The steroid C-17,20-lyase complex in isolated Graafian follicles: effects of human chorionic gonadotropin

Androstenedione synthesis was studied in isolated rat preovulatory follicles and compared with that of rat testicular tissue using [14C]progesterone together with 17 alpha-hydroxy-[3H]progesterone as substrates in the presence of NADH or NADPH as cofactors. The amount of androstenedione formed was measured by addition of carrier, reisolation, and crystallization to constant specific activity. The labeling patterns of androstenedione and 17 alpha-hydroxyprogesterone (17-OHP) confirmed that both tissues preferentially catalyzed the synthesis of androstenedione from progesterone rather than from 17-OHP. It appears, therefore, that free 17-OHP was not an obligatory intermediate in this reaction. When hCG (5 IU) was administered sc and the follicles were isolated 3 h later, androstenedione synthesis was inhibited whether NADH or NADPH was added as cofactors. By contrast, 17-hydroxylase activity was inhibited only with NADH as cofactor. Hence, the gonadotropin, with NADH as cofactor, specifically reduced progesterone incorporation into androstenedione without affecting incorporation of 17-OHP. Thus, hCG appears to affect androstenedione production from progesterone at two different sites of the lyase complex.