Biotransformation Xenobiotic metabolism

BiotransformationXenobiotic metabolism

“Essentials of Toxicology”

by Klaassen Curtis D. and Watkins John B Chapter 6

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

Biotransformation

Potentially toxic xenobiotic

Inactive metabolite

Relatively harmless

Reactive intermediate

DetoxificationMetabolic activation

Converting lipophilic to water soluble compounds

Xenobiotic

Reactive intermediate

Conjugate

Phase I - Activation

Phase II - Conjugation

Excretion

Lipophilic

(non-polar)

Water soluble(polar)

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

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

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

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”

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)

The CYP-450 reaction cycleA

E

D

C

F

G(B)

Oxidation of vinyl chloride to an epoxide

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

Cooxidation of acetaminophenby prostaglandin endoperoxide synthetase

Compare to fig. 6-2

Hydrolysis of esters and amides

Hydrolysis of organophosphates

Hydrolysis of epoxides

Stereoselectivehydroxylation

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

Azo- and nitro- reduction

Flora action

Intestinal flora as part of biotransformation

reabso

rptio

n

Ready for elimination

Oxidation reactions

Benzene trasformation to leukemia-causing metabolite

Flavin mono-oxygenases(FMO) catalyzed reactions

Nitrogen compounds

Phase II reactions

• Glycoside conjugation - glucuronidation• Sulfate - sulfation• Glutathione (GSH)• Methylation• Acylation

– Acetylation– Amino acid conjugation– Deacetylation

• Phosphate conjugation

Glucuronidation of phenol

Sulfation of phenol and toluene

GSH conjugation of acetaminophen

Glutathione

-glutamyl-cysteinyl-glycine

Active site of a GST: