The carbohydrate moiety of immunoglobulins: biosynthesis and function



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A model immunological system was established. It was found to contain a pool of K-chains to which glucose would be added as 89% sialic acid at the site of the plasma membrane. Several immunosuppressive reagents were used and their effect upon glucose incorporation into K-chains was determined. Methylthioinosine periodate oxidation product was found to suppress intracellular attachment of carbohydrate to L-chains, but it did not affect the plasma membrane or extracellular incorporation. Puromycin inhibited both protein synthesis and carbohydrate attachment. Arabinosylcytosine showed no effect on either protein or glucose incorporation. Plasma membranes were isolated and used as a sialyltransferase source to attach sialic acid to desialyzed K-chains. The role of the sialic acid and ß-galactosidase residues in IgG was not in antibody specificity or activity as detected by complement fixation and hemagglutination inhibition titers. In IgM, the carbohydrate moiety appears to play a role in conformation of the pentamer. More interestingly, the mannose and glucosamine residues of the Fc region function in noncovalent association of monomers into a pentameric molecule.