Identification of the cytochrome P-450 isozymes responsible for testosterone oxidation in rat lung, kidney, and testis: evidence that cytochrome P-450a (P450IIA1) is the physiologically important testosterone 7 alpha-hydroxylase in rat testis.

Previous studies have shown that several forms of cytochrome P-450 present in rat liver microsomes oxidize testosterone with a high degree of regio- and stereospecificity. The aim of this study was to characterize the pathways of testosterone oxidation catalyzed by rat extrahepatic microsomes. Lung, kidney, testis, prostate, and brain were isolated from 3- and 14-week-old-male Sprague-Dawley rats. Microsomes from lung, kidney, and testis catalyzed distinctly different pathways of testosterone oxidation, whereas microsomes from prostate and brain failed to hydroxylate testosterone directly in a time- and protein-dependent manner. Lung microsomes from immature and mature rats converted testosterone to 16 alpha-hydroxytestosterone, 16 beta-hydroxytestosterone, and androstenedione. Lung microsomes were shown by Western immunoblot to contain cytochrome P-450b (P450IIB1), which has been shown previously to catalyze these three pathways of testosterone oxidation. Antibody against cytochrome P-450b strongly inhibited (greater than 80%) androstenedione formation and completely inhibited (greater than 95%) the 16 alpha- and 16 beta-hydroxylation of testosterone catalyzed by lung microsomes (as did carbon monoxide and antibody against NADPH-cytochrome P-450 reductase). Kidney microsomes from mature male rats converted testosterone to 2 alpha-hydroxytestosterone, 16 alpha-hydroxytestosterone, and androstenedione, whereas only the latter pathway was catalyzed by kidney microsomes from immature rats. Kidney microsomes from mature male rats were shown by Western immunoblot to contain cytochrome P-450h (P450IIC11), which has been shown previously to convert testosterone to 2 alpha-hydroxytestosterone, 16 alpha-hydroxytestosterone, and androstenedione. Antibody against cytochrome P-450h completely inhibited (greater than 95%) the 2 alpha- and 16 alpha-hydroxylation of testosterone by kidney microsomes, but had little effect on androstenedione formation, which is catalyzed by 17 beta-hydroxysteroid dehydrogenase. Testicular microsomes from mature, but not immature, rats catalyzed the 7 alpha-hydroxylation of testosterone. Previous studies have shown that this reaction is catalyzed in liver microsomes by cytochrome P-450a (P450IIA1). Testicular microsomes from mature, but not immature, rats were shown by Western immunoblot to contain cytochrome P-450a. Antibody against cytochrome P-450a or NADPH-cytochrome P-450 reductase completely inhibited (greater than 95%) the 7 alpha-hydroxylation of testosterone by testicular microsomes. A 90:10 atmosphere of carbon monoxide and oxygen did not appreciably block the 7 alpha-hydroxylation of testosterone by testicular microsomes, wh