Steryl glycoside biosynthesis

Abstract

The existence of steroid glycosides has been known for many years and, more recently, their derivatives have been described. Steroid glycosides have been isolated from a number of organisms, however, the largest number of these compounds are found in plants. As to glycoside biosynthesis, the sterols are the most extensively studied steroid group. Of the sterols, only the 4-demethyl sterols have been isolated as glycosides. The glycosidic bond formation is mediated through nucleotide sugars, and UDP-glucose appears to be the most active glycosyl donor. In cell-free studies, the pH of the incubation medium is quite critical and depends on the tissue under investigation, but generally the optimum is near pH 7.0. Formation of steryl glycosides is particulate in nature and is stimulated by ATP, Ca²⁺, and Mg²⁺. Similar results are obtained, regardless whether the sterol or the sugar moiety is labeled. Formation of acylsteryl glycosides could occur via two pathways: through the acylation of steryl glycosides or through the transfer of an acylglycosyl unit to a sterol moiety. Results from in vitro experiments suggest that acylsteryl glycoside formation occurs via steryl glycosides. Two acyl transfer reactions have been demonstrated; one is microsomal in nature and involves phosphatidylethanolamine, while the other reaction involves a soluble enzyme and requires galactolipids. In vivo experiments, however, indicate that a second pathway may also exist. If cholesterol-4-¹⁴C is used as substrate, a highly radioactive component can be isolated which is readily converted to acylsteryl glycoside, but not to free sterol or steryl glycoside. It is suggested that this component is an intermediate in acylsteryl glycoside biocynthesis. At present, the nature of the component is unknown. It is quite stable, and acid hydrolysis produces free sterol. Saponification produces two products which in thin layer chromatograms closely resemble acylsteryl glycoside.