Biosynthesis of the 24-methylcholesterols dihydrobrassicasterol and campesterol in cultured cells of Amsonia elliptica: incorporation of [1,2-13C2]acetate and [2-13C,2H3]acetate
Feeding experiments with [1,2-13C2]acetate and [2-13C,2H3]acetate in suspension cultures of A. elliptica demonstrate that campesterol has C-26 (the pro-R methyl group at C-25) and C-27 (the pro-S methyl group at C-25) arising from C-6 and C-2 of mevalonate (MVA), respectively. In contrast, dihydrobrassicasterol has C-26 and C-27 derived from C-2 and C-6 of MVA, respectively. In both 24-methylcholesterols, the methyl group derived fron C-2 and C-6 of MVA retained two and three deuterium atoms, respectively. Furthermore, no signal due to a deuterium atom at C-24 or C-25 originating from [2-13C,2H3]acetate was found for campesterol and dihydrobrassicasterol. These data suggest that both campesterol and dihydrobrassicasterol are biosynthesized by regio-facial specific reduction of the 24(25)-double bond of the 24-methyl-24(25)-ene side-chain which is formed by double-bond isomerization of Δ24(28)-sterol.