The localization and some characteristics of mouse adrenal C19-steroid 5 β-reductase were determined by the incubation of subcellular fractions of mouse adrenal tissue with [7 α-3H]androst-4-ene-3,17-dione. This enzyme was present only in the soluble fraction and was NADPH-dependent, although a small activity in the presence of NADH was also detected. The soluble fraction also contained 3α-, 3β- and a small amount of 17 β-hydroxy steroid dehydrogenase. These and other steroid-metabolizing enzymes present in the remaining subcelluar fractions are also described briefly. To measure 5 β-androstane-3,17-dione production by the mouse adrenal soluble fraction, all 5 β products first had to be oxidized to 5 β-androstane-3,17-dione, and the recovery of radio-activity between the substrate androst-4-ene-3,17-dione and product 5 β-androstane-3,17-dione of 96.1 +/-3.2% validated this technique. C19-steroid 5 β-reductase has a pH optimum of 6.5 and at low substrate concentrations the Km and Vmax. for 5 β reduction of [7 α-3H]androst-4-ene-ene-3,17-dione was 2.22 times 10(-6) ±0.48 times 10(-6) M and 450+/- 53 pmol/min per mg of protein respectively. At high substrate concentration, inhibition of the reaction occurred, which was shown to be due to increasing product concentration.
