Structural study of the carbohydrate moieties of two human immunoglobulin subclasses (IgG2 and IgG4)

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Abstract
Asparagine-linked sugar chains were quantitatively released as oligosaccharides from human IgG2 and IgG4 myeloma proteins by hydrazinolysis followed byN-acetylation and NaB3H4 reduction. Each oligosaccharide was isolated by serial lectin column chromatography. Study of their structures by sequential exoglycosidase digestion and methylation analysis, revealed that all of them were of the bi-antennary complex-type containing Manα1-6(±GlcNacß1-4)(Manα1-3)Manß1-4GlcNAcß1-4(±Fucα1-6)GlcNAc, as core structures, and GlcNAcß1-, Galß1-4GlcNacß1- and Siaα2-6Galß1- in their outer chain moieties. However, the molar ratio of each oligosaccharide was different in each IgG sample, indicating that clonal variation is included in the sugar chain moieties of IgG molecules. One of the IgG2 contained four asparagine-linked sugar chains in one molecule, two on the Fc fragment and the remainder on the Fab fragment. The sugar chains in the Fc fragment contained much less galactose as compared with the Fab fragment.