Regulation of testosterone biosynthesis in rat testes by 7α-hydroxylated C 19-steroids

Abstract

7α-Hydroxyandrostenedione and 7α-hydroxytestosterone inhibited the transformation, in vitro, of pregnenolone to progesterone by the Δ 5-3β-hydroxysteroid dehydrogenase coupled with the Δ 5-Δ 4 isomerase in rat testicular microsomal fraction (10000–105000 × g precipitate). The inhibition by the 7α-hydroxylated steroids on the enzyme system was competitive, and the inhibitor constant was estimated to be 56 μM for 7α-hydroxyandrostenedione and 385 μM for 7α-hydroxytestosterone. Substrate constants for the Δ 5-3β-hydroxysteroid dehydrogenase for pregnenolone were 67 and 64 μM, respectively, in the presence of 7α-hydroxyandrostenedione and 7αhydroxytestosterone. 7α-Hydroxytestosterone inhibited the testicular 7α-hydroxylase activity, but not significantly the 17β-hydroxysteroid dehydrogenase, whereas 7αhydroxyandrostenedione inhibited both enzymic activities. Other testicular enzymic activities related to steroid metabolism, such as the testicular 17α-hydroxylase, C 17-C 20-lyase and 20α-hydroxysteroid dehydrogenase were not influenced in vitro by the two 7α-hydroxylated steroids. When 7α-hydroxyandrostenedione was incubated with subcellular fractions of rat testes, no metabolite other than 7α-hydroxytestosterone was obtained. When the testicular microsomal fraction suspended in 0.25 M sucrose solution was stored at 0°, the activity of the 17β-hydroxysteroid dehydrogenase decreased nearly exponentially with the time of storage, but the activity of 7α-hydroxylase remained without decrease up to the 5th day of storage. From the results obtained, the possibility of an intracellular regulation of testosterone production by 7α-hydroxylation and its 7α-hydroxylated steroids is proposed.