The effects of 31 different steroids on the formation of 3α-androstanediol from [1,2-3H]-dihydrotestosterone were assessed in both microsomal and cytosolic fractions of dog prostate. Several features of the substrate specificity of the reaction were deduced from these studies. First, 4-ene-3-keto steroids and 5β-reduced steroids were weak or inactive inhibitors of the reaction. Second, oxo substitution at C6 or C11 abolished the inhibitory ability of 5α-reduced steroids. Third, 2α-halogenated androstanes were good inhibitors, and, in particular, the 2α-bromo steroids (2α-bromo-androstanedione and -dihydrotestosterone) were at least five fold more inhibitory than the parent androstane. Fourth, the 3-keto function of the steroid nucleus is critical whereas a 17-oxo-group (hydroxyl or keto) is not essential. Fifth, the relative inhibition of the microsomal and cytosolic enzyme correlated closely, a finding compatible with enzymic identity of the two intracellular fractions.
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