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 gl ucocorticoids cortisol and corticosterone to their inactive 11-oxo met abolites cortisone and dehydrocorticosterone, respectively. The proper ties of the human placental 11beta-hydroxysteroid dehydrogenase (11bet a-HSD) were studied. The enzyme was active in the oxidative and reduct ive directions. pH optimum for 11beta-dehydrogenase activity was in th e 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 th e enzyme to retain activity. 11beta-HSD a membrane bound enzyme was di stributed in all particulate subcellular fractions. Addition of deterg ent released latent activity of 11beta-dehydrogenase and inactivated 1 1-reductase activity. Both corticosterone and cortisol were substrates for the enzyme. The K(m) value with corticosterone as substrate was m uch lower than with cortisol. The K(m) values with cortisone and dehyd rocorticosterone were similar.