access to their internal environment. Polar

compounds cannot diffuse across these cell membranes, and the uptake

of useful molecules is mediated through transport proteins that

specifically select substrates from the extracellular mixture. Thisselective uptake means that most hydrophilic molecules cannot enter

cells, since they are not recognised by any specific transporters.[2] In

contrast, the diffusion of hydrophobic compounds across these barriers

cannot be controlled, and organisms, therefore, cannot exclude lipid-

soluble xenobiotics using membrane barriers.


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However, the existence of a permeability barrier means that organisms were able to

evolve detoxification systems that exploit the hydrophobicity common to membrane-

permeable xenobiotics. These systems therefore solve the specificity problem by

possessing such broad substrate specificities that they metabolise almost any non-polar

compound.[1] Useful metabolites are excluded since they are polar, and in general

contain one or more charged groups.

The detoxification of the reactive by-products of normal metabolism cannot be

achieved by the systems outlined above, because these species are derived from normal

cellular constituents and usually share their polar characteristics. However, since these

compounds are few in number, specific enzymes can recognize and remove them.

Examples of these specific detoxification systems are the glyoxalase system, which

removes the reactive aldehyde methylglyoxal,[3] and the various antioxidant systemsthat eliminate reactive oxygen species.[4]


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Endogenous toxins

The detoxification of endogenous reactive metabolites such as peroxides and reactive

aldehydes often cannot be achieved by the system described above. This is the result

of these species' being derived from normal cellular constituents and usually sharing

their polar characteristics. However, since these compounds are few in number, it is

possible for enzymatic systems to utilize specific molecular recognition to recognizeand remove them. The similarity of these molecules to useful metabolites therefore

means that different detoxification enzymes are usually required for the metabolism

of each group of endogenous toxins. Examples of these specific detoxification

systems are the glyoxalase system, which acts to dispose of the reactive aldehyde

methylglyoxal, and the various antioxidant systems that remove reactive oxygenspecies.

Ph I


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Phase I

introduction of functional group

hydrophilicity increases slightly

may inactivateor

activate original compound major player is CYP or mixed function oxygenase

(MFO) system in conjunction with NAD(P)H

location of reactions is smooth endoplasmic reticulum


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Ph I ti


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Phase I reactions

OxidationHydroxylation (addition of -OH group)

N- and O- Dealkylation (removal of -CH side chains)

Deamination (removal of -NH side chains)

Epoxidation (formation of epoxides)

Oxygen addition (sulfoxidation, N-oxidation)

Hydrogen removal

Reduction

Hydrogen addition (unsaturated bonds to saturated)

Donor molecules include GSH, FAD, NAD(P)H

Oxygen removal

Hydrolysis

Splitting of C-N-C (amide) and C-O-C (ester) bonds

O

C C

OC

See also Chapter 6 of Casarett and Doulls Toxicology

Table 6.1

epoxide

Bi t f ti


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Biotransformation

Activation of xenobiotics is a key element(e.g. benzene, vinyl chloride)

Reactive intermediates include epoxides and freeradical species (unpaired electrons) that are short-lived

and hence highly reactive Protection is provided by

endogenous antioxidant substances, e.g. GSH

vitamins C and E

antioxidant enzymes, SOD, GPX, CAT in coupled reactions

Antioxidant molecules are oxidized in the process buthave the capacity to regenerate the reduced form fromthe oxidized - NAD(P)H is a key player

See also p. 40-44 of Casarett and Doulls Toxicology

Th CYP 450 ti l


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The CYP-450 reaction cycle

A

E

D

C

F

G(B)

O id i f i l hl id


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Oxidation of vinyl chloride to an

epoxide

Hydrolysis of esters and amides


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Hydrolysis of organophosphates


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Hydrolysis of epoxides


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M b li f


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Metabolism of

benzo(a)pyrene to

9,10 epoxide:

Potent mutagen

that binds DNA

Azo- and nitro- reduction


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Intestinal flora as part of biotransformation


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Flora action

Ready for elimination


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Oxidation reactions


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Flavin mono-oxygenases

(FMO) catalyzed reactions

Nitrogen compounds


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Phase II reactions

Glycoside conjugation - glucuronidation

Sulfate - sulfation

Glutathione (GSH)

Methylation

Acylation

Acetylation

Amino acid conjugation Deacetylation

Phosphate conjugation

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Xenobiotic-FKBiotransformation