The occurrence and subcellular localization of 3 -hydroxysteroid dehydrogenase in adult rat brain

The subcellular localization of 3α-hydroxysteroid dehydrogenase, the dependency of the rate of reaction on time, concentration of protein, cofactor requirements and the substrate stereospecificity were investigated in the adult rat brain. The $\text{in vitro}$ conversion of 3-keto-5β-cholanoic-24-¹⁴C acid to lithocholic acid was shown to occur in the cytosol without added cofactors. Incubation of ¹⁴C labeled 3α,7α-dihydroxy-5β-cholanoic and 3α,12α-dihydroxy-5β-cholanoic acids with adult rat brain cell-free preparations resulted in the production of less polar metabolites identified as 3-keto-7α-hydroxy-5β-cholanoic and 3-keto-12α-hydroxy-5β-cholanoic acids by TLC, GLC combined with a radioactive monitoring detection system and by cocrystallization to constant specific activity.