Studies of the nuclear 5α-reductase of human hyperplastic prostatic tissue

The 5α-reduction of testosterone was studied using the nuclear fraction of hyperplastic human prostatic tissue. Standard assay conditions were: non-radioactive testosterone (4 μM) plus 3.0–4.0 × 10 5 d.p.m. [ 3H]-testosterone, 0.4 mM NADPH, 2.0 mM EDTA and the nuclear fraction equivalent to 200 mg of prostatic tissue, in a final volume of 3.0 ml of 0.1 M HEPES buffer, pH 7.4. Incubations were performed at 37°C for 30 min. Under the conditions of this assay, the conversion of testosterone to androstenedione, dihydrotestosterone (DHT) to 3α-androstanediol (3α Adiol) or 3α Adiol to DHT was negligible. Optimum 5α-reduction was achieved under standard assay conditions. In the absence of EDTA: enzyme activity was 15–25% of the standard assay, 2 mM Ca 2+ or 2mM Zn 2+ further diminished enzyme activity to below 10% of the standard assay, while 2 mM Mg 2+ or 2 μM Zn 2+ increased 5α-reduction to 30% of the standard assay. If NADPH were omitted, 5α-reductase activity virtually was eliminated. NADH (0.4 mM) could not be substituted for NADPH to obtain maximum enzyme activity. The pH optimum of the enzyme was between pH 7.2 and 7.8. The K m of the enzyme was 1.5 μM. The V max, under standard assay conditions, using tissue obtained from 36 men, varied from 30–80 pmol/mg protein/30 min. Cyproterone acetate at a concentration of 1.0 μM had no effect on 5α-reductase activity, but competitively inhibited the enzyme at concentrations of 10 and 100 μM.