20-ALPHA-HYDROXYSTEROID OXIDOREDUCTASE ACTIVITY AND 20-ALPHA-DIHYDROPROGESTERONE CONCENTRATION IN HUMAN-PLACENTA BEFORE AND AFTER PARTURITION

The objective of this investigation was to determine if decreased biosynthesis or increased catabolism of progesterone (P) during labor was responsible for the decreased concentration of hormone observed in the human placenta after labor and vaginal delivery. No significant difference was found in P biosynthesis by placental tissues examined before and after labor as evidenced by a similar activity of 3β-hydroxysteroid dehydrogenase:Δ5-isomerase. In contrast, there was a marked increase in P catabolism during labor, as shown by a significant (p<0.05) change in placental 20α-hydroxysteroid oxidoreductase activity which increased from 835.5±103 (mean±SEM) pmoles of 20α-dihydroprogesterone (20α-DHP) generated per milligram of protein per hour before labor to 1,160.4±101 pmoles/mg/hr after labor. A similar significant (p<0.02) increase in enzyme activity was noticed in parallel assays of the reverse (20α-DHP→P) reaction. As a consequence of increased P catabolism, placentas after labor had a 20α-DHP concentration of 63.7±9.5 (mean±SEM) ng/mg protein, a value which was significantly (p<0.05) larger than that found before labor (37.8±8.3 ng/mg protein). These changes resulted in a modification of the placental tissue P/20α-DHP ratio which decreased from 2/1 before to 1/1 after labor. The results indicate that the catabolism of P to 20α-DHP increases significantly during human parturition. This phenomenon may be of importance in the mechanism of initiation and continuation of labor.