17 beta-hydroxysteroid and 20 alpha-hydroxysteroid dehydrogenase activities of human placental microsomes: kinetic evidence for two enzymes differing in substrate specificity.
During storage at 4 degrees C, the 17 beta-hydroxysteroid dehydrogenase activity of human placental microsomes with estradiol-17 beta was more stable than that with testosterone. In order to evaluate the basis for this difference, kinetics with C18-, C19-, and C21- steroids as substrates and/or inhibitors was studied in conjunction with an analysis of the effects of detergents. Both 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) and 20 alpha-hydroxysteroid dehydrogenase (20 alpha-HSD) activities were detected. At pH 9.0, apparent Michaelis constants were 0.8, 1.3, and 2.3 microM for estradiol-17 beta, testosterone, and 20 alpha-dihydroprogesterone, respectively, 17 beta-HSD activity with testosterone was inhibited by estradiol-17 beta, 5 alpha-dihydrotestosterone, 5 beta-dihydrotestosterone, 20 alpha-dihydroprogesterone, and progesterone. In each case 90 to 100% inhibition was observed at 50 to 200 microM steroid. Activity with 20 alpha-dihydroprogesterone was similarly sensitive to inhibition by C19-steroids. By contrast, 25 to 45% of the activity with estradiol-17 beta was not inhibited by high concentrations of C19- or C21-steroids and differed from the 17 beta-HSD activity with testosterone and the major fraction of that with estradiol-17 beta by being insensitive to solubilization by detergent. These results are consistent with an association of two dehydrogenase activities with human placental microsomes. One recognizes C18-, C19-, and C21-steroids as substrates with comparable affinities. The second appears to be highly specific for estradiol-17 beta. The former activity may account for most if not all of the oxidation-reduction at C-17 of C19-steroids and at C-20 of C21-compounds at physiological concentrations by term placental tissue.
D015067:17-Hydroxysteroid Dehydrogenases / Q000037:antagonists & inhibitors / Q000302:isolation & purification*; D015089:20-Hydroxysteroid Dehydrogenases / Q000037:antagonists & inhibitors / Q000302:isolation & purification*; D043607:20-alpha-Hydroxysteroid Dehydrogenase; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology*; D010920:Placenta / Q000201:enzymology*; D011247:Pregnancy; D012995:Solubility; D013379:Substrate Specificity; D013739:Testosterone / Q000494:pharmacology
D013739:Testosterone; D004958:Estradiol; D015067:17-Hydroxysteroid Dehydrogenases; D015089:20-Hydroxysteroid Dehydrogenases; D043607:20-alpha-Hydroxysteroid Dehydrogenase