Danazol Inhibits Steroidogenesis in the Rat Testis in Vitro

The effects of danazol on steroidogenesis in vitro in the rat testis were examined by studying: 1) androgen synthesis in rat Leydig cells cultured with danazol, 2) danazol binding to rat testis microsomal cytochrome P-450, and 3) enzyme kinetics of danazol inhibition of the microsomal enzymes of testicular steroidogenesis. Concentrations of danazol as low as 1 μM suppressed LHstimulated testosterone and androstenedione production in cultured Leydig cells. The addition of danazol to a preparation of testicular microsomes elicited a type I cytochrome P-450 binding spectrum, with an apparent spectral dissociation constant (K_s) of 4.8 μM. Danazol inhibited progesterone and 17α-hydroxyprogesterone binding to microsomal P-450 with apparent spectral inhibition constants of 2.4 μM and 2.8 μM, respectively. Danazol competitively inhibited 3β-hydroxy-Δ⁵-steroid dehydrogenase-isomerase (apparent enzymatic inhibition constant, K₁ = 5.8 μM), 17α-hydroxylase (K₁ = 2.4 μM), 17,20 lyase (K₁ = 1.9 μM), and 17β-hydroxysteroid dehydrogenase (K₁ = 4.4 μM). These findings indicate that low concentrations of danazol directly inhibit steroidogenesis in the rat testis in vitro.