Evidence for Involvement of Cytochrome P-450-Linked Oxygenase System in the Conversion of C21-Steroids to Δ16-C19-Steroids Catalyzed by Pig Testicular Microsomes

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Abstract
Formation of androstadienone or androstadienol, a Δ16-C19-steroid, from progesterone or pregnenolone is catalyzed by the so-called Δ16-C19-steroid synthesizing enzyme in the pig testicular microsomes. The enzyme activity was also present in the testicular microsomes prepared from neonatal pig. The enzyme activity was considerably inhibited by CO, and such cytochrome P-450 inhibitors as SU 8000, SU 10603, and metyrapone. Δ16-C19-Steroid synthesizing enzyme activity was extracted from the testicular microsomes by sodium cholate in potassium phosphate buffer, pH 7.4, containing EDTA and dithiothreitol, and the solubilized enzyme activity was partially purified by DEAE-cellulose column chromatography. It was shown by reconstitution of the enzyme activity that Δ16-C19-steroid synthesizing enzyme is a cytochrome P-450-linked oxygenase system dependent on cytochrome P-450-reductase and cytochrome b5. In particular, cytochrome b5 was an essential component for the activity of Δ16-C19-steroid synthesizing enzyme.