title: Stereo-selectivity and regio-selectivity in the metabolism of 7,8-dihydrobenzo[a]pyrene by cytochrome P450, epoxide hydrolase and hepatic microsomes from 3-methylcholanthrene-treated rats.
abstract: The active site of cytochrome P450 1A1 has been probed with the substrate 7,8-dihydrobenzo[a]pyrene using a purified, reconstituted system composed of cytochrome P450 1A1, NADPH-cytochrome c reductase and lipid in the presence or absence of epoxide hydrolase. The turnover of the substrate was found to be 38 nmol/nmol of cytochrome P450/min. The metabolic products that were identified are: a phenolic 7,8-dihydrobenzo[a]pyrene (20-29%); 9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (17-28%); benzo[a]pyrene (12-19%); 7-hydroxy-7,8-dihydrobenzo[a]pyrene (13-16%); 8-hydroxy-7,8-dihydrobenzo[a]pyrene (7-15%); 3-hydroxybenzo[a]pyrene (7-15%); 4,5-epoxy-4,5,7,8-tetrahydrobenzo[a]pyrene (0-4%); and a triol of 7,8,9,10-tetrahydrobenzo[a]pyrene (0-4%). 9,10-Epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene undergoes rapid hydrolysis to cis- and trans-9,10-dihydroxy-dihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (2:1) by benzylic attack of water at C-10. Approximately 71% of the trans diols are derived from (+)-(9S,10R)-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, indicating that cytochrome P450 1A1 has more than a 2:1 preference for selective epoxidation of an enantiotopic face of 7,8-dihydrobenzo[a]pyrene. This stereo-selectivity agrees with the postulated stereo-selectivity predicted by a previously described active site model for cytochrome P450 1A1. Epoxide hydrolase in pure form or in hepatic microsomes catalyzes the hydrolysis of 9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, which is inhibited by 1,1,1-trichloropropane 2,3-oxide. The (+)-(9S,10R)-isomer of the epoxide is slightly preferred as a substrate over its enantiomer and is cleaved by benzylic and nonbenzylic attack. Only benzylic attack was found with (-)-(9R,10S)-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene.
authors.affiliation: University of Southern California, School of Pharmacy, Los Angeles 90033.
authors.forename: J D
authors.initials: JD
authors.lastname: Adams
authors.forename: H
authors.initials: H
authors.lastname: Yagi
authors.forename: W
authors.initials: W
authors.lastname: Levin
authors.forename: D M
authors.initials: DM
authors.lastname: Jerina
mesh_terms: D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000378:metabolism; D000818:Animals; D001580:Benzopyrenes / Q000378:metabolism*; D001665:Binding Sites; D003577:Cytochrome P-450 Enzyme System / Q000494:pharmacology*; D004851:Epoxide Hydrolases / Q000378:metabolism*; D008297:Male; D008748:Methylcholanthrene / Q000494:pharmacology*; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013237:Stereoisomerism
chemical_list: D001580:Benzopyrenes; C023388:7,8-dihydrobenzo(a)pyrene; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide; D008748:Methylcholanthrene; D003577:Cytochrome P-450 Enzyme System; D004851:Epoxide Hydrolases