title: Differential rates of metabolic activation and detoxication of the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by different cytochrome P450 enzymes.
abstract: Rat liver microsomes metabolized the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) to the genotoxic metabolite 2-hydroxamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (2-hydroxamino-PhIP) and to the detoxified product 2-amino-4'-hydroxy-1-methyl-6-phenylimidazo[4,5-b]pyridine (4'-hydroxy-PhIP). A 25-fold higher rate of metabolism was measured in microsomes from polychlorinated-biphenyl-treated rats (94 nmol/mg proteins/30 min) in comparison with those from untreated rats. Other effective inducers of PhIP metabolism were beta-naphthoflavone and isosafrole (ISF), whereas phenobarbital was ineffective. About twice as much 2-hydroxamino-PhIP as 4'-hydroxy-PhIP was formed in microsomes irrespective of the inducer the rats had been treated with. The metabolism was dependent on NADPH and was abolished by the cytochrome P450 inhibitor alpha-naphthoflavone. In a reconstituted enzyme system purified rat cytochrome P450 IA2 (P450ISF-G) had the highest N-hydroxylation rate (30 nmol/nmol P450/30 min) closely followed by the rat cytochrome P450 IA1 (P450BNF-B). Less activity was seen with rat P450 IIC11 (P450UT-A) and rabbit P450 IA2 (P450 LM4). Rat P450 IIE1 (P450j), P450 IIB1 (P450PB-B) and rabbit P450 IIB4 (P450 LM-2) and P450 IIE1 (P450 LM3a) were essentially inactive. Rat P450 IA1 (P450BNF-B) produced five times more 4'-hydroxy-PhIP (32 +/- 2 nmol/nmol P450/30 min) than did P450 IA2 (P450ISF-G). Hence, the measured ratio of activation to detoxication for rat P450 IA2 (P450ISF-G) enzyme was 7-fold higher than that of the other active P450 enzymes.
authors.affiliation: Department of Toxicology, National Institute of Public Health, Oslo, Norway.
authors.forename: H
authors.initials: H
authors.lastname: Wallin
authors.forename: A
authors.initials: A
authors.lastname: Mikalsen
authors.forename: F P
authors.initials: FP
authors.lastname: Guengerich
authors.forename: M
authors.initials: M
authors.lastname: Ingelman-Sundberg
authors.forename: K E
authors.initials: KE
authors.lastname: Solberg
authors.forename: O J
authors.initials: OJ
authors.lastname: Rossland
authors.forename: J
authors.initials: J
authors.lastname: Alexander
mesh_terms: D000818:Animals; D001711:Biotransformation; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology*; D004790:Enzyme Induction; D006900:Hydroxylation; D007093:Imidazoles / Q000378:metabolism*; D008658:Inactivation, Metabolic; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism; D009153:Mutagens / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D007093:Imidazoles; D009153:Mutagens; D003577:Cytochrome P-450 Enzyme System; C049584:2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine