Biosynthesis of sitosterol, cycloartenol, and 24-methylenecycloartanol in tissue cultures of higher plants and of ergosterol in yeast from [1,2-13C2]- and [2-13C2H3]-acetate and [5-13C2H2]MVA

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Abstract
The [1,2]-methyl migrations postulated in the ‘biogenetic isoprene rule’ proposed by Ruzicka et al. have been verified by 13C n.m.r. spectroscopy in the biosynthesis of cycloartenol (10a), 24-methylenecycloartanol (11a), and sitosterol (12a) using cultured cells of higher plants,Rabdosia japonica and Physalis peruviana, and of ergosterol (14a) in yeast fed with [1,2-13C2] acetate. The [1,2]-hydride shifts from C-17 to C-20, and C-13 to C-17 have also been demonstrated in the biosynthesis of sitosterol (12b) in R. japonica and of ergosterol (14b) in yeast fed with [2-13C2H3]acetate. The [1,2]-hydride shift from C-9 to C-8 has also been verified in 24-methylenecycloartanol (11b) fed [2-13C2H3]acetate to tissue cultures of Trichosanthes kirilowii Maxim. var. japonica. In the side-chain formation of 24-methylenecycloartanol (11b) and ergosterol (14b), a [1,2]-hydride (deuteride) shift from C-24 to C-25 is observed. Conversely, no deuterium atom at C-24 or C-25 is observed in sitosterol (12b) formation. Both C-11 and C-12 of sitosterol (12c) labelled as 13C-2H2 and 13C-2H1H, biosynthesized from [5-13C2H2]MVA in R. japonica suggest that squalene is released from an enzyme and the following oxidation does not distinguish a terminal double bond of one farnesyl moiety from the other to form epoxysqualenes (8A) and (8B).

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