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Molecular & Cellular Proteomics 6:2132-2138, 2007.
© 2007 by The American Society for Biochemistry and Molecular Biology, Inc.

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Research

N-Glycosylation Site Occupancy in Serum Glycoproteins Using Multiple Reaction Monitoring Liquid Chromatography-Mass Spectrometry^*

Andreas J. Hülsmeier^,, Patricie Paesold-Burda^¶ and Thierry Hennet

From the Institute of Physiology and Center for Integrative Human Physiology, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland and ^¶ Division of Metabolism and Molecular Pediatrics, University Children's Hospital Zürich, Steinwiesstrasse 75, 8032 Zürich, Switzerland

Congenital disorders of glycosylation (CDGs) are a family of N-linked glycosylation defects associated with severe clinical manifestations. In CDG type-I, deficiency of lipid-linked oligosaccharide assembly leads to the underoccupancy of N-glycosylation sites on glycoproteins. Although the level of residual glycosylation activity is known to correlate with the clinical phenotype linked to individual CDG mutations, it is not known whether the degree of N-glycosylation site occupancy by itself correlates with the severity of the disease. To quantify the extent of underglycosylation in healthy control and in CDG samples, we developed a quantitative method of N-glycosylation site occupancy based on multiple reaction monitoring LC-MS/MS. Using isotopically labeled standard peptides, we directly quantified the level of N-glycosylation site occupancy on selected serum proteins. In healthy control samples, we determined 98–100% occupancy for all N-glycosylation sites of transferrin and ₁-antitrypsin. In CDG type-I samples, we observed a reduction in N-glycosylation site occupancy that correlated with the severity of the disease. In addition, we noticed a selective underglycosylation of N-glycosylation sites, indicating preferential glycosylation of acceptor sequons of a given glycoprotein. In transferrin, a preferred occupancy for the first N-glycosylation site was observed, and a decreasing preference for the first, third, and second N-glycosylation sites was observed in ₁-antitrypsin. This multiple reaction monitoring LC-MS/MS method can be extended to multiple glycoproteins, thereby enabling a glycoproteomics survey of N-glycosylation site occupancies in biological samples.

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