Competition between benzo[a]pyrene and various steroids for cytochrome P-450-dependent rat liver monooxygenases.

Cytochrome P-450-dependent monooxygenases are able to oxidize a large variety of endogenous and exogenous substrates. This paper describes the in vitro interaction between benzopyrene and steroids at the level of two rat liver monooxygenases: steroid-16 alpha-hydroxylase and aryl hydrocarbon hydroxylase (AHH). The results obtained suggest the following conclusions: (1) Steroid-16 alpha-hydroxylase is partially supported by a specific cytochrome P-450 form which is not inhibited in vitro by exogenous substrates. Steroid-16 alpha-hydroxylase is completely independent from cytochrome P1-450 (or P-448), as it is insensitive, in vitro, to alpha-naphthoflavone; (2) AHH is supported by two cytochrome P-450 forms: a specific form which is inducible by methylcholanthrene and inhibited in vitro by alpha-naphthoflavone, but is insensitive to metyrapone and steroids; and another less specific form which is inhibited by metyrapone and steroids in vitro.