LOW AROMATASE-ACTIVITY IN MICROSOMES FROM COMPLETE HYDATIDIFORM MOLE

The complete hydatidiform mole (CHM) is characterized by the presence of aberrant placenta, with hyperplasia of cyto- and syncytiotrophoblas ts and the absence of maternal genetic information. Steroidogenesis in this condition is, thus, of special interest. In this study we invest igated the kinetic parameters of aromatase in microsomes from CHM comp ared with those in normal early placenta (NEP). The enzyme activity wa s determined by measuring the conversion of [H-3] testosterone to [H-3 ]estradiol plus [H-3]estrone. The K(m) value for testosterone was 33 n mol/L in CHM and 17 nmol/L in NEP of similar gestational ages. Aminogl utethimide, a nonsteroideal inhibitor, decreased in a dose-dependent m anner and with the same potency the aromatization of testosterone in b oth tissues (ID50, 2 vs. 1 mumol/L in CHM and NEP, respectively). Thes e results suggest that the enzymes from the two sources are kineticall y similar. However, the enzyme efficiency, expressed as the maximum ve locity/K(m) ratio, was 17-fold lower in CHM than in NEP tissue (1.22/3 3 vs. 10.68/17 min/mg . mL). These findings suggest that in molar preg nancy the decreased capacity of trophoblast tissue for the formation o f estrogen could increase the testosterone concentration inside the mo lar vesicle, which, in turn, as we previously reported, inhibits proge sterone formation. All of these data could provide an explanation for the low circulating level of progesterone, which may directly or indir ectly affect the spontaneous expulsion of this aberrant tissue in the second trimester of pregnancy.