Regio - and Stereoselective Metabolism of Two C 19 Steroids by Five Highly Purified and Reconstituted Rat Hepatic Cytochrome P - 450

High pressure liquid chromatographic systems capable of resolving at least 28 known and potential metabolites of 17@-hydroxy-4-androsten-3-one (testosterone) and 4-androstene-3,17-dione (androstenedione) were used to quantitatively assess the metabolism of the two steroids in monooxygenase systems reconstituted with five purified rat liver cytochrome P-450 isozymes. Cytochromes P-450a, -b, -c, -d, and -e catalyzed the oxidation of testosterone at overall rates of 21, 27, 2, 0.7, and 3 nmol/min/nmol of cytochrome P-450, respectively; while the corresponding rates for total androstenedione metabolism were 12, 62, 1.5, 0.3, and 5. Cytochrome P-450a catalyzed the oxidation of -testosterone and androstenedione almost exclusively to their respective 7a-hydroxy metabolites. Cytochrome P-450b catalyzed the oxidation of testosterone to androstenedione and 16nand 16&hydroxytestosterone in approximately equal molar ratios. However, this same hemoprotein exhibited a marked stereoselectivity in the metabolism of androstenedione since the molar ratio of 16aand 16s-hydroxyandrostenedione was greater than 1: lO. Cytochrome P-450e catalyzed the oxidation of both steroids to the same products as cytochrome P-450b, but at approximately 10% of the rate. Cytochromes P-45Oc and P-450d catalyzed the oxidation of testosterone and androstenedione regioand stereospecifically to their respective 60hydroxy metabolites. These results indicate that certain cytochrome P-450 isozymes show marked positional specificity in the metabolism of both testosterone and androstenedione, and that the rate as well as stereoselectivity of the oxidative reactions can be markedly dependent on subtle differences in the structure of the steroid substrate.