Control of placental 3 beta-hydroxy-delta 5-steroid dehydrogenase: an apparent endogenous regulator of porgesterone synthesis.

Dilution of tissue fractions from human placenta resulted in an increase in the specific activites of 3 beta-hydroxy-delta 5-steroid dehydrongease and of 20 alpha-hydroxysteroid dehydrogenase. The apparent Ks and V max of 3 beta-hydroxy-delta 5-steroid dehydrogenase were altered. It is postulated that this dilution effect is caused by the presence in the placenta of an endogenous inhibitor of steroid synthesis which may be important in the regulation of steroid output by this organ. A mathematical model of such an inhibitor corresponds well to the experimental results.