Lectin analysis of human immunoglobulin G N-glycan sialylation

The lectins Sambucus nigra agglutinin (SNA) and Ricinus communis agglutinin (RCA), specific for alpha2,6 linked sialylation, and terminal galactose re spectively were used to study the occurrence, linkage and distribution of h uman immunoglobulin G (IgG) sialylation. SNA was shown to bind N-glycan alp ha2,6-linked sialic acid only. Sialidase analysis confirmed that this is th e dominant, if not exclusive linkage. Total IgG sialylation was estimated a t 1.0 mug SA/mg IgG (or about 0.5 mole per mole) using a biochemical sialic acid assay. SNA displayed strong binding to the IgG Fab fragment in both i ts native and denatured state. In contrast, SNA failed to bind the IgG Fc f ragment in its native form, but displayed strong binding after the Fc was d enatured. This allowed the construction of quantitative assays capable of m easuring both IgG Fab and Fc alpha2,6-sialylation without the need for enzy matic peptide digestion.