In vitro androgen metabolism in mouse kidney: High 3-keto-reductase (3α-hydroxysteroid dehydrogenase) activity relative to 5α-reductase

The $\text{in vitro}$ metabolism of testosterone and dihydrotestosterone was studied in slices and cell fractions of mouse kidney. When testosterone was used as substrate, very little metabolism to dihydrotestosterone occurred suggesting very low 5α-reductase activity. When dihydrotestosterone was substrate, it was rapidly converted to 5α-androstane-3α, 17β-diol by a potent 3-keto-reductase. Ninety-five percent of this latter enzyme is located in cytosol and it requires NADPH as cofactor. The 3-keto-reductase may exist in two molecular forms which can be separated by polyacrylamide gel electrophoresis. Form A and B have mean molecular radii which correspond to molecular weights of 38, 700 and 28, 700, respectively. Sufficient 3-keto-reductase activity is present in cytosol at 0°C to reduce physiological concentrations (2×10^?9 M) of dihydrotestosterone without the addition of cofactor. 3-Keto-reductase activity is higher in intact male than in castrate male or female mice and increases with androgen treatment.From these studies we conclude: (a) The virtual absence of 5α-reductase in mouse kidney is consistent with the thesis that testosterone rather than dihydrotestosterone may be the intracellular androgen in this organ. (b) Kinetic studies which depend upon the $\text{in vitro}$ uptake and retention of dihydrotestosterone by receptor proteins may be difficult to interpret due to rapid metabolism of ligand.