Delta5,3beta-hydroxysteroid dehydrogenase activities in rat trophoblast and ovary during pregnancy.

Delta5,3beta-Hydroxysteroid dehydrogenase (3beta-HSD) activity has been measured in homogenates of ovary and trophoblast from 9th to 21st day pregnant rats under conditions where there was little or no further metabolism of the progesterone formed from pregnenolone. Trophoblast 3beta-HSD activity is first detectable on the 11th day of gestation; the specific activity peaks on the 13th day, and falls thereafter. The ovary enzyme activity rises gradually to the 19th day and falls on the 21st day. Decidua homogenates contain very low levels of 3beta-HSD activity, while embryo and yolk sac homogenates do not posses significant amounts. Total 3beta-HSD activities were also determined on a per conceptus basis. At no time during pregnancy does the total trophoblast 3beta-HSD activity exceed 40% the activity of the ovary. This may explain why the progesterone-producing capacity of trophoblast in the rat is inadequate to support normal pregnancy following ovariectomy. The temporal patterns of trophoblast and ovary 3beta-HSD activity suggest that the enzymes are not coordinately controlled in the two tissues.