Sialylated N-glycans in adult rat brain tissueÐa widespread distribution of disialylated antennae in complex and hybrid structures

This paper extends our earlier work on the analysis of neutral N-glycans from adult rat brain to glycans carrying NeuAc residues as their sole charged groups. These structures comprised at least 40% of the total (acidic and neutral) N-glycan pool. Compounds were identified by a combination of endogly-cosidase and exoglycosidase digestions, anion-exchange chromatography, normal and reverse-phase high-performance liquid chromatography, matrix-assisted laser desorption/ionisation-mass spectrometry and combined gas chromatography/mass spectrometry. Mono-, di- and trisialylated components, together with components substituted with four (or more) NeuAc residues, showed abundances of approximately 12, 10, 7 and 7%, respectively, relative to the total N-glycan pool. In addition, neuraminidase digestion resulted in the neutralisation of a fraction of highly charged species, possibly indicating the presence of N-glycans substituted with short chains of polysialic acid. Sialylated bi-, tri- [mainly the (2,4)-branched isomer], tetraantennary complex, polylactosamine and hybrid structures were detected. Typically, for ‘brain-type ’ N-glycosylation, these sialylated structures were variously modified by the presence of core A126-linked and outer-arm A123-linked fucose residues and by a bisecting GlcNAc. Structural groups such as sialyl Lewisx and NeuAcA223 substituted Galâ124GlcNAc antennae were common. In contrast to the neutral glycans, however, a widespread distribution of terminal â123-linked galactose residue