Removal of the 4,4-Dimethyl Carbons in the Enzymic Conversion of Lanosterol to Cholesterol

The sequence of removal of methyl substituents at C-4 of 4,4,14a-trimethyl-5ar-cholesta-8,24-dien-3fl-o1 (lanosterol) in the course of its conversion to cholesterol has been re-examined.From a homogenate of rat liver incubated with or,-mevalonic acid-2J4C-5-SH, squalene, lanosterol, 4,4,14cr-trimethyl-5cu-cholest-S-en-3/S01, 4cy-methyl-5occholest-7-en-3P-01, and cholesterol have been isolated.It is concluded from measurements of the 3H:r4C ratios found in these various compounds that the 4a-methyl group of lanosterol is removed before the 4/3-methyl group and that the 4cy-methyl group of 4a!-methyldar-cholest-7-en-3@-ol originates as the 4/Smethyl group of lanosterol.Recent studies in these laboratories of the hepatic metabolism in vitro of the saturated sterol, 4,4-dimethyl-5ar-cholestan-3/3-ol, and several of its derivatives, have led us to conclude that the oxidative removal of the C-4 methyl groups from such substrates involves initial attack on the 4o!-methyl group which is, in consequence, the first of the C-4 substituents to be lost (1, 2).This conclusion, derived from studies with analogues of the naturally occurring sterols in the pathway of biosynthesis of cholesterol from 4,4,14a-trimethyl-5ol-cholesta-8,24-dien-3~-ol, conflicts with that of Gaylor and Delwiche with regard to the order of removal of the 4,4-dimethyl substituents from the natural substrate (3).The question of whether our findings might reflect the activities of enzyme systems that have stereospecificities different from those of the enzymes responsible for the process of demethylation at C-4 in the natural system could only be answered by a new examination of the sequence of events