17,20-LYASE - ANALYSIS OF THE CYTOCHROME-P-450 BINDING-SITE USING MULTIPLE INHIBITORS

The activity of 17,20-lyase of rat testis microsomes is measured using 17α-hydroxyprogesterone as substrate. Both metyrapone and 20α-hydroxy-4-pregnen-3-one and the 20β-epimer are competitive inhibitors with regard to steroid substrate. Current evidence supports the involvement of cytochrome P-450 in this enzyme activity. The steroid substrate, steroid inhibitors and metyrapone bind to this cytochrome. An analysis of the cytochrome binding of inhibitors was carried out by simultaneous addition of two inhibitors. It was found that when the two steroid inhibitors were added that binding was mutually exclusive and no complex of enzyme-I1-I2 was formed. In contrast, a tertiary complex of enzyme-metyrapone-steroid inhibitor was formed although the binding of these structurally dissimilar inhibitors was antagonistic. © 1979.