HUMAN PLACENTAL 11-BETA-HYDROXYSTEROID DEHYDROGENASE - EVIDENCE FOR AND PARTIAL-PURIFICATION OF A DISTINCT NAD-DEPENDENT ISOFORM

Excess glucocorticoids impair fetal growth and cause teratogenesis. Placental 11beta-hydroxysteroid dehydrogenase (11betaHSD) catalyzes the inactivation of cortisol to cortisone, preventing the high maternal cortisol levels from reaching the fetal circulation and thus preserving the low cortisol fetal environment. In previous work, an NADP-dependent isoform of 11betaHSD has been purified from rat liver, a cDNA isolated, and the human homolog cloned. However, much evidence suggests tissue-specific 11betaHSD activities that cannot be explained by the liver-type isoform. Therefore, we have partially purified human placental 11betaHSD and compared it to the enzyme in rat liver. Human placental subcellular fractions exhibited NAD-dependent 11betaHSD activity, but showed little activity with NADP. The enzyme had a pH optimum of 7-8.5 (peak, 7.7), was only sparingly soluble in detergents (solubility with Triton X-100 was very poor), and exhibited little latency or change in pH profile in detergent solution. By contrast, rat liver 11betaHSD was exclusively NADP dependent and was easily solubilized by a wide range of detergents (including Triton X-100), revealing substantial latency and altered pH profile [optimum of 10, becoming 7-10 (peak, 9.5) in detergent]. These data do not merely reflect species differences, as rat placental 11betaHSD was similiar to the human placental isoform. AMP affinity chromatography, which was completely without affinity for rat liver 11betaHSD, achieved a 1000-fold purification of human placental 11betaHSD. This had K(m) values for corticosterone (mean +/- SE, 14 +/- 1 nM) and cortisol (approximately 55 nM) that were over 100 times lower than that for liver 11betaHSD. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis allowed identification of a band (apparent mol wt, 40,000) that correlated consistently with human placental 11betaHSD activity (contrasting with a mol wt of 34,000 for rat liver 11betaHSD). Thus, the NAD-dependent human placental 11betaHSD is distinct from the previously characterized rat liver isoform and may be the product of a separate gene.