The Biosynthesis of Steryl Glucosides in Plants

Mitochondrial preparations from pea root (Pisum sativum L. var. Alaska) cauliflower inflorescence (Brassica cauliflora Gars.) and avocado inner mesocarp (Persea americana Mill. var. Fuerte), and chloroplast preparations from spinach leaf (Spinacia oleracea L. var. Bloomsdale) incorporate glucose into steryl glucoside and acylated steryl glucoside when either uridine diphosphate-glucose or uridine diphosphate-galactose is supplied as precursor. In the case of pea root mitochondria, galactosyl diglycerides are not formed from either nucleotide sugar. In the case of spinach chloroplasts only 3% of the metabolized uridine diphosphate-galactose is found as steryl glycosides. Time course experiments indicate that the steryl glucoside is the precursor of the acylated steryl glucoside. The effect of pH on the over-all reaction and analysis of the reaction products suggest that the glucosylation of the sterol has a pH optimum of 8 to 9, and the pH optimum for the acylation of the steryl glucoside is 6.5 to 7. The synthesis of steryl glucoside and acylated steryl glucoside, catalyzed by acetone powders of pea root mitochondria, is stimuated by added sitosterol and stigmasterol.