CHARACTERIZATION OF 11-BETA-HYDROXYSTEROID DEHYDROGENASE OF HUMAN PLACENTA - EVIDENCE FOR THE EXISTENCE OF 2 SPECIES OF 11-BETA-HYDROXYSTEROID DEHYDROGENASE

The enzyme, 11beta-hydroxysteroid dehydrogenase converts the active glucocorticoids cortisol and corticosterone to their inactive 11-oxo metabolites cortisone and dehydrocorticosterone, respectively. The properties of the human placental 11beta-hydroxysteroid dehydrogenase (11beta-HSD) were studied. The enzyme was active in the oxidative and reductive directions. pH optimum for 11beta-dehydrogenase activity was in the range of 7-10 and for 11-oxoreductase it was in the range of 5.5-6.0. The crude placental homogenate was unstable. Reductase activity was more labile than dehydrogenase activity. Removal of cytosol enabled the enzyme to retain activity. 11beta-HSD a membrane bound enzyme was distributed in all particulate subcellular fractions. Addition of detergent released latent activity of 11beta-dehydrogenase and inactivated 11-reductase activity. Both corticosterone and cortisol were substrates for the enzyme. The K(m) value with corticosterone as substrate was much lower than with cortisol. The K(m) values with cortisone and dehydrocorticosterone were similar.