Metabolism of 7,12-dimethylbenz[ a ]anthracene in hepatic microsomal membranes from rats treated with isoenzyme-selective inducers of cytochromes P450

Previous work has shown that member(s) of the cytochrome P450IIC sub-family play significant roles in the formation of diols of 7,12-dimethylbenz[ a ]anthracene (DMBA) and are particularly important in formation of the proximate carcinogen (DMBA-3,4-diol). To further characterize the role of members of this subfamily in DMBA-diol formation and to assess the part played by other P450s, DMBA metabolism has been investigated in microsomes prepared from animals pre-treated with isoenzyme selective inducers. The rates of formation of DMBA-diols in membranes from phenobarbital-treated rats were very low when NADH was used as reductant and rates were not altered when NADPH and NADH were used in combination rather than using NADPH alone. This suggests that cytochrome b 5 is not involved in DMBA-diol formation in these membranes. Treatment of animals with clofibrate, pyrazole and dexamethasone produced regio-selective alterations in the rates of formation of DMBA-diols at the -3,4-, -5,6- and -8,9- positions. However, none of the inducers caused increases in the rates of DMBA-diol formation of any great magnitude suggesting that the isoforms which are the major induced proteins (P450IVA1, P450IIE1 and P450IIIA1) do not play a significant role in diol formation. The content of other P450s in these membrane are also altered and these were investigated by Western blot using antibodies to P450IIC6, P450IIB1 and P450IIIA1. The results of the Western blots show that the effects of the inducing agents on DMBA-diol formation can be explained by alterations of members of the P450IIC and P450IIB subfamilies.