Inhibition of estrogen 2-hydroxylase

Inhibitors of estrogen 2-hydroxylase can be useful agents for studying the kinetics of this enzyme system, in the elucidation of the structural requirements of the active site, and in examining the importance of the enzyme and the 2-hydroxyestrogens in various biochemical processes. The initial series of inhibitors evaluated included synthetic halogenated estrogens, numerous available steroids, and several nonsteroidal agents. These compounds were examined for their ability to block the conversion of estradiol to 2-hydroxyestradiol by male rat liver microsomal preparations using two radiotracer methods--the conversion of [4-14C]estradiol to [4-14C]2-hydroxyestradiol and the release of 3H2O from [2-3H]estradiol. The A-ring halogenated estrogens were effective inhibitors of estrogen 2-hydroxylase. The product of the enzymic reaction, 2-hydroxyestradiol, did not show feedback inhibition of the enzyme; however, 4-hydroxyestradiol and 2-methoxyestradiol did inhibit the enzyme to a moderate degree. In addition to inhibition by estrogen analogs, 2-hydroxylase was inhibited by androgens like testosterone and 5 alpha-dihydrotestosterone. Finally, various P450 inhibitors demonstrated varying degrees of inhibition of the enzymatic activity. Thus, qualitative structure-activity relationships concerning the interaction of steroidal and nonsteroidal compounds with the estrogen 2-hydroxylase were developed.