Metabolism of conjugated sterols in eggplant. Part 2. Phospholipid : steryl glucoside acyltransferase.

A membrane-bound phospholipid : steryl glucoside acyltransferase from Solanum melongena leaves was partially purified and its specificity and molecular as well as kinetic properties were defined. Among the steryl glycosides tested (e.g. typical plant steryl glucosides, steryl galactosides and cholesteryl xyloside) the highest activity was found with cholesteryl glucoside, but some structurally related compounds such as sito- and stigmasteryl glucoside or galactoside as well as cholesteryl galactoside were also acylated, albeit at lower rates. The investigated enzyme was able to use all classes of phosphoglycerolipids (phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol) as an acyl source for biosynthesis of acylated steryl glucoside. Among them 1,2-dimirystoylphosphatidylic acid appeared to be the best acyl donor. Apart from phosphoglycerolipids, 1,2-diacylglycerols were also used as acyl donor for steryl glucoside acylation, although at a distinctly lower rate. The acyl moiety was transferred from the C-1 position of phospholipid molecule. The investigated acyltransferase activity was stimulated by 2-mercaptoethanol, Triton X-100, 1-monoacylglycerols and inhibited in the presence of divalent cations such as Ca(2+), Mn(2+), Zn(2+) or Co(2+), some lipids (MDGD, ceramide), detergents (Tween 20, 40, 60 and 80, Tyloxapol, sodium deoxycholate) and high ionic strength.

Storage Does Not Influence Steryl Glucoside Concentration in Cycas micronesica Seeds

The concentration of two steryl glucosides and their sterol precursors were measured in gametophyte tissue of Cycas micronesica K.D. Hill seeds before and after two storage treatments. For one study, intact 14-month-old seeds were stored at room temperature for 10 months. For the second study, cleaned (sarcotesta tissue removed) 22-month-old seeds were sown in perlite propagation beds, and ungerminated seeds were harvested after 14 months. Concentration of the steryl glucosides and sterols in the gametophyte tissue did not differ between the fresh seeds and the seeds after storage. The function of these metabolites is not known, but their unexpected stability for up to 14 months of storage indicates they are not metabolized during storage and may be vital during germination and early seedling growth. Implications on human exposure to their neurotoxic effects from gametophyte flour ingestion are discussed.

Compartmentation and topology of glucosylceramide synthesis

Evidence is presented supporting a model for glucosylceramide formation on the apoplastic side of the plasma membrane in plants. Glucosylceramide synthase and sterol glucosyltransferase were both localized to the plasma membrane. Whereas sterol glucosylation was sensitive to proteolytic enzymes, ceramide glucosylation was not. These results are consistent with our model in which steryl glucoside is synthesized on the cytosolic side of the membrane and then translocated across the membrane where it donates glucose to ceramide.