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In vitro effect of 16alpha-hydroxyprogesterone on the enzyme activities related to androgen production in human testes.

Progesterone was converted in vitro to 16alpha- and 17alpha-hydroxyprogesterones in the presence of NADPH by the testicular microsomal fraction (precipitate at 10 000 x g-105 000 x g) obtained from patients with prostatic carcinoma. 16alpha-Hydroxyprogesterone was not metabolized by either the microsomal or the cytosol fractions, and accumulated in the incubation medium. 16alpha-Hydroxyprogesterone competitively inhibited the activity of the C-17-C-20 lyase in the testicular microsomal fraction with an estimated inhibitor constant of 72 micron. Moreover, the 16alpha-hydroxyprogesterone non-competitively inhibited the activity of the 20alpha-hydroxysteroid dehydrogenase in the testicular cytosol fraction and had an estimated inhibitor constant of 52.9 micron. Other testicular enzymes related to steroid metabolism, such as delta5-3beta-hydroxysteroid dehydrogenase coupled with the delta4-delta5 isomerase, 16alpha-hydroxylase, 17alpha-hydroxylase, and 17beta-hydroxysteroid dehydrogenase were not influenced in vitro by 16alpha-hydroxyprogesterone at the concentration of 0.1 mM. From these findings, it is concluded that 16alpha-hydroxyprogesterone inhibit specifically the cleavage of the side-chain of 17alpha-hydroxypregnenes in the course of androgen formation from pregnenolone in vitro.