Download Biotransformation Xenobiotic metabolism

Biotransformation
Xenobiotic 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
Detoxification
Inactive metabolite
Relatively harmless
Metabolic
activation
Reactive intermediate
Converting lipophilic to water
soluble compounds
Lipophilic
Xenobiotic
(non-polar)
Phase I - Activation
Reactive intermediate
Phase II - Conjugation
Conjugate
Excretion
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
Table 6.1
 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)
C
 Oxygen addition (sulfoxidation, N-oxidation)
 Hydrogen removal
O
epoxide
C
 Reduction
 Hydrogen addition (unsaturated bonds to saturated)
 Donor molecules include GSH, FAD, NAD(P)H
 Oxygen removal
 Hydrolysis
C
O
 Splitting of C-N-C (amide) and C-O-C (ester) bonds
See also Chapter 6 of Casarett and Doull’s “Toxicology”
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
– Dioxin/PCBs
– Barbiturates
CYP2E1
CYP1A
CYP2B
• CYP genes have multiple alleles (2D6 has 53, and 2E1 has
13)
The CYP-450 reaction cycle
A
G
(B)
C
F
E
D
Oxidation of vinyl chloride to an
epoxide
Metabolic enzymes
1.
Microsomal:
1.
2.
2.
Non-microsomal
1.
2.
3.
3.
CYP450 monooxygenases
Flavin monooxygenase
Alcohol dehydrogenase
Aldehyde dehydrogenase
Monoamine and diamine oxidases
Both
1.
2.
3.
Esterases and Amidases
Prostaglandin synthase
Peroxidases
Cooxidation of
acetaminophen
by prostaglandin
endoperoxide
synthetase
Compare to fig. 6-2
Hydrolysis of esters and amides
Hydrolysis of organophosphates
Hydrolysis of epoxides
Stereoselective
hydroxylation
Metabolism of
benzo(a)pyrene to
9,10 epoxide:
Potent mutagen
that binds DNA
Azo- and nitro- reduction
Intestinal flora as part of biotransformation
Ready for elimination
Flora action
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: