β-Sitosterol is one of several phytosterols with chemical structures similar to that of cholesterol. Sitosterols are white, waxy powders with a characteristic odor. They are hydrophobic and soluble in alcohols. The regulation of the biosynthesis of both sterols and some specific lipids occurs during membrane biogenesis. Through 13C-labeling patterns, it has been determined that both the mevalonate and deoxyxylulose pathways are involved in the formation of β-sitosterol. The precise mechanism of β-sitosterol formation varies according to the organism, but is generally found to come from cycloartenol. The biosynthesis of cycloartenol begins as one molecule of isopentenyl diphosphate and two molecules of dimethylallyl diphosphate form farnesyl diphosphate. Two molecules of FPP are then joined tail-to-tail to yield squalene, a triterpene. Squalene, through a cyclization reaction with 2,3-oxidosqualene 6 as an intermediate forms cycloartenol.
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