Enzymatic synthesis of a glucose-containing oligosaccharide-lipid involved in glycosylation of proteins

Abstract

Earlier studies on the biosynthesis of an oligosaccharide-lipid with the structure (alpha-Man)4-6-beta-Man-(1 leads to 4)-beta-GlcNAc-(1 leads to 4)-GlcNAc-P-P-dolichol have been extended to show that enzymes in membrane preparations of hen oviduct catalyze synthesis of a Glc-containing oligosaccharide-lipid that is similar in properties to the Glc-free oligosaccharide-lipid. Conditions that enable enzymatic preparation of Man-, GlcNAc-, or Glc-labeled oligosaccharide-lipid have been established. Experiments with isolated [Glc-14C]oligosaccharide-lipid as substrate demonstrated that the oligosaccharide chain was transferred en bloc from the lipid to an endogenous membrane protein of apparent Mr = 25,000. In addition, an exogenous soluble protein, S-carboxymethylated alpha-lactalbumin, was shown to serve as acceptor of the oligosaccharide chain from the Glc-containing oligosaccharide-lipid. Studies on the metabolic fate of the Glc residues in these proteins are reported in the accompanying paper (Chen, W. W., and Lennarz, W. J. (1978) J. Biol. Chem. 253, 5780-5785).