Characterization of Human Placental 11β-Hydroxy steroid Dehydrogenase, a Key Enzyme of Corticosteroid Metabolism

Abstract

Human placenta exhibits high levels of 11β-HSD oxidative activity. The enzymatic activity was tested in tissue slices, homogenates, microsomes and CHAPS-solubilized microsomal protein of spontaneously delivered fresh human placenta. Compared to liver and kidney the placenta exhibits the highest specific 11β-HSD activity in microsomal preparations. In contrast to liver and kidney, no strict cosubstrate dependence but some preference for NAD⁺ over NADP⁺ as cosubstrate was detectable. Conversion of corticosterone by microsomes with NAD⁺ as cofactor was 51.1 ± 3.7 ^. 10^–13 mol/(mg·min), withNADP+ 15.0 ± 0.8·10–13 mol/(mg·min). In contrast to tissue slices, subcellular fractions of placental preparations exhibited a small but significant reductive activity. A kinetic analysis of the conversion of corticosterone to 11-dehydrocorticosterone by Eadie-Hofstee plots revealed a biphasic course in the presence of NADP+ indicating the existence of isoenzymes. The k_M values were 6.3 ^. 10^–9 and 3.4 ^. 10^–8 mol/l respectively. In the presence of NAD⁺, a linear Eadie-Hofstee plot was obtained with a single k_M value of 4.4 ^. 10^–9 mol/l. Placental 11β-HSD was inhibited by β-glycyrrhetinic acid and by the steroidal detergent CHAPS. With NADP+ as cosubstrate, the IC50 values were 3.0 ^. 10^–7 and 1.2 ^. 10^-4 mol/l, with NAD⁺ 9.7 ^. 10^-8 and 5.6 ^. 10^–3 mol/l respectively. We conclude that the placenta constitutes an important barrier for 11-OH steroids during pregnancy between the maternal and fetal organism.