Relative Contributions of CYP1A2 and CYP2E1 to the Bioactivation and Clearance of 4-Aminobiphenyl in Adult Mice.
4-Aminobiphenyl (ABP), a prototypical aromatic amine carcinogen in rodents and humans, requires bioactivation to manifest its toxic effects. A traditional model of ABP bioactivation, based on in vitro enzyme kinetic evidence, had postulated initial N-hydroxylation by the cytochrome P450 isoform CYP1A2. This is followed by phase 2 O-conjugation and hydrolysis to form a reactive nitrenium ion that covalently binds to DNA and produces tumor-initiating mutations. However, Cyp1a2(-/-) mice still possess significant liver ABP N-hydroxylation activity, DNA damage, and incidence of ABP-induced liver tumors, and in vivo induction of CYP1A2 paradoxically reduces levels of ABP-induced DNA damage. Competing ABP detoxification pathways can include N-acetylation by arylamine N-acetyltransferase 1 (NAT1) and/or NAT2; however, wild-type and Nat1/2(-/-) mice have similar in vivo ABP clearance rates. Together, these studies suggest the existence of novel ABP bioactivating and clearance/detoxification enzymes. In the present study, we detected similar reductions in Vmax for ABP N-hydroxylation by liver microsomes from Cyp1a2(-/-) and Cyp2e1(-/-) mice when compared with wild-type mice. In addition, recombinant mouse CYP1A2 and CYP2E1 were both able to N-hydroxylate ABP in mouse hepatoma cells. However, the in vivo clearance of ABP was significantly reduced in Cyp1a2(-/-) but not in Cyp2e1(-/-) mice. Our results support a significant role for CYP2E1 as a novel ABP N-oxidizing enzyme in adult mice, and suggest a more important contribution of CYP1A2 to the in vivo plasma clearance and thus detoxification of ABP.
Department of Pharmacology & Toxicology, University of Toronto, Toronto, Ontario, Canada.
Shuang
S
Wang
Department of Pharmacology & Toxicology, University of Toronto, Toronto, Ontario, Canada.
Debbie
D
Bott
Department of Pharmacology & Toxicology, University of Toronto, Toronto, Ontario, Canada.
Aveline
A
Tung
Department of Pharmacology & Toxicology, University of Toronto, Toronto, Ontario, Canada.
Kim S
KS
Sugamori
Department of Pharmacology & Toxicology, University of Toronto, Toronto, Ontario, Canada denis.grant@utoronto.ca.
Denis M
DM
Grant
D000107:Acetylation; D065767:Activation, Metabolic / Q000235:genetics; D000611:Aminobiphenyl Compounds / Q000378:metabolism* / Q000493:pharmacokinetics; D000818:Animals; D001191:Arylamine N-Acetyltransferase / Q000235:genetics / Q000378:metabolism; D045744:Cell Line, Tumor; D019388:Cytochrome P-450 CYP1A2 / Q000235:genetics* / Q000378:metabolism*; D019392:Cytochrome P-450 CYP2E1 / Q000235:genetics* / Q000378:metabolism*; D004249:DNA Damage; D006900:Hydroxylation; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008862:Microsomes, Liver / Q000201:enzymology
D000611:Aminobiphenyl Compounds; D007527:Isoenzymes; C006757:4-biphenylamine; D019392:Cytochrome P-450 CYP2E1; D019388:Cytochrome P-450 CYP1A2; D001191:Arylamine N-Acetyltransferase; C089384:N-acetyltransferase 1; C524206:Nat2 enzyme, mouse