CONTRIBUTION OF HUMAN CYTOCHROME-P450 TO BENZO[A]PYRENE AND BENZO[A]PYRENE-7,8-DIHYDRODIOL METABOLISM, AS PREDICTED FROM HETEROLOGOUS EXPRESSION IN YEAST

The metabolism of benzo[a]pyrene (B[a]P) and its proximate mutagen B[a ]P-7,8-dihydrodiol (7,8-diol) was investigated in the presence of huma n microsomal epoxide hydrolase and P450 1A1, 1A2, 2C8, 2C9, 2C18, 2C19 , 2D6 and 3A4 expressed in the yeast Saccharomyces cerevisiae. P450 1A l had the highest turnover numbers for the formation of all B[a]P meta bolites, including phenols and dihydrodiols. P450 1A2, 2C8, 2C9, 2C18, 2C19 and 3A4, which are well represented in the liver, gave rise to t he formation of appreciable amounts of 3-hydroxy-B[a]P and of some dih ydrodiols from B[a]P. When 7,8-diol was used as substrate, P4501A1 als o exhibited the highest turnover numbers for the formation of tetrols, the hydrolysis products of the diolepoxides, whereas P450 1A2, 2C8, 2 C19 and 3A4 showed moderate activities. In order to test the validity of the yeast system, the contribution of each P450 isoform to B[a]P an d 7,8-diol metabolism was evaluated as the product of the turnover num bers of recombinant P450s by specific contents of each P450 in human l iver microsomes. Calculated formation rates for each B[a]P and 7,8-dio l metabolite globally matched experimental values. There is evidence t hat P450 3A4 and 2C9 play a major role in the formation of 3-hydroxy-B [a]P from B[a]P. Accumulation of the proximate mutagen 7,8-diol was pr edicted to be mainly driven by P450 1A2, 2C9 and 2C19, while formation of the genotoxic diolepoxides from 7,8-diol appeared to be dependent on P450 1A2 and 3A4 in the liver.