Regulation of the 3β-hydroxysteroid dehydrogenase activity in tissue fragments and microsomes from human term placenta: Kinetic analysis and inhibition by steroids

The effects of 50 μM of progesterone (P 4), estradiol (E 2), estrone (E 1), estriol (E 3), dehydroepiandrosterone (DHIA), androstenedione (Δ 4) and testosterone (T) on the bioconversion of [ 3H]pregnenolone (6 nM) to [ 3H]P 4 were investigated by incubating 200 mg of tissue fragments as well as equivalent aliquots of microsomes from human term placenta during 30 min. All the steroids assayed, except E 3, significantly inhibited the [ 3H]P 4 formation in a microsome incubation system with respect to the control assay ( P < 0.001). Conversely in a tissue incubation system. Pin4, E 1] as well as E 3 had no effect on [ 3H]pregnenolone bioconversion while E 2 slightly decreased the [ 3H]P 4 formation ( P < 0.05) compared with the control. A significant inhibition was observed in this system with the other steroids ( P < 0.001). To investigate these apparent different results of inhibition-noninhibition of the same steroids irrespective of the system of incubation used, the effects of P 4, E 2 and T on 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD) activity were studied in tissue fragments and microsomes in kinetic terms. The results found indicate that these steroids inhibited in a competitive fashion the 3β-HSD activity in both systems. The different K i values found in tissue fragments and microsomes respectively for P 4 (1.8 μM vs 0.5 μM), E 2 (2.3 μM vs 0.6μM) and T (0.25μM vs 0.3 μM) explain the bioconversion results obtained in presence of 50 μM of the same steroids. These results include inhibition of [ 3H]P 4 formation by T in tissue fragments as well as in microsomes whereas P 4 and E 2 inhibited the [ 3H]P 4 formation only in microsomes. Furthermore, the comparison of these K i values with the available data of intraplacental and circulating concentrations of the same steroids in human term pregnancy suggest that only P 4 would be expected to cause marked 3β-HSD inhibition in physiological conditions.