Human placental delta5-3beta hydroxysteroid dehydrogenase activity (delta5-3beta HSDH): intracellular distribution, kinetic properties, retroinhibition and influence of membrane delipidation.

Delta5-3beta HSDH activity has been assayed either by spectrophotometric method or by use of radioactive substrates. The enzymatic activity is equally distributed between mitochondrial and microsomal fractions verified by electronic microscopy. The specific activity is comparable in both fractions, as well as the optimal pH and the Km for NAD and for the substrates. The delta5-3beta Hut optimal pH, specific activity and sensitivity to the inhibitory action of various steroids are different when C19 and C21 steroids are used as substrates. Estrogens and cyclic AMP have also an inhibitory action on the oxidation of C21 steroids. Treatment of microsomal or mitochondrial membranes with phospholipase A releases fatty acids (mainly arachidonic) and decreases the enzymatic activity. "Adsorbtion" of the fatty acids on bovine serum albumin partially reactivates the delta5-3beta HSDH.