Enzymatic excision of glucosyl units linked to the oligosaccharide chains of glycoproteins.

Studies in the accompanying paper (Chen, W. W., and Lennarz, W. J. (1978) J. Biol. Chem. 253, 5774-5779) showed that hen oviduct membranes catalyze synthesis of a Glc-containing oligosaccharide-lipid and that the oligosaccharide moiety of this compound is transferred en bloc to an endogenous protein as well as to an exogenous, soluble protein. In this study we have established that the endogeneous proteins in oviduct membranes are devoid of Glc. In view of these findings, the possibility that Glc was excised from the oligosaccharide chain after it had been transferred to protein was examined. Kinetic studies involving the use of endogenous membrane proteins as acceptors of the oligosaccharide chain suggested that the Glc residues were released without significant degradation of the core oligosaccharide. Direct evidence for the presence of a membrane-bound glucosidase was obtained using either free, Glc-containing oligosaccharide or Glc-containing oligosaccharide linked to S-carboxymethylated alpha-lactalbumin as substrates. Experiments utilizing [3H]Glc- and [14C]GlcNAc-labeled, glycosylated S-carboxymethylated alpha-lactalbumin established that, under conditions leading to extensive removal of [3H]Glc, there was essentially no degradation of the [14C]GlcNAc-labeled oligosaccharide core.