Biosynthesis of beta-sitosterol and stigmasterol in Croton sublyratus proceeds via a mixed origin of isoprene units.

A green callus culture of Croton sublyratus Kurz established from the leaf explants appeared to actively synthesize two well-known phytosterols, beta-sitosterol and stigmasterol. The phytosterol biosynthesis was highly active during the linear phase of the culture. Feeding of [1-13C]glucose into the callus culture at this growth phase showed that the label from glucose was highly incorporated into both phytosterols. Isolation of the labeled products followed by 13C NMR analysis revealed that the phytosterols had their 13C-labeling patterns consistent with the acquisition of isoprene units via both the mevalonate pathway and the deoxyxylulose pathway with relatively equal contribution. Since the biosynthesis of phytosterol has so far been reported to be mainly from the classical mevalonate pathway, this study provides a new evidence on the biosynthesis of phytosterols via the novel deoxyxylulose pathway.