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Recent investigations on the nature of sterol intermediates in the biosynthesis of cholesterol

Lanosterol(4,4,14α-trimethyl-cholesta-8,24-dien-3β-ol) has been proposed as the primary product of the cyclization of 2,3-epoxysqualene in animal tissues. Enzymic conversion of lanosterol to cholesterol requires reduction of the ∆24 double bond, removal of the three extra methyl groups, and shift of the nuclear double bond from ∆8 position to the ∆5 position. Until very recently, all of the proposed sterol intermediates in the biosynthesis of cholesterol possessed nuclear double bonds in the ∆8, ∆7, ∆5,7 or ∆5 positions. Consideration of possible mechanisms for the removal of the methyl group at carbon atom 14 of sterol precursors led to our demonstration of the presence of cholest-8(14)-en-3β-ol in animal tissues and establishment of the convertibility of this sterol to cholesterol in rat liver homogenate preparations. Reports (from other laboratories) of the stereospecific loss of the 15α-hydrogen of lanosterol upon enzymic conversion to cholesterol led to the demonstration of the convertibility of cholesta-8,14-dien-3β-ol, cholesta-7,14-dien-3β-ol, 14α-methyl-cholest-7-en-3β,15-diol, cholest-8(14)-en-3β,15α-diol, and cholest-8(14)-en-3β,15β-diol to cholesterol in rat liver preparations. We have recently developed chromatographic methods permitting the resolution of all of the C27 sterols in question. The results of recent experiments directed towards an understanding of the detailed metabolism of these compounds are presented herein.