Changes in 17 beta,20 alpha-hydroxysteroid dehydrogenase activity supporting an increase in the estrogen/progesterone ratio of human fetal membranes at parturition.

Abstract

Our purpose was to measure the activity of the reversible enzyme 17 beta,20 alpha-hydroxysteroid dehydrogenase around parturition with estrogen and progestogen substrates. 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. Affinity constant values for estrone, estradiol, and 20 alpha-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 (20 alpha-dihydroprogesterone), suggesting an increase in the local estrogen/progesterone ratio. The enzyme 17 beta,20 alpha-hydroxysteroid dehydrogenase may be an important regulator of the local estrogen/progesterone ratio in fetal membranes around the time of parturition.