Structure of the Carbohydrate Units of IgA, Immunoglobulin

The carbohydrate composition of an IgA (a, subtype) myeloma protein has been determined. The carbohydrate, present only on the heavy chain, was found to consist of 3 moles of sialic acid, 9 mole of galactose, 5.4 moles of mannose, 0.8 moles of fucose, 8.8 moles of N-acetylglucosamine, and 5 moles of N-acetylgalactosamine per mole of heavy chain. Four major glycopeptide-containing fractions were isolated following pronase degradation of the protein. Glycopeptide I consisted of galactose, N-acetylgalactosamine, threonine, serine, and proline in the molar ratio 4:s :4:5 :9. Each of the N-acetylgalactosamine residues was found to be involved in an 0-glycosidic linkage, demonstrating that there are five 0-glycosidically linked oligosaccharide units per heavy chain. The structure of this glycopeptide is presented in the following paper (BAENZIGER, J., AND KORNFELD, S. (1974) J. Biol. Chem. 249, 7270-7281. The three other glycopeptide fractions contained 1 to 2 residues of sialic acid, 0 to 0.7 residue of fucose, 2 residues of galactose, 3 residues of mannose, 4 to 5 residues of Nacetylglucosamine, and 1 residue of asparagine. Glycopeptide IIA was a homogeneous glycopeptide with the following structure: