Kinetic studies of inhibition of estradiol 2- and 16 alpha-hydroxylases in rat liver microsomes with various cytochrome P-450 inhibitors

Kinetic studies of inhibition of estradiol 2- and 16 alpha-hydroxylase activities in male rat liver microsomes with cytochrome P-450 inhibitors, alpha-naphthoflavone, DL-aminoglutethimide, SKF-525A and metyrapone, were extensively carried out. All of the inhibitors competitively blocked the two enzyme activities. The former three inhibitors preferentially inhibited the 16 alpha-hydroxylase activity while the reverse result was obtained in the case of metyrapone, and SKF-525A was the most potent inhibitors for the two enzyme among the four inhibitors. The kinetic data, the apparent Ki's for the four inhibitors and Km's for the substrate estradiol in the assays, along with the inhibition results with carbon monooxide suggest that different forms of cytochrome P-450 may be involved in the two hydroxylations. Kinetic parameters of the two hydroxylase activities in female rat liver microsomes were then determined to be an apparent Km of 23.0 and 158 microM and Vmax of 99.0 and 5.65 pmol/min/mg protein for the 2-hydroxylation and the 16 alpha-hydroxylation, respectively. The kinetic data show that the 2-hydroxylation may be quantitatively an exclusive hydroxylative pathway in estrogen metabolism in female.