Incorporation of label from 18O2 into acetate during side-chain cleavage catalysed by cytochrome P-45017α(17α-hydroxylase-17,20-lyase)

Abstract

Samples of pregnenolone and 17α-hydroxypregnenolone deuteriated at the C-21 methyl group have been prepared and subjected to side-chain cleavage with a pig testes microsomal preparation under ¹⁸O₂. 17α-Hydroxypregnenolone was exclusively cleaved to dehydroisoandrosterone and the acetic acid released during the process was found to incorporate 0.90 atom of ¹⁸O. When a similar incubation was performed with pregnenolone two steroidal products, dehydroisoandrosterone and androsta-5,16-dien-3β-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 ¹⁸O. Complementary experiments in which the two substrates labelled with ¹⁸O at the C-20 carbonyl group were incubated under ¹⁶O₂ 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: [graphic omitted]