Uptake and metabolism of [3H]testosterone, mainly to 5\g=a\-dihydrotestosterone (5\g=a\-DHT) and 5\g=a\-androstanediolwere higher in the caput than in the cauda epididymidis in vitro. The metabolites represented 57, 49 and 47% of the total radioactivity in the caput, corpus and cauda epididymidis, respectively; subcellular distributions of the metabolites in each segment showed 67% of total radioactivity in cytosol and 18% in the nuclei. In both fractions, the amount of 5\g=a\-DHTwas greater

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... n that of androstanediol while the reverse was true for the mitochondria and microsomes. The distribution of 5\g=a\-reductaseactivity in subcellular fractions was similar to that of the microsomal marker enzyme NADPH: cytochrome C reductase, whilst 3 \ g = a \ -h y d r o x y s t e r o i d dehydrogenase was found mainly in the cytosol. Maximal 5\g=a\-reductaseactivity was at pH 5\m=.\3,apparent Km values in the microsomal and nuclear fractions were 1\m=.\65 \ m=+-\0\m=.\7and 1\m=.\75\ m=+-\0\m=.\36\m=x\ 10\m=-\6 M respectively, and the Vmax in these preparations was 5\m=.\28\ m=+-\1\m=.\19 and 3\m=.\1\ m=+-\ 0\m=.\52pmol/mg protein/min, respectively. The activity of 5\g=a\-reductase was inhibited by Zn2+, Cu2+, Ba2+ and Cd2+ and by epitestosterone, progesterone and 4-androstene-3-one-17\g=b\-carboxylic acid.