KINETIC-ANALYSIS OF 3-BETA-HYDROXYSTEROID DEHYDROGENASE-ACTIVITY IN MICROSOMES FROM COMPLETE HYDATIDIFORM MOLE

Microsomes isolated from complete hydatidiform moles (CHM) were able to convert [3H]pregnenolone to [3H]progesterone which indicates the presence of 3β-hydroxy-steroid dehydrogenase/isomerase (3β-HSD) activity. The kinetic parameters found (Km=0.63 μM and Vmax = 1-3.05 nmol/min/mg of protein) were like those observed in microsomes from normal early placenta (NEP) of similar gestational age (herein) and term placenta [1] suggesting that the enzymes from the three sources are kinetically similar. Testosterone, progesterone and estradiol in a dose range of 0.05-5 μmol/1 inhibited differently the in vitro conversion of [3H]pregnenolone to [3H]progesterone in a dose-dependent manner. The steroid concentrations necessary to inhibit the conversion of pregnenolone to progesterone by 50% (ID50) in CHM were 0.1 μM for testosterone, 0.6 μM for progesterone and 3 μM for estradiol, whereas in NEP they were 2.5, 1 and 5 μM respectively. The Ki values calculated from these ID50 in CHM together with the reported levels of endogenous steroids indicate that the accumulation of testosterone and progesterone inside the molar vesicle could physiologically regulate the rate of further conversion of pregnenolone to progesterone. The present findings could provide an explanation for the low level of progesterone in patients with CHM in the second trimester of pregnancy which in turn may directly or indirectly affect the spontaneous expulsion of this aberrant tissue. © 1990.