3 Alpha-hydroxysteroid dehydrogenase activity in rat liver and skin.

3 alpha-Hydroxysteroid dehydrogenase (3 alpha HSD) is one of the main enzymes involved in the metabolism of the active androgen, dihydrotestosterone (DHT). 3 alpha HSD catalyzes the reversible reduction of DHT to 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha DIOL). The equilibrium of 3 alpha HSD reductive and oxidative activity is an important factor in the regulation of intracellular levels of DHT. In this study, we determined the kinetic characteristics of 3 alpha HSD in the subcellular fractions of female rat liver and abdominal skin. The enzyme expressed its activity in the cytosol and microsomal fractions of both of these tissues. It showed higher activity with the phosphorylated cofactors, NADPH and NADP, and was inhibited by indomethacin. The Vmax values of 3 alpha HSD in the cytosol were 10-fold higher than the Vmax values in the microsomes in both the liver and skin. In both tissues, the Km values with DHT as the substrate (reductive) were lower than the Km with 3 alpha DIOL as the substrate (oxidative). Although the Vmax values of the oxidative reaction were higher than the Vmax values of the reductive reaction in both liver and skin, the low Km values and the higher Vmax/Km ratio for DHT indicated that the reduction of DHT to 3 alpha DIOL was the favored reaction. The enzyme kinetics of 3 alpha HSD suggest that neither tissue accumulates DHT, but promptly converts it to 3 alpha DIOL.(ABSTRACT TRUNCATED AT 250 WORDS)