Effects of UDP-glucose addition on the synthesis of mannosyl lipid-linked oligosaccharides by cell-free fibroblast preparations.

The pattern of mannosyl lipid-linked oligosaccharides synthesized by cell-free enzyme preparations from cultured fibroblasts is altered substantially when 0.2 muM UDP-glucose is added to the incubation medium. Inclusion of UDP-glucose results in the appearance of a new labeled oligosaccharide, which is 1 or 2 glycose units larger than the lipid-linked oligosaccharide synthesized in the presence of only GDP-mannose (2 muM) and UDP-N-acetylglucosamine (20 muM). Label from UDP-[3H]glucose is incorporated into the same larger oligosaccharide size class. The results can be explained most easily by assuming that the new mannosyl lipid-linked oligosaccharide contains 1 or 2 glucose residues in addition to 5 to 6 mannose residues. The results are compatible with the recent finding by Spiro et al. (Spiro, M.J., Spiro, R.G., and Bhoyroo, V.D. (1976) Fed Proc. 35, 1375; Spiro M.J. Spiro, R.G., and Bhoyroo, V.D. (1976) J. Biol. Chem. 251, 6400-6408; Spiro, R.G., and Spiro, M.J. AND Bhoyroo, V.D. (1976) J. Biol. Chem. 251, 6409-6419; Spiro, M.J., Spiro R.G., and Bhoyroo, V.D. (1976) J. Biol. Chem. 251, 6420-6425) that naturally occurring mannosyl lipid-linked oligosaccharides contain glucose. In addition to being incorporated into lipid oligosaccharides, glucose residues are also incorporated into endogenous glycoproteins. Incorporation of glucose into glycoproteins that give rise to pronase glycopeptides of the typical asparagine-linked size classes is almost completely dependent on the presence of GDP-mannose.