A high affinity inhibitor of pituitary progesterone 5 alpha-reductase

The capacity of the 5 alpha-dihydroprogesterone analog, 4-aza-4-methyl-5 alpha-pregnane-3,20-dione (AMPD), to inhibit progesterone 5 alpha-reductase and both 5 alpha-dihydroprogesterone 3 alpha-hydroxysteroid oxidoreductase activities (NADPH- and NADH-linked) from the female rat anterior pituitary has been investigated. Dose response studies demonstrate that AMPD is a powerful inhibitor of pituitary progesterone 5 alpha-reduction but is ineffective at inhibiting either of the 3 alpha-hydroxysteroid oxidoreductase activities, even at concentrations up to 10 microM. A kinetic analysis of the interaction of AMPD with progesterone 5 alpha-reductase indicates that it is a competitive inhibitor vs. progesterone [Kislope = 7.2 +/- 0.6 nM; apparent Michaelis-Menten constant (Km) (progesterone) = 193 +/- 18 nM] and an uncompetitive inhibitor vs. NADPH (Kiintercept = 17.9 +/- 1.4 nM). These inhibition patterns are consistent with the view that NADPH binding precedes that of either AMPD or progesterone. Furthermore, AMPD does not appear to be an irreversible inhibitor since preincubation of the enzyme (at 37 C) with AMPD and NADPH, for periods of time up to 60 min, does not lead to a time-dependent loss of activity. The inhibition can also be readily removed by dilution, even after a 60-min preincubation with the inhibitor and NADPH. It is postulated that the selective and potent inhibition of the 5 alpha-reduction of progesterone by AMPD may be due to the steroid functioning as a transition state analog. This inhibitor should prove useful in studying the properties of progesterone 5 alpha-reductase and the function of anterior pituitary progestin metabolism.