Glycosyl transfers from UDP-sugars to lipids of plant membranes: identification and specificity of the transferases.

Microsome preparations extracted from wheat roots or sycamore cell suspensions catalyzed the transfer of sugar from nucleotide-sugars to endogenous lipidic acceptors. The nature of the products biosynthesized from UDP-Glc, GDP-Glc, UDP-Gal, UDP-Xyl or UDP-Arab was examined. Sterylglycosides were obtained from UDP-Gglc, GDP-Glc or UDP-Xyl. Galactosyldiglycerides were synthesized from UDP-Gal. When UDP-Glc or UDP-Gal was used as a substrate, a membrane-bound 4-epimerase interconverted the epimeric nucleotide-sugars, thereby allowing the simultaneous biosynthesis of galactosyldiglycerides and sterylglucosides. The biosynthesis of free and acylated sterylglucosides from UDP-Glc, without interference of other glycosyl transfer reactions, was obtained by the omission of Mg++ ions from the incubation medium. The biosynthesis of galactosyldiglycerides from UDP-Gal without interference of other transfer reactions was obtained when digitonin was added to the incubation medium of sycamore microsomes.