Activity of 3-Beta-hydroxysteroid Dehydrogenase-5,4-En Isomerase in Chorioallantois and Amnion from Pregnant Sheep

Activity of 3 beta-hydroxysteroid dehydrogenase-5,4-en isomerase (3 beta-HSD; pregnenolone to progesterone) was investigated in homogenates of chorioallantois and amnion obtained from sheep at three gestational ages (day 50, day 100 and term) using an isotope assay. Chorioallantois and, to a lesser extent, amnion possessed 3 beta-HSD at each gestational age studied. Activity in the chorioallantoic membrane was greater at term than at days 50 and 100, and was similar to that in the placenta at day 100 and term. Activity in the placenta was considerably greater at day 50 than at day 100 and term. The subcellular distribution of 3 beta-HSD in chorioallantoic membrane was particulate in nature. The mean Km values (+/- S.E.M.) were 56.8 +/- 5.5 and 39.0 +/- 8.7 nmol/l at day 50 and term respectively. The mean Vmax values were 14.1 +/- 3.1 and 62.7 +/- 11.4 pmol/min per mg protein at day 50 and term respectively. Several steroids inhibited the conversion of pregnenolone to progesterone, particularly dehydroepiandrosterone (DHEA), oestrone and oestradiol. Oestrone and DHEA were competitive inhibitors. The Ki values were 19.6 +/- 5.9 and 40.2 +/- 14.7 nmol/l for oestrone and DHEA inhibition respectively. The results show that ovine fetal membranes have the capacity to convert pregnenolone to progesterone and demonstrate the possibility of inhibition by other steroids. Progesterone produced by the fetal membranes might influence the local intra-uterine steroid milieu during pregnancy and before parturition.