11 beta-Hydroxysteroid dehydrogenase activity of the human placenta during pregnancy.

A method for the quantitative estimation of 11 beta-hydroxysteroid dehydrogenase activity (11 beta-HSD; EC.1.1.146) in human placental homogenates is described. This method is based on the separation of cortisol and cortisone by high performance liquid chromatography after extraction from homogenates incubated in the presence of cortisol and NADP. 11 beta-HSD activity (pmol/g wet weight per min) averaged 900 +/- 150 (mean +/- SEM) at 10 +/- 2 weeks of gestation, 915 +/- 35 at 17 +/- 2 weeks and 790 +/- 42 at 40 +/- 2 weeks, thus supporting the view that the placenta is an effective barrier to materno-fetal cortisol transfer throughout gestation.