Follicle regulatory protein noncompetitively inhibits microsomal 3 beta-hydroxysteroid dehydrogenase activity

A heat- and trypsin-labile follicular fluid protein (FRP) extracted from both human and porcine follicular fluid has been shown to modulate ovarian steroidogenesis. To further investigate the effects of FRP, its effect on the kinetics of 3 beta-hydroxysteroid dehydrogenase activity (3 beta-HSD) was evaluated in cell-free microsomal preparations from human placenta. Test fractions of follicular fluid protein were preincubated with placental microsomes followed by the addition of various substrate concentrations (pregnenolone + NAD). Subsequent progesterone formation was interpreted as the velocity of the reaction. The 50% inhibitory dose (ID50) of FRP for 3 beta-HSD for the three substrate concentrations was 300 micrograms/ml. Although a clear decrease in 3 beta-HSD activity typically occurred after pre-incubation with 730 micrograms/ml of FRP, a paradoxical augmentation in 3 beta-HSD activity was present with the lower concentrations of FRP (10-30 micrograms/ml) and the more concentrated microsomal preparations. Double reciprocal plots of these reactions demonstrated a Km for 3 beta-HSD of 1.8-2.1 X 10(-6) M. Analysis of all reactions was found to be consistent with a noncompetitive mode of enzyme inhibition with an apparent Ki of 120 ng/ml or approximately 10(-8) M assuming a mol. wt of 16,000 Daltons for FRP. This derived Ki for FRP is within the biological concentration of FRP in follicular fluid.