3 beta-Hydroxysteroid dehydrogenase activity and gestagen concentrations in bovine cotyledons and caruncles during gestation and parturition.

3 beta-Hydroxysteroid dehydrogenase (3 beta-HSD) activity in bovine cotyledons was much higher than in caruncles throughout the gestation period. The activity of this enzyme in cotyledons increased greatly to 150.6 +/- 5.8 pmol min-1 mg-1 protein during the seventh month of gestation, reached a peak of 221.0 +/- 34.9 pmol min-1 mg-1 protein during the eighth month, and decreased at parturition. Progesterone and 20 alpha-hydroxyprogesterone concentrations in cotyledons also increased sharply to 2.69 +/- 0.30 and 2.15 +/- 0.42 ng mg-1 protein, respectively, during the seventh month of gestation, reaching peaks of 2.86 +/- 0.47 and 2.51 +/- 0.36 ng mg-1 protein, respectively, during the eighth month and decreasing at parturition, in a manner similar to the activity of 3 beta-hydroxysteroid dehydrogenase. The fluctuation of 17 alpha-hydroxyprogesterone concentration in cotyledons was different from that of progesterone and 20 alpha-hydroxyprogesterone. These findings indicate that the activity of 3 beta-hydroxysteroid dehydrogenase in the placenta is enhanced during the third trimester, and progesterone synthesized in the cotyledons is converted concurrently to 20 alpha-hydroxyprogesterone before progesterone is transferred to the fetal blood.