Inhibition of human placental progesterone synthesis by aminoglutethimide in vitro

Aminoglutethimide (AG) is an inhibitor of P-450 linked steroid hydroxylation reactions including the cholesterol side chain cleavage, which is the rate-limiting step by which the synthesis of pregnenolone and also progesterone can be controlled. d,1-AG is a weak inhibitor of the cholesterol side chain cleavage of human term placenta in vitro. The inhibitory potency of AG was greater in tests with mitochondrial preparations than in organ culture tests. The cholesterol side chain cleavage enzyme in mitochondria from a human term placenta showed a 10% residual activity in the presence of 300 microM AG. A 50% inhibition was obtained using 90 microM AG. Due to large unknown quantities of unlabeled cholesterol a Ki for AG could not be determined. In organ culture tests, the progesterone concentration in culture medium of a human term placenta decreased to 50% of control values with rising amounts of AG (3-300 microM) and increased to 60-70% of control values with higher AG concentrations (up to 3 mM). The slow AG-dose dependent decrease of progesterone in the incubation medium of early human pregnancy can be explained by an inhibition of progesterone degradation by means of hydroxylation. The increase of progesterone at higher AG (300-3000 microM) concentrations could be the result of the fact that degradation is inhibited more than progesterone is formed. AG (100 microM) showed no inhibition of mitochondrial 3 beta-hydroxysteroid dehydrogenase and cytoplasmic 20 alpha-hydroxysteroid dehydrogenase, both isolated from a human term placenta.