SPECIFICITY OF CDNA-EXPRESSED HUMAN AND RODENT CYTOCHROME P450S IN THE OXIDATIVE-METABOLISM OF THE POTENT CARCINOGEN 7,12-DIMETHYLBENZ[A]ANTHRACENE

7,12-Dimethylbenz[a]anthracene (DMBA), a potent carcinogen, requires m etabolic activation by cytochrome P450s (P450s) to electrophilic metab olites that result in DNA modification, mutagenicity, and carcinogenic ity. In this study, we used eight human forms, four rodent forms, and one rabbit form of P450 expressed from recombinant vaccinia or baculov irus vectors to define their specificity for metabolizing DMBA. Of the eight human P450s, 1A1 was the most active (specific activity = 14.7 nmol/min/nmol of P450) in total metabolism of DMBA and showed approxim ately 6- to 33-fold more activity than other P450s. 2B6, 2C9, and 1A2 were also capable of metabolizing DMBA (2.0-2.5 nmol/min/nmol of P450) , whereas 2C8, 2E1, 3A4, and 3A5 exhibited relatively low activities. Among animal P450s, mouse 1A1 exhibited activity similar to that of hu man 1A1 and had 5.0- to 37-fold more activity than other rodent and ra bbit P450s. In regard to enzyme regioselectivity, most human and roden t P450s predominantly formed the 8,9-diol, but human 2B6 and rat 2B1 p referentially formed the 5,6-diol. In the production of monohydroxymet hyl metabolites, all the enzymes yielded more 7-hydroxymethyl-12-methy lbenz[a]anthracene (7HOM12MBA) than 12-hydroxymethyl-7-methylbenz[a]an thracene (7M12HOMBA), except for human 1A1, which presented the revers e selectivity. Human liver microsomes from 10 organ donors were shown to metabolize DM BA and in most circumstances generated the meta belie profile DM BA trans-8,9-d dihydrodiol > 7HOM12MBA greater than or equ al to DMBA trans-5,6-dihydrodiol greater than or equal to 7,12-dihydro xymethylbenz[a]anthracene > 7M12HOMBA > DMBA trans-3,4-dihydrodiol. Th us, the combined activity of hepatic microsomal 2C9, 1A2, and 2B6 may contribute to the metabolic activation and the metabolism of DMBA in n ormal human liver. (C) 1996 Wiley-Liss, Inc.