CHANGES IN 17-BETA,20-ALPHA-HYDROXYSTEROID DEHYDROGENASE-ACTIVITY SUPPORTING AN INCREASE IN THE ESTROGEN PROGESTERONE RATIO OF HUMAN FETAL MEMBRANES AT PARTURITION

OBJECTIVE: Our purpose was to measure the activity of the reversible enzyme 17beta,20alpha-hydroxysteroid dehydrogenase around parturition with estrogen and progestogen substrates. STUDY DESIGN: Classic kinetic studies and explant cultures were used to determine kinetic parameters and net enzyme activities in both oxidative and reductive directions for both sets of substrates. RESULTS: Affinity constant values for estrone, estradiol, and 20alpha-dihydroprogesterone were 1 to 8 mumol/L. Affinity constant for progesterone was 9 to 25 mumol/L. Maximal velocities for all substrates in the chorion were 20- to 70-fold higher than in amnion and severalfold higher for estrogen substrates compared with the progestins. Around parturition there was a significant change toward net formation of the stronger estrogen (estradiol) and the weaker progestin (20alpha-dihydroprogesterone), suggesting an increase in the local estrogen/progesterone ratio. CONCLUSION: The enzyme 17beta,20alpha-hydroxysteroid dehydrogenase may be an important regulator of the local estrogen/progesterone ratio in fetal membranes around the time of parturition.