Characterization of the purified pituitary cytosolic NADPH: 5α-dihydroprogesterone 3α-hydroxysteroid oxidoreductase

The purified cytosolic 3α-hydroxysteroid oxidoreductase (3α-HSOR) from female rat pituitary which catalyzes the reversible conversion of 5α-dihydroprogesterone (5α-DHP) to 3α,5α-tetrahydroprogesterone (3α,5α-THP) has been characterized in terms of its steroid substrate specificity, dihydrodiol dehydrogenase activity and inhibition by drugs such as medroxyprogesterone and indomethacin. The purified enzyme has a strong preference for the C 21 progestin steroid substrates, 5α-DHP and 3α,5α-THP, over the corresponding C 19 androgenic steroid substrates, 5α-dihydrotestosterone (5α-DHT) and 3α,5α-tetrahydrotestosterone (3α,5α-THT). The apparent K m for 5α-DHP (80 nM) is about 250 times lower than the K m for the androgenic steroid, 5α-DHT (21 μM). In the oxidative direction, the apparent K m for 3α,5α-TP (1.4 μM) is about 3-fold lower than the K m for the androgenic steroid, 3α,5α-THT (4.2 μM). A number of other naturally occurring 3-keto- and 3α(β)-hydroxy-steroids were assessed for their ability to act as inhibitors (alternate substrates) of the 3α-reduction of 5α-DHP catalyzed by the purified 3α-HSOR. None of the 3β- or 5β-isomers had any effect. Of the other 3-keto and 3α- steroids tested, only deoxycorticosterone and the ovarian progestins showed any significant inhibition. These may be acting as inhibitors since there was little, if any, direct 3α-reduction of progesterone to 3α-hydroxy-4-pregnen-20-one. Unlike the liver cytosolic 3α-HSOR, the pituitary enzyme has no associated dihydrodiol (quinone) dehydrogenase activity. This enzyme is similar to other cytosolic 3α-HSORs from liver and brain in that it is potentially inhibited by indomethacin and by medroxyprogesterone.