Partial characterization of 5 alpha-reductase in the human epididymis

The properties of 5 alpha-reductase in the human epididymis were studied. When testosterone-4-14C was incubated with the whole homogenate of human epididymis in the presence of NADPH, the identifed metabolites of the substrate were 5 alpha-dihydrotestosterone and 5 alpha-androstane-3 alpha, 17 beta-diol. 5 alpha-reductase activity was mainly localized in the microsomal and nuclear fractions. The Km values of the 5 alpha-reductase for testosterone in the microsomal and nuclear fractions were 1.07 x 10(-8) M and 0.83 x 10(-8) M, respectively. The optimum pH of the 5 alpha-reductase activity was 5.7 for the microsomal fraction and 5.5 for the nuclear fraction. The optimum temperature was about 48 degrees C for 5 alpha-reductase in both the microsomal and nuclear fractions. The most preferable cofactor for the microsomal 5 alpha-reductase was NADPH and the Km value of the enzyme for NADPH was 2.1 x 10(-6) M. These results confirm the presence of 5 alpha-reductase in the human epididymis and suggest that the enzyme has considerable affinity for testosterone and that the properties of 5 alpha-reductase in the microsomal fraction are similar to those in the nuclear fraction.