The Structure of the Glycopeptides of a Human γM-Immunoglobulin

Abstract Seven glycopeptide fractions, representing five different glycopeptides, were isolated from a γM-immunoglobulin obtained from a patient with Waldenstrom's macroglobulinemia. Two of the glycopeptides contained only mannose and N-acetylglucosamine residues in the molar ratio of 7:2, while the other three glycopeptides contained sialic acid, galactose, mannose, N-acetylglucosamine, and fucose in the molar ratio of 1:2:3 to 4:4 to 4.5:0.5. The structure of the oligosaccharide portion of four of the glycopeptides was investigated as follows. Sequential enzymatic degradation of the oligosaccharide chains with purified glycosidases helped to determine the sequences of the sugars. The linkages between the sugars were established by serial periodate oxidation and by methylation of the glycopeptides, followed by identification of the alditol acetate derivatives of the methylated sugars by gas chromatography and mass spectrometry. The methylation data demonstrated the existence of microheterogeneity in one of the oligosaccharide units which contains only mannose and N-acetylglucosamine residues. Microheterogeneity was also shown to occur in the outer branches of the more complex oligosaccharide units which lacked a full complement of fucose and sialic acid residue. Partial structures are proposed for four of the glycopeptides.