Quantification of the α2-6 Sialic Acid Linkage in Branched N-Glycan Structures with Capillary Nanogel Electrophoresis

Sialylation and sialic acid linkage in N-glycans are markers of disease but are analytically challenging to quantify. A capillary electrophoresis method is reported that integrates a unique combination of enzymes and lectins to modify sialylated N-glycans in real time in the capillary so that N-glycan structures containing alpha 2-6-linked sialic acid are easily separated, detected, and quantified. In this study, N-glycans were sequentially cleaved by enzymes at the head of the separation capillary so that the presence of alpha 2-6-linked sialic acids corresponded to a shift in the analyte migration time in a manner that enabled interpretation of the N-glycan structure. Following injection, only afucosylated N-glycan structures were passed through enzyme zones that contained alpha 2-3 sialidase, followed by beta 1-3,4 galactosidase, which cleaved any terminal alpha 2-3-linked sialic acid and underlying galactose yielding a terminal N-acetyl glucosamine. With this treatment complete, a third zone of alpha 2-3,6,8 sialidase converted the remaining alpha 2-6-linked sialic acid to terminal galactose. With these enzyme processing steps the alpha 2-6-linked sialic acid residues on an N-glycan correlated directly to the number of terminal galactose residues that remained. The number of terminal galactose residues could be interpreted as a stepwise decrease in the migration time. Complex N-glycans from alpha-1-acid glycoprotein were analyzed using this approach, revealing that a limited number of alpha 2-6 linked sialic acids were present with biantennary, triantennary, and tetraantennary N-glycans of alpha-1-acid glycoprotein generally containing 0 or 1 alpha 2-6-linked sialic acid.