Structure of the Carbohydrate Units of IgA, Immunoglobulin I. COMPOSITION, GLYCOPEPTIDE ISOLATION, AND STRUCTURE OF THE ASPARAGINE-LINKED OLIGOSACCHARIDE

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: