Intact Human Alpha-Acid Glycoprotein Analyzed by ESI-qTOF-MS: Simultaneous Determination of the Glycan Composition of Multiple Glycosylation Sites.

Alpha-1-acid glycoprotein (AGP) is a highly glycosylated protein from human plasma with five N-type glycosylation sites carrying dominantly higher antennary structures and thus represents a challenging target for characterization of glycan heterogeneity. Here, we show that glycan composition over all five glycosylation sites can be determined quantitatively by ESI-qTOF-MS of the intact glycoprotein in negative ion mode. We find numerous glycan species extending the mass range of the glycoprotein species from 35.0 to 38.5 kDa. The dominant glycan compositions contain tri- and tetraantennary structures on all glycosylation sites. The mass degeneracy of two fucosyl units versus one sialic acid was resolved by treating the sample with sialidase and analyzing the resulting desialylated AGP by electrospray ionization-mass spectrometry in positive ion mode. The pattern of nonsialylated oligosaccharides was used for interpretation of the fully sialylated species using bioinformatics tools. From pooled human plasma, we find 90, 101, and 64 different glycan compositions for genetic variants ORM1*F1, ORM1*S, and ORM2, respectively. Glycan structures carry dominantly between 15 and 16 sialic acids indicating an almost complete termination of all antenae with sialic acid. AGP from human plasma samples of single individuals was analyzed as desialylated glycoproteins and showed variations in fucosylation and in the amount of antennary structures between individuals.