Structure of the carbohydrate units of IgA₁ immunoglobulin. I. Composition, glycopeptide isolation, and structure of the asparagine linked oligosaccharide units

The carbohydrate composition of an IgA (α₁ subtype) myeloma protein was determined. The carbohydrate, present only on the heavy chain, was found to consist of 3 moles of sialic acid, 9 moles of galactose, 5.4 moles of mannose, 0.8 moles of fucose, 8.8 moles of N acetylglucosaine, 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 4:5:4:5:9. Each of the N acetylgalactosamine residues was found to be involved in an O glycosidic linkage, demonstrating that there are 5 O glycosidically linked oligosaccharide units per heavy chain. The structure of this glycopeptide is presented in a following paper. The 3 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 N acetylglucosamine, and 1 residue of asparagine. Glycopeptide IIA was a homogeneous glycopeptide. Glycopeptide fractions IIB and IIC did not appear to be homogeneous, but instead each contained 2 glycopeptides. In both cases, one of the glycopeptides was probably the disialyl form of IIA containing a residue of sialic acid linked α2,6 to the terminal galactose. The other glycopeptide had 2 rather than 3 nonreducing termini arising from the core mannose. Both of these terminated in sialic acid and had the same sequence as that found for the sialic acid containing terminus of IIA. These termini arose from positions 3 and 6 of the core mannose. The core of this glycopeptide differed from that of IIA in having a residue of fucose which was most likely linked α1,6 to the N acetylglucosamine involved in the N glycosidic linkage to the peptide.