Activity of 3β-Hydroxy-Δ5-Steroid Dehydrogenase/Δ 5-Δ4-Isomerase in the Ovine Corpus Luteum1

Abstract

The 3β-hydroxy-Δ⁵-steroid dehydrogenase/Δ⁵-Δ⁴-isomerase complex (HSD) is responsible for the conversion of pregnenolone to progesterone. In the ovine corpus luteum, HSD activity was found to be localized in the microsomal fraction and the activity displayed a broad pH optimum between pH 6.5 and 8.0. The HSD activity was extremely sensitive to feedback inhibition by its own product, progesterone, and the inhibition appears to be competitive in nature. The inhibition could be relieved and pseudo-zero order kinetics established with the addition of normal serum to the reaction mixture. The steroid binding proteins in serum presumably bound the progesterone as it was produced. The affinity of HSD for progesterone (product) appears to be several fold higher than for pregnenolone (substrate). Testosterone and estradiol were also potent inhibitors of HSD activity. The inhibition of HSD activity by progesterone could be demonstrated in suspensions of isolated luteal cells. Since this phenomenon was observed in preparations of intact cells, the possibility of a local autoregulation of progesterone secretion involving circulating steroids and feedback inhibition was considered. To create locally high concentrations of inhibitory steroids, testosterone was infused into the ovarian arterial circulation. When this was done there was no effect on progesterone secretion, suggesting the absence of feedback regulation of progesterone production by circulating steroids at the ovarian level.