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 kM 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 kM 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.