Partial characterization of 5.ALPHA.-reductase in the human epididymis.

The properties of 5α-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 identified metabolites of the substrate were 5αdihydrotestosterone and 5α-androstane-3a, 17β-diol. 5α-reductase activity was mainly localized in the microsomal and nuclear fractions. The Km values of the 5α-reductase for testosterone in the microsomal and nuclear fractions were 1.07×10-8M and 0.83×10-8M, respectively. The optimum pH of the 5α-reductase activity was 5.7 for the microsomal fraction and 5.5 for the nuclear fraction. The optimum temperature was about 48°C for 5α-reductase in both the microsomal and nuclear fractions. The most preferable cofactor for the microsomal 5α-reductase was NADPH and the Km value of the enzyme for NADPH was 2.1×10-6M.These results confirm the presence of 5α-reductase in the human epididymis and suggest that the enzyme has considerable affinity for testosterone and that the properties of 5α-reductase in the microsomal fraction are similar to those in the nuclear fraction.