Reduction of Δ²⁴ of Lanosterol in the Biosynthesis of Cholesterol by Rat Liver Enzymes. II. Stereochemistry of Addition of the C-25 Proton

Mycobacterium smegmatis converts cholesterol to (25S)-26-hydroxycholest-4-en-3-one. It was shown, with the use of [¹⁴C₅]cholesterol biosynthesized from [2-¹⁴C]mevalonic acid in the S-10 fraction of rat livers, that the 26-¹⁴C atom bore the oxygen atom. Evidence is provided that the 26-hydroxylation by M. smegmatis proceeds without loss of the C-25 hydrogen. Barring rearrangement and migration of this hydrogen, it may be inferred that the C-26-hydroxylation proceeds without epimerization at C-25. Previously, we demonstrated that in the reduction of Δ²⁴ of lanosterol a 24-pro-S hydrogen is added. It follows that the reduction of Δ²⁴ of lanosterol in the biosynthesis of cholesterol in the S-10 fraction of rat liver entails the overall cis addition of two hydrogen atoms.