Analysis of protein glycosylation by mass spectrometry

There is a growing pharmaceutical market for protein-based drugs for use in therapy and diagnosis. The rapid developments in molecular and cell biology have resulted in production of expression systems for manufacturing of recombinant proteins and monoclonal antibodies. These proteins are glycosylated when expressed in cell systems with glycosylation ability. For glycoproteins intended for therapeutic administration it is important to have knowledge about the structure of the carbohydrate side chains to avoid cell systems that produce structures, which in humans can cause undesired reactions, e.g., immunological and unfavorable serum clearance rate. Structural analysis of glycoprotein oligosaccharides requires sophisticated instruments like mass spectrometers and nuclear magnetic resonance spectrometers. However, before the structural analysis can be conducted, the carbohydrate chains have to be released from the protein and purified to homogeneity, and this is often the most time-consuming step. Mass spectrometry has played and still plays an important role in analysis of protein glycosylation. The superior sensitivity compared to other spectroscopic methods is its main asset. Structural analysis of carbohydrates faces several problems, however, due to the chemical nature of the constituent monosaccharide residues. For oligosaccharides or glycoconjugates, the structural information from mass spectrometry is essentially limited to monosaccharide sequence, molecular weight, an only in exceptional cases glycosidic linkage positions can be obtained. In order to completely establish an oligosaccharide structure, several other structural parameters have to be determined, e.g., linkage positions, anomeric configuration and identification of the monosaccharide building blocks. One way to address some of these problems is to work on chemical pretreatment of the glycoconjugate, to specifically modify the carbohydrate chain. In order to introduce specific modifications, we have used periodate oxidation and trifluoroacetolysis with the objective of determining glycosidic linkage positions by mass spectrometry.

Mass spectrometry of high-mannose oligosaccharides after trifluoroacetolysis and periodate oxidation

High-mannose oligosaccharides were treated with a mixture of trifluoroacetic acid and trifluoroacetic anhydride under conditions where reducing terminal N-acetylglucosamine residues are specifically degraded. The resulting mannose-containing products were, after further chemical modifications, analyzed by mass spectrometry in fast atom bombardment and electron ionization modes. Binding positions between monosaccharide residues were deduced from mass spectra of peracetylated compounds, which, prior to the derivatization, had been subjected to periodate oxidation and borodeuteride reduction.

Mass spectrometry of hexosamine containing oligosaccharides as permethylated N-trifluoroacetyl derivatives

Permethylated N-trifluoroacetyl hexosamine containing oligosaccharide alditols are favorable for analysis by mass spectrometry. Electron impact and fast atom bombardment mass spectra of these derivatives are characterized by strong primary and secondary fragments in which a positive charge is localized on the N-trifluoroacetyl hexosamine residue. From the mass spectra the monosaccharide sequence and the position(s) of substitution of a N-trifluoroacetyl hexosamine can be determined.

Structural studies on the O-glycosidically linked carbohydrate chains of glucoamylase G1 from Aspergillus niger

Glucoamylase G1 from Aspergillus niger contains an unusual type of carbohydrate-protein linkage, involving mannose O-glycosidically linked to serine and threonine. The majority of the neutral oligosaccharides of glucoamylase G1 are located in a region of about 70 amino acid residues which carries about 35 oligosaccharide units [(1983) Carlsberg Res. Commun. 48, 517-527]. Structural analysis was performed on the O-linked carbohydrates of a tryptic fragment from glucoamylase G1 comprising the segment characterized by a high degree of glycosylation. The carbohydrate structures released by trifluoroacetolysis were elucidated using sugar analysis, methylation analysis, mass spectrometry, chromium trioxide oxidation, digestion with alpha-mannosidase and 1H-NMR spectroscopy. The following structures could be identified. (formula; see text)

Degradation of chondroitin 4-sulphate by tri-fluoroacetolysis: isolation of oligosaccharides from the carbohydrate-protein linkage region

Oligosaccharides from the linkage-region tetrasaccharide, beta-D-GlcpA-(1----3)-beta-D-Galp-(1----3)-beta-D-Galp-(1----4)-D- Xylp, of chondroitin 4-sulphate were isolated after trifluoroacetolysis. The oligosaccharides were purified by ion-exchange chromatography and paper chromatography and subjected to sugar and methylation analysis and g.l.c.-m.s. The recovery of linkage-region oligosaccharides was approximately 45% after trifluoroacetolysis, calculated according to the D-xylose present in the chondroitin 4-sulphate preparation. The following structures were identified: beta-D-Galp-(1----4)-D-Xylp, beta-D-Galp-(1----3)-D-Galp, beta-D-Galp-(1----3)-beta-D-Galp-(1----4)-D-Xylp, beta-D-GlcpA-(1----3)-beta-D-Galp-(1----3)-beta-D-Galp-(1----4)-D- Xylp.

Structural studies on the carbohydrate portion of human alpha 1-microglobulin

Human alpha 1-microglobulin has been shown to have three N-glycosidically linked carbohydrate chains of the following structure: (formula: see text) This structure was established by sugar and methylation analysis before and after removal of the N-acetylneuraminic acid residue and by using specific chemical degradation techniques such as Smith degradation, chromium trioxide oxidation and trifluoroacetolysis.