METABOLISM OF BENZO[A]PYRENE AND BENZO[A]PYRENE-7,8-DIOL BY HUMAN CYTOCHROME-P450 1B1

Benzo[a]pyrene (B[a]P), a ubiquitous environmental, tobacco and dietar y carcinogen, has been implicated in human cancer etiology, The role o f human cytochrome P450 1B1 in the metabolism of B[a]P is poorly under stood. Using microsomal preparations of human P450 1A1, 1A2 and 1B1 ex pressed in baculovirus-infected insect cells, as well as human and rat P450 1B1 expressed in yeast, we have determined the metabolism of B[a ]P, with and without the addition of exogenous epoxide hydrolase, and B[a]P-7,8-dihydrodiol (7,8-diol), each substrate at a concentration of 10 mu M. HPLC analysis detected eight major metabolites of B[a]P and four metabolites of the 7,8-diol, The results of these studies indicat e that cytochrome P450 1B1 carries out metabolism of B[a]P along the p athway to the postulated ultimate carcinogen, the diol epoxide 2, at r ates much higher than P450 1A2 but less than P450 1A1, The rates of fo rmation of the 7,8-diol metabolite in incubations with epoxide hydrola se are 0.17 and 0.38 nmol/min/nmol P450 for human P450 1B1 and 1A1, re spectively, and undetectable for 1A2, The rates of total tetrol metabo lite formation from the 7,8-diol, which are indicative of diol epoxide formation, are 0.60, 0.43 and 2.58 nmol/min/nmol P450 for 1B1, 1A2 an d 1A1 respectively. In agreement with other reports of rat P450 1B1 ac tivity, our data show this rat enzyme to be very active for B[a]P and 7,8-diol, with rates higher than human P450 1B1, In addition to the es tablished role of P450 1A1 in B[a]P metabolism P450 1B1 may significan tly contribute to B[a]P and 7,8-diol metabolism and carcinogenesis in rodent tumor models and in humans.