Mechanism of the second methylation in sitosterol side-chain biosynthesis in higher plants: metabolic fate of 28-hydrogens of 24-methylenecholesterol in Morus alba cell cultures

Biosynthesis of the side-chain of sitosterol in higher plants involves two methylation steps by attack of S-adenosylmethionine. The stereochemical features of the second methylation, namely, of the conversion from 24-methylenecholesterol to isofucosterol in higher plants has been investigated. Feeding studies of synthesized [28E-2H]- and [28Z-2H]-24-methylenecholesterols to cultured cells of Morus alba followed by 2H NMR analysis of the resulting isofucosterol established that the second methylation proceeded in such a manner that addition of the methyl group and proton loss occur on opposite faces of the original Δ24(28)-double bond.