Kinetic analysis of the placental microsomal 3 beta-hydroxysteroid dehydrogenase activity

A sensitive accurate assay for the placental microsomal 3 beta-hydroxysteroid dehydrogenase (E.C.1.1.1.51) has been developed using tritiated substrates. Kinetic analysis of the enzyme with 3 beta-hydroxy-5-androsten-17-one and 3 beta-hydroxy-5-pregnen-20-one indicates that the apparent Km values for these substrates are orders of magnitude less than previously described. Analyses were carried out with microsomal preparations from two different placentas. For placenta 1 the apparent Km value for 3 beta-hydroxy-5-androsten-17-one was 14 nM and for 3 beta-hydroxy-5-pregnen-20-one was 36 nM; for placental 2 apparent Km values were 19 nM and 42 nM respectively. The analyses were performed over wide ranges of substrate concentration (about 200 fold), both above and below the Km values and no deviation from linearity of Eadie-Hoftsee plots was observed.