OXIDATION OF BENZO[A]PYRENE BY RECOMBINANT HUMAN CYTOCHROME-P450 ENZYMES

The oxidation of benzo[a]pyrene (B[a]P) was examined using reconstitut ed systems prepared with recombinant human cytochrome P450 (P450) enzy mes 1A1, 1A2, 2C8, 2C10, 2E1, and 3A4 and with microsomes prepared fro m Saccharomyces cerevisiae expressing recombinant human P450s 2C8, 2C9 , and 2C18. Products measured by HPLC included the 3- and 9-phenols, t he 4,5-, 7,8-, and 9,10-dihydrodiols (detected in the presence of epox ide hydrolase), and products in the polar fraction eluting immediately after the void volume. The most active enzyme in all reactions was P4 50 1A1. P450 3A4 and P450 1A2 formed appreciable amounts of several of the products, including the 3-phenol. P450 2C enzymes and P450 2E1 fo rmed relatively low amounts of all B[a]P products. Consideration of th ese patterns along with knowledge of levels of expression of the P450s in human tissues and previous results with microsomes leads to the co nclusion that P450 1A1 should dominate the oxidation of B[a]P in tissu es where it is present and inducible. In human liver the level of P450 1A1 is low and P450 3A4, P450 2C subfamily enzymes, and P450 1A2 prob ably all contribute. Of the human P450s considered here, P450 1A2 was the most active hepatic enzyme forming the 7,8-dihydrodiol. 7,8-Benzof lavone stimulated the oxidation of B[a]P by P450 3A4 and inhibited the oxidations catalyzed by P450 1A2. The extent of inhibition of P450 1A 1 was less (than with P450 1A2), probably due to the rapid oxidation o f 7,8-benzoflavone by P450 1A1. The major 7,8-benzoflavone product app ears to be the 5,6-oxide.