Binding of a 4-Methyl-4-Aza-Steroid to 5α-Reductase of Rat Liver and Prostate Microsomes

A tritium labeled 5α-reductase inhibitor, [1,2- 317βN,N-diethylcarbamoyl-4-inethyl-4-aza-5α-androstan-3- one ([3H]4-MA), binds reversibly to a high affinity site (Kd) 6.5 nM) in liver microsomes from male rats. The binding requires a nicotinamide nucleotide coenzyme; NADH is at least 100 times less potent than NADPH, and NADP+, NAD+, flavin adenine dinucleotide, coenzyme A, and ADP are inactive. The relative potencies of 13 steroids as inhibitors of the binding of [3H]4- MA to liver microsomes correlate with their relative potencies as inhibitors of the conversion of [14C]testosterone to [14C]5αdihydrotestosterone by liver microsomes. Comparison of liver icrosomes from mature female rats and microsomes from mature male rat liver, ventral prostate, spleen, kidney, and skeletal muscle shows that their NADPH-dependent [3H]4-MA binding capacities correlate with their levels of 5α-reductase activity. These results suggest that [3H]4-MA binds specifically to 5α-reductase in a NADPH-dependent manner. 5α-Reductas...