Transcript
Page 1: Biotransformation Xenobiotic metabolism

BiotransformationXenobiotic metabolism

“Essentials of Toxicology”

by Klaassen Curtis D. and Watkins John B Chapter 6

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Biotransformation• Water soluble xenobiotics are easier to

eliminate ( 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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Biotransformation

Potentially toxic xenobiotic

Inactive metabolite

Relatively harmless

Reactive intermediate

DetoxificationMetabolic activation

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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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Phase I

• introduction of functional group

• hydrophilicity increases slightly• may inactivate or 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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Phase II

• conjugation with endogenous molecules(GSH, glycine, cystein, glucuronic acid)

• hydrophilicity increases substantially• neutralization of active metabolic intermediates• facilitation of elimination • location of reactions is cytoplasm

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Phase I reactions

Oxidation Hydroxylation (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

OC C

OC

See also Chapter 6 of Casarett and Doull’s “Toxicology”

Table 6.1

epoxide

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Biotransformation

• Activation of xenobiotics is a key element (e.g. benzene, vinyl chloride)– Reactive intermediates include epoxides and free

radical 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 but have the capacity to regenerate the reduced form from the oxidized - NAD(P)H is a key player

See also p. 40-44 of Casarett and Doull’s “Toxicology”

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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 has 13)

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The CYP-450 reaction cycleA

E

D

C

F

G(B)

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Oxidation of vinyl chloride to an epoxide

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Metabolic enzymes

1. Microsomal:1. CYP450 monooxygenases2. Flavin monooxygenase

2. Non-microsomal1. Alcohol dehydrogenase2. Aldehyde dehydrogenase3. Monoamine and diamine oxidases

3. Both1. Esterases and Amidases2. Prostaglandin synthase 3. Peroxidases

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Cooxidation of acetaminophenby prostaglandin endoperoxide synthetase

Compare to fig. 6-2

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Hydrolysis of esters and amides

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Hydrolysis of organophosphates

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Hydrolysis of epoxides

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Stereoselectivehydroxylation

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Metabolism of benzo(a)pyrene to 9,10 epoxide:Potent mutagen that binds DNA

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Azo- and nitro- reduction

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Flora action

Intestinal flora as part of biotransformation

reabso

rptio

n

Ready for elimination

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Oxidation reactions

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Benzene trasformation to leukemia-causing metabolite

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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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Glucuronidation of phenol

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Sulfation of phenol and toluene

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GSH conjugation of acetaminophen

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Glutathione

-glutamyl-cysteinyl-glycine

Active site of a GST:


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