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Uptake and metabolism of androgen by the human epididymis in vitro

Summary. Uptake and metabolism of [³H]testosterone, mainly to 5-dihydrotestosterone (5-DHT) and 5-androstanediol were 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-DHT was greater than that of androstanediol while the reverse was true for the mitochondria and microsomes.

The distribution of 5-reductase activity in subcellular fractions was similar to that of the microsomal marker enzyme NADPH: cytochrome C reductase, whilst 3-hydroxysteroid dehydrogenase was found mainly in the cytosol. Maximal 5-reductase activity was at pH 5·3, apparent K_m values in the microsomal and nuclear fractions were 1·65 ± 0·7 and 1·75 ± 0·36 x 10^–6 M respectively, and the V_max in these preparations was 5·28 ± 1·19 and 3·1 ± 0·52 pmol/mg protein/min, respectively. The activity of 5-reductase was inhibited by Zn²⁺, Cu²⁺, Ba²⁺ and Cd²⁺ and by epitestosterone, progesterone and 4-androstene-3-one-17β-carboxylic acid.