INCORPORATION OF LABEL FROM O-18(2) INTO ACETATE DURING SIDE-CHAIN CLEAVAGE CATALYZED BY CYTOCHROME P-450(17-ALPHA) (17-ALPHA-HYDROXYLASE-17,20-LYASE)

Samples of pregnenolone and 17 alpha-hydroxypregnenolone deuteriated at the C-21 methyl group have been prepared and subjected to side-chain cleavage with a pig testes microsomal preparation under O-18(2). 17 alpha-Hydroxypregnenolone was exclusively cleaved to dehydroisoandrosterone and the acetic acid released during the process was found to incorporate 0.90 atom of O-18. When a similar incubation was performed with pregnenolone two steroidal products, dehydroisoandrosterone and androsta-5,16-dien-3 beta-ol, were formed in an approximate ratio of 1:2-3 and the acetic acid formed in the process was again shown to contain 0.85-0.90 atom of O-18; Complementary experiments in which the two substrates labelled with O-18 at the C-20 carbonyl group were incubated under O-16(2) gave acetic acid retaining between 65-85% of the original carbonyl oxygen. The experiments strengthen the hypothesis that the two C(17)-C(20) bond cleavages described above correspond to the acyl-carbon fission process of the equation below showing the indicated fate of the various oxygen atoms: [GRAPHICS]