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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: