Exclusion of 20(22)-dehydrocholesterol as an intermediate in the biosynthesis of pregnenolone in bovine adrenocortical mitochondrial acetone-dried powder preparations

Abstract

Incubation of (22R)-(22-180)20-hydroxycholesterol with a bovine adrenocortical mitochondrial acetone-dried powder preparation in air yielded (20R, 22R)-20, (22-18O)22-dihydroxy-cholesterol. Incubation of (20S)-(20-18O)22-hydroxycholesterol yielded (20R, 22R)-(20-18O)20,22-dihydroxycholesterol. The formed glycols and the substrates reisolated at the end of the incubations had the same 18O abundance as the starting materials. No significant (20R, 22R)-20,22-dihydroxycholesterol was formed following incubation with either (E)-or (Z)-20, (22)-dehydrocholesterol. (20R,22S)-20, 22-Epoxycholesterol yielded approximately 1/5 of the amount of pregnenolone obtained in a similar incubation with cholesterol. No significant pregnenolone formation was observed with (20R, 22R)-20,22-epoxycholesterol. These results exclude a mechanism for the biosynthesis of (20R, 22R)-20,22-dihydroxycholesterol from the monohydroxylated cholesterol derivatives by way of dehydration followed by epoxidation and hydration. Similarly, the participation of an olefin and an epoxide as intermediates in the transformation of cholesterol to pregnenolone in acetone-dried powder preparations of adrenal cortex mitochondria is unlikely.