Photoaffinity labeling of steroid 5 alpha-reductase of rat liver and prostate microsomes

21-Diazo-4-methyl-4-aza-5 alpha-pregnane-3,20-dione (Diazo-MAPD) inhibits steroid 5 alpha-reductase in liver microsomes of female rats with a Ki value of 8.7 +/- 1.7 nM, and the inhibition is competitive with testosterone. It also inhibits the binding of a 5 alpha-reductase inhibitor, [3H] 17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstan-3-one ([3H]4-MA), to the enzyme in liver microsomes. The inhibition of 5 alpha-reductase activity and of inhibitor binding activity by diazo-MAPD becomes irreversible upon UV irradiation. [1,2-3H]Diazo-MAPD binds to a single high affinity site (Kd 8 nM, 125 pmol binding sites/mg of protein) in liver microsomes of female rats, and this binding requires NADPH. Without UV irradiation, this binding is reversible, and it becomes irreversible upon UV irradiation. Both the initial reversible binding and the subsequent irreversible conjugation after UV irradiation are inhibited by inhibitors (diazo-MAPD and 4-MA) and substrates (progesterone and testosterone) of 5 alpha-reductase, but they are not inhibited by 5 alpha-reduced steroids (5 alpha-dihydrotestosterone and 5 alpha-androstan-3 alpha, 17 beta-diol). NADPH stimulates the binding of [3H] diazo-MAPD to microsomes of male rat liver and prostate. UV irradiation also induces conjugation of [3H] diazo-MAPD to these microsomes. Photoaffinity labeled liver microsomes of female rats were solubilized and fractionated by high performance gel filtration. The radioactive conjugate eluted in one major peak at Mr 50,000.