Comparison of 3 alpha-hydroxysteroid dehydrogenase activities in the microsomal fractions of hyperplastic, malignant...

The conversion of dihydrotestosterone (DHT) to 3 alpha-androstanediol (3 alpha-adiol) was studied using the microsomal fractions of 15 hyperplastic, 5 malignant and 6 normal human prostatis tissues. Standard assay conditions were: 0.2 microM DHT, 1.0 mM NADPH, 1.0 mM NADH, 2.0 mM EDTA and the microsomal fractions equivalent to 200 mg of prostatic tissue, in 0.1 M MES buffer, pH 6.5. Under the conditions of this assay, the back-conversion of 3 alpha-adiol to DHT or the conversion of DHT to androstanediol were negligible. Optimum enzyme activity was achieved under standard assay conditions. In the absence of EDTA: enzyme activity was 65% of the standard assay; activity was diminished further by 2 mM Ca2+ and virtually eliminated by 2 mM Mg2+ or 2 microM Zn2+. Activity in the absence of either NADPH or NADH was only 50% of the activities seen in the presence of both cofactors. The pH optimum of the enzyme was between 6.0 and 6.5. The apparent Km values of the enzymes in hyperplastic, malignant and normal tissues were 0.03, 0.02 and 0.03 microM, respectively. The Vmax values for these tissues were 6.0 +/- 2.1, 1.6 +/- 0.5 and 14.0 +/- 3.0 pmol/mg protein/20 min incubation, respectively. The results of these experiments offer further explanation for the differences in DHT and 3 alpha-adiol levels seen in the 3 prostatic tissues.