Transfer of glucose in the biosynthesis of thyroid glycoproteins. II. Possibility of a direct transfer of glucose from UDP-glucose to proteins.

When thyroid rough microsomes are incubated with labeled UDPglucose, extensive labeling occurs on endogenous protein. The present study reports various time-course experiments of glucose incorporation from UDPglucose either into endogenous acceptors or into exogenous synthetic peptide, whether or not conditions are used which depress dolichol-P-glucose synthesis. The results are not compatible with the presently accepted idea of oligosaccharide lipids as sole donors of labeled glucose to proteins. Some structural properties of glucose-labeled oligosaccharides released from proteins by endo-beta-N-acetylglucosaminidase H were also investigated. They appeared to be of a single species and less degradable by alpha-mannosidase than their lipid-linked counterpart. Since glucose-labeled glycopeptides were found susceptible to a beta-glucosidase, it is proposed that some labeled glucose residues are involved in beta-linkages. The possibility is raised that glucose might be directly transferred from UDPglucose to endogenous glycoproteins.