OXIDATION OF XENOBIOTICS BY RECOMBINANT HUMAN CYTOCHROME-P450 1B1

Human cytochrome P450 (P450) 1B1 (CYP1B1) has recently been shown to b e an important enzyme in the activation of diverse procarcinogens such as arylarenes, nitroarenes, and arylamines to reactive metabolites th at cause DNA damage in the cells. However, it is not known whether thi s P450 enzyme also plays roles in the oxidation of certain drugs or mo del substrates commonly used in P450 assays. We examined the substrate oxidation activities of recombinant human CYP1B1 in yeast microsomes and compared these activities with those catalyzed by reconstituted sy stems containing recombinant CYP1A1 and CYP1A2 which were isolated fro m membranes of Escherichia coli in which respective cDNAs have been ex pressed. Catalytic activities towards some of the model substrates of other human P450 enzymes including CYP2A6, 2C9, 2C19, 2D6, 2E1, and 3A 4 were also determined and compared. CYP1B1 catalyzed benzo[a]pyrene 3 -hydroxylation at rates lower than those of CYP1A1 but higher than tho se of CYP1A2. The activity towards 7-ethoxyresorufin O-deethylation ca talyzed by CYP1B1 was about one-tenth of that of CYP1A1, but the K-m v alues were lower for CYP1B1 than those for CYP1A1 and CYP1A2. CYP1B1 w as also able to catalyze the oxidation of theophylline and caffeine, t wo prototypic substrates for CYP1A2. CYP1B1 did not oxidize other typi cal P450 substrates such as coumarin, tolbutamide, S-mephenytoin, chlo rzoxazone, nifedipine, and testosterone, while low rates of oxidation of bufuralol and 7-ethoxycoumarin were found for CYP1B1. These results indicate that CYP1B1 has catalytic activities overlapping CYP1A1 and CYP1A2 with respect to the oxidation of drugs and model P450 substrate s, although the relative catalytic roles in these three P450 enzymes d iffer depending upon the substrates examined. A distinct marker activi ty of CYP1B1 has not been identified.