Inhibition of placental steroid synthesis by steroid metabolites: possible feedback control

Two placental steroid synthesizing enzymes are non-competitively inhibited by steroids normally present in the placenta. The conversion of 4-¹⁴C-3β -hydroxy-5-pregnene-20-one to progesterone by Δ⁵ -3β -hydroxysteroid dehydrogenase is inhibited by 20α-hydroxy-4-pregnene-3-one. The K_i of the inhibition is of the same order of magnitude as is placental inhibitor concentration. The 20α-reduction of 4-¹⁴C-progesterone is inhibited by estriol. The K_i of this inhibition is higher than normal placental estriol levels.We propose that these inhibitions constitute part of the control mechanism for placental steroid synthesis.