In vitro metabolism of 5α-dihydrotestosterone to 5α-androstane-3α,17β-diol in rat heart, diaphragm, skeletal muscle and bulbocavernosus/ levator ani: Enzyme characterization and quantification

Abstract

3α-Hydroxysteroid dehydrogenase activity in rat heart muscle, diaphragm, skeletal muscle and bulbocavernosus/levator ani (BCLA) was assayed by measuring initial conversion rates of [ 3 H]-5α-dihydrotestosterone (DHT) to [ 3 H]-5α-androstane-3α,17β-diol (3α-diol). Little, or no, [ 3 H]-5α-androstane-3β, 17β-diol was formed. The enzyme in each muscle was located predominantly in the cytosol, showed a preference for NADPH as added cofactor and at an ammonium sulphate concentration of 40–80% approx. 60–75% of the enzyme activity could be precipitated. The apparent K M values varied between 0.13–1.29 μM. Quantitatively the following order of enzyme activity was found: heart muscle > diaphragm > skeletal muscle = BCLA. The enzyme is sexually differentiated, females having a higher activity than males. Short-term (3 and 7 days) castration of male rats resulted in an enzyme activity approximating that of the respective female rats. Long-term (7 weeks) castration of males also increased the enzyme activity, but the increases were less than those in the 3- and 7-day castrated males. Prepubertal (18–20 day old) male rats showed an enzyme activity in each of the four muscles which was significantly higher than that of adult males, females and castrated males. In “old” (2 years of age) male rats the enzyme activity, when compared to the uncastrated adult males, was significantly lower only in the BCLA. This study indicates that the enzyme is qualitatively similar, but quantitatively different in the various muscle types. In addition, the enzyme activity is dependent upon the hormonal status of the animal.