The elimination of O-linked glycans from glycoproteins under non-reducing conditions

Simple Routes to Stable Isotope-Coded Native Glycans

Understanding the biological role of protein-linked glycans requires the reliable identification of glycans. Isomer separation and characterization often entail mass spectrometric detection preceded by high-performance chromatography on porous graphitic carbon. To this end, stable isotope-labeled glycans have emerged as powerful tools for retention time normalization. Hitherto, such standards were obtained by chemoenzymatic or purely enzymatic methods, which introduce, e.g., 13C-containing N-acetyl groups or galactose into native glycans. Glycan release with anhydrous hydrazine opens another route for heavy isotope introduction via concomitant de-N-acetylation. Here, we describe that de-N-acetylation can also be achieved with hydrazine hydrate, which is a more affordable and less hazardous reagent. Despite the slower reaction rate, complete conversion is achievable in 72 h at 100 °C for glycans with biantennary glycans with or without sialic acids. Shorter incubation times allow for the isolation of intermediate products with a defined degree of free amino groups, facilitating introduction of different numbers of heavy isotopes. Mass encoded glycans obtained by this versatile approach can serve a broad range of applications, e.g., as internal standards for isomer-specific studies of N-glycans, O-glycans, and human milk oligosaccharide by LC-MS on either porous graphitic carbon or─following permethylation─on reversed phase.

Analytical Scheme Leading to Integrated High-Sensitivity Profiling of Glycosphingolipids Together with N- and O-Glycans from One Sample

Glycoconjugates are directly or indirectly involved in many biological processes. Due to their complex structures, the structural elucidation of glycans and the exploration of their role in biological systems have been challenging. Glycan pools generated through release from glycoprotein or glycolipid mixtures can often be very complex. For the sake of procedural simplicity, many glycan profiling studies choose to concentrate on a single class of glycoconjugates. In this paper, we demonstrate it feasible to cover glycosphingolipids, N-glycans, and O-glycans isolated from the same sample. Small volumes of human blood serum and ascites fluid as well as small mouse brain tissue samples are sufficient to profile sequentially glycans from all three classes of glycoconjugates and even positively identify some mixture components through MALDI-MS and LC-ESI-MS. The results show that comprehensive glycan profiles can be obtained from the equivalent of 500-μg protein starting material or possibly less. These methodological improvements can help accelerating future glycomic comprehensive studies, especially for precious clinical samples. Graphical Abstract Outline of glycan profiling procedures.

LW-AFC Effects on N-glycan Profile in Senescence-Accelerated Mouse Prone 8 Strain, a Mouse Model of Alzheimer's Disease

Glycosylation is one of the most common eukaryotic post-translational modifications, and aberrant glycosylation has been linked to many diseases. However, glycosylation and glycome analysis is a significantly challenging task. Although several lines of evidence have indicated that protein glycosylation is defective in Alzheimer’s disease (AD), only a few studies have focused on AD glycomics. The etiology of AD is unclear and there are no effective disease-modifying treatments for AD. In this study, we found that the object recognition memory, passive avoidance, and spatial learning and memory of senescence-accelerated mouse prone 8 (SAMP8) strain, an AD animal model, were deficient, and LW-AFC, which was prepared from the traditional Chinese medicine prescription Liuwei Dihuang decoction, showed beneficial effects on the deterioration of cognitive capability in SAMP8 mice. Forty-three and 56 N-glycan were identified in the cerebral cortex and serum of SAMP8 mice, respectively. The N-glycan profile in SAMP8 mice was significantly different from that of senescence accelerated mouse resistant 1 (SAMR1) strains, the control of SAMP8 mice. Treatment with LW-AFC modulated the abundance of 21 and 6 N-glycan in the cerebral cortex and serum of SAMP8 mice, respectively. The abundance of (Hex)3(HexNAc)5(Fuc)1(Neu5Ac)1 and (Hex)2(HexNAc)4 decreased in the cerebral cortex and serum of SAMP8 mice compared with SAMR1 mice, decreases that were significantly correlated with learning and memory measures. The administration of LW-AFC could reverse or increase these levels in SAMP8 mice. These results indicated that the effects of LW-AFC on cognitive impairments in SAMP8 mice might be through modulation of N-glycan patterns, and LW-AFC may be a potential anti-AD agent.

Improved nonreductive O-glycan release by hydrazinolysis with ethylenediaminetetraacetic acid addition

The study of protein O-glycosylation is receiving increasing attention in biological, medical, and biopharmaceutical research. Improved techniques are required to allow reproducible and quantitative analysis of O-glycans. An established approach for O-glycan analysis relies on their chemical release in high yield by hydrazinolysis, followed by fluorescent labeling at the reducing terminus and high-performance liquid chromatography (HPLC) profiling. However, an unwanted degradation known as "peeling" often compromises hydrazinolysis for O-glycan analysis. Here we addressed this problem using low-molarity solutions of ethylenediaminetetraacetic acid (EDTA) in hydrazine for O-glycan release. O-linked glycans from a range of different glycoproteins were analyzed, including bovine fetuin, bovine submaxillary gland mucin, and serum immunoglobulin A (IgA). The data for the O-glycans released by hydrazine with anhydrous EDTA or disodium salt dihydrate EDTA show high yields of the native O-glycans compared with the peeled product, resulting in a markedly increased robustness of the O-glycan profiling method. The presented method for O-glycan release demonstrates significant reduction in peeling and reduces the number of sample handling steps prior to release.

LC-MS3 quantification of O-glycopeptides in human serum

Quantitative analysis of site-specific glycosylation of proteins is a challenging part of glycoproteomic research. Multiple enrichment steps are typically required in the analytical workflows to achieve adequate characterization of the site-specific glycoforms. In spite of recent advances, quantitative workflows need further development. Here, we report a selective and sensitive MS2 followed by further fragmentation in the linear IT-MS analyzer (MS3) multiple reaction monitoring workflow mass spectrometric method for direct analysis of O-glycopeptides in difficult matrix such as serum. Method optimization was performed using two serum glycoproteins, hemopexin (HPX) and sex hormone binding globulin. With the optimized MS3 workflow, we were able to analyze major glycoforms of HPX directly in human serum. Quantification of the minor glycoforms of HPX and glycoforms of sex hormone binding globulin required enrichment of the protein because these analytes were below the sensitivity of the 4000 quadrupole ion trap hybrid mass spectrometer in the complex serum background. In conclusion, we present a quantitative method for site-specific analysis of O-glycosylation with general applicability to mucin-type glycoproteins. Our results document reliable application of the optimized MS3 multiple reaction monitoring workflow to the relative quantification of O-glycosylation microheterogeneity of HPX in human serum. Introduction of isotopically labeled standards would be desirable to achieve absolute quantification of the analytes. The possibility to analyze serum samples directly represents a significant improvement of the quantitative glycopeptide workflows with the potential for use in clinical applications.

One-pot nonreductive O-glycan release and labeling with 1-phenyl-3-methyl-5-pyrazolone followed by ESI-MS analysis

A novel one-pot procedure for the nonreductive release of O-linked glycans from glycoproteins and the simultaneous derivatization of released glycans with 1-phenyl-3-methyl-5-pyrazolone (PMP) is described. Unlike the traditional reductive β-elimination, which produces alditols, this new method employs PMP/ammonia aqueous solution as the reaction medium. The O-glycans are released from glycoproteins and derivatized with PMP nonreductively, specifically, and quantitatively. Samples can be easily purified from ammonia, excess PMP, and peptide residues by evaporation, chloroform extraction, and solid-phase extraction (SPE) column fractionation for HPLC, CE, or MS analysis. The procedure has been elaborated with two purified glycoproteins, porcine stomach mucin and bovine fetuin, and successfully applied to O-glycan profiling of a challenging biological specimen, healthy human plasma. This new procedure has shown methodological significance in O-glycan analysis.

Negative ion CID fragmentation of O-linked oligosaccharide aldoses--charge induced and charge remote fragmentation

Collision induced dissociation (CID) fragmentation was compared between reducing and reduced sulfated, sialylated, and neutral O-linked oligosaccharides. It was found that fragmentation of the [M - H](-) ions of aldoses with acidic residues gave unique Z-fragmentation of the reducing end GalNAc containing the acidic C-6 branch, where the entire C-3 branch was lost. This fragmentation pathway, which is not seen in the alditols, showed that the process involved charge remote fragmentation catalyzed by a reducing end acidic anomeric proton. With structures containing sialic acid on both the C-3 and C-6 branch, the [M - H](-) ions were dominated by the loss of sialic acid. This fragmentation pathway was also pronounced in the [M - 2H](2-) ions revealing both the C-6 Z-fragment plus its complementary C-3 C-fragment in addition to glycosidic and cross ring fragmentation. This generation of the Z/C-fragment pairs from GalNAc showed that the charges were not participating in their generation. Fragmentation of neutral aldoses showed pronounced Z-fragmentation believed to be generated by proton migration from the C-6 branch to the negatively charged GalNAc residue followed by charge remote fragmentation similar to the acidic oligosaccharides. In addition, A-type fragments generated by charge induced fragmentation of neutral oligosaccharides were observed when the charge migrated from C-1 of the GalNAc to the GlcNAc residue followed by rearrangement to accommodate the (0,2)A-fragmentation. LC-MS also showed that O-linked aldoses existed as interchangeable α/β pyranose anomers, in addition to a third isomer (25% of the total free aldose) believed to be the furanose form.

Application of the 50% Hydrazine Solution Method for O-Glycans Release, their Chemical Labeling, and HPLC Separation

Mucins are high molecular mass glycoproteins with oligosaccharides O-bonded to the protein core. beta-elimination is the most popular method used for releasing of O-glycans. However to such released glycoforms it is difficult to introduce a label to amplify a signal for oligosaccharide detection.In our study we used a combination of the beta-elimination and hydrazinolysis methods. Released glycoforms were labeled with para-amino benzooic acid ethyl ester (ABBE) and fractionated on HPLC column.This combined procedure seems to be a good tool for O-glycans analysis.

Probing mucin-type O-linked glycosylation in living animals

Changes in O-linked protein glycosylation are known to correlate with disease states but are difficult to monitor in a physiological setting because of a lack of experimental tools. Here, we report a technique for rapid profiling of O-linked glycoproteins in living animals by metabolic labeling with N-azidoacetylgalactosamine (GalNAz) followed by Staudinger ligation with phosphine probes. After injection of mice with a peracetylated form of GalNAz, azide-labeled glycoproteins were observed in a variety of tissues, including liver, kidney, and heart, in serum, and on isolated splenocytes. B cell glycoproteins were robustly labeled with GalNAz but T cell glycoproteins were not, suggesting fundamental differences in glycosylation machinery or metabolism. Furthermore, GalNAz-labeled B cells could be selectively targeted with a phosphine probe by Staudinger ligation within the living animal. Metabolic labeling with GalNAz followed by Staudinger ligation provides a means for proteomic analysis of this posttranslational modification and for identifying O-linked glycoprotein fingerprints associated with disease.

Mass spectrometry--a key technology in proteome research

The rapid developments in the field of mass spectrometry have transformed it into a key technology in proteome research. Increased sensitivity in mass spectrometry, as a result of more efficient ionisation techniques and better detectors, has allowed the stepwise reduction of protein quantity for analysis. Protein spots of 2D-PAGE separated samples are now quantitatively sufficient for an unequivocal identification of a protein by mass spectrometry. In addition to protein identification a closer look at posttranslational modifications is now also possible. It is speculated that modifications like phosphorylation or glycosylation exist on every second protein and that they are important for the protein function. This review highlights the different mass spectrometric methods and gives a brief overview of strategies and methods used to identify modifications.

The reaction of acetylacetone with amino sugars: implications for the formation of glycosylpyrazole derivatives

Glycosylpyrazoles are efficiently formed by reaction of saccharide hydrazones with pentan-2,4-dione (acetylacetone), but in aqueous buffer, pyrazole derivatives of amino sugars couple with a further equivalent of acetylacetone affording high yields of ketoenamines. These ketoenamines were considerably more stable than the ketoenamines formed from 2-amino-2-deoxy aldoses that have been described as intermediates in the classical Elson-Morgan reaction. Moreover, high yields of perketoenamine derivatives were achieved with oligosaccharides derived from hydrolysis of chitosan. The removal of the ketoenamine moieties to regenerate the free amine was readily accomplished with aqueous hydrazine.

Analysis of O-linked reducing oligosaccharides released by an in-line flow system

Reducing O-linked oligosaccharides from bovine submaxillary mucin, bovine fetuin, and porcine gastric mucin were recovered by nonreductive alkaline beta-elimination from an in-line flow system. Glycoproteins where attached to a solid support using hydrophobic interaction with alkali-resistant Poros reversed phase beads and a flow of alkali released the oligosaccharides. The alkali was subsequently neutralized by a continuous flow through cation exchange resin. The released oligosaccharides in the flow were trapped in a cartridge filled with graphitized carbon. Salt-free oligosaccharides could be recovered as a concentrated solution by elution with organic solvents from the cartridge. The glycosylation pattern of the released oligosaccharides was compared with the conventionally released and reduced oligosaccharides recovered from alkaline beta-elimination in the presence of borohydride. In general, the recovery from the in-line release was sometimes lower than from the reductive elimination method, but it was shown that alkaline degradation of reducing oligosaccharides was limited in this system. Liquid chromatography using graphitized carbon packing and high pH mobile phases together with negative ion electrospray mass spectrometry showed that both neutral and acidic reducing oligosaccharides could be analyzed in a single run. Reducing O-linked oligosaccharides could also be recovered in this way from human glycophorin separated by SDS-PAGE. The polyacrylamide was sufficient to retain the glycoprotein in the gel while the flow of alkali released the oligosaccharides. It was also shown that the alkaline conditions for releasing O-linked oligosaccharides from fetuin would partially release some N-linked oligosaccharides, particularly in the presence of reducing agent.

Heterocyclic derivatives of sugars: the formation of 1-glycosyl-3-methylpyrazol-5-ones from hydrazones

Conditions to effect the conversion of monosaccharide and disaccharide hydrazones to 1-glycosyl-3-methylpyrazol-5-ones were examined. The sugar pyrazolone derivatives were sensitive to oxidation, but high yields were achieved with 2,2,2-trifluoroethyl acetoacetate in mildly acidic solution. Azo coupling of the pyrazolones produced highly coloured azopyrazolone derivatives that prevented further degradation, and these may prove useful labels for chromatographic analysis of carbohydrates.

Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates

This review describes the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to carbohydrate analysis and covers the period 1991-1998. The technique is particularly valuable for carbohydrates because it enables underivatised, as well as derivatised compounds to be examined. The various MALDI matrices that have been used for carbohydrate analysis are described, and the use of derivatization for improving mass spectral detection limits is also discussed. Methods for sample preparation and for extracting carbohydrates from biological media prior to mass spectrometric analysis are compared with emphasis on highly sensitive mass spectrometric methods. Quantitative aspects of MALDI are covered with respect to the relationship between signal strength and both mass and compound structure. The value of mass measurements by MALDI to provide a carbohydrate composition is stressed, together with the ability of the technique to provide fragmentation spectra. The use of in-source and post-source decay and collision-induced fragmentation in this context is described with emphasis on ions that provide information on the linkage and branching patterns of carbohydrates. The use of MALDI mass spectrometry, linked with exoglycosidase sequencing, is described for N-linked glycans derived from glycoproteins, and methods for the analysis of O-linked glycans are also covered. The review ends with a description of various applications of the technique to carbohydrates found as constituents of glycoproteins, bacterial glycolipids, sphingolipids, and glycolipid anchors.

Structural heterogeneity in the core oligosaccharide of the S-layer glycoprotein from Aneurinibacillus thermoaerophilus DSM 10155

The surface layer glycoprotein of Aneurinibacillus thermoaerophilus DSM 10155 has a total carbohydrate content of 15% (by mass), consisting of O-linked oligosaccharide chains. After proteolytic digestion of the S-layer glycoprotein byPronase E and subsequent purification of the digestion products by gel permeation chromatography, chromatofocusing and high-performance liquid chromatography two glycopeptide pools A and B with identical glycans and the repeating unit structure -->4)-alpha-l-Rha p -(1-->3)-beta-d- glycero -d- manno -Hep p -(1--> (Kosma et al., 1995b, Glycobiology, 5, 791-796) were obtained. Combined evidence from modified Edman-degradation in combination with liquid chromatography electrospray mass-spectrometry and nuclear magnetic resonance spectroscopy revealed that both glycopeptides contain equal amounts of the complete core structure alpha-l-Rha p -(1-->3)-alpha-l-Rha p -(1-->3)-beta-d-Gal p NAc-(1-->O)-Thr/Ser and the truncated forms alpha-l-Rha p -(1-->3)-beta-d-Gal p NAc-(1-->O)-Thr/Ser and beta-d-Gal p NAc-(1-->O)-Thr/Ser. All glycopeptides possessed the novel linkage types beta-d-Gal p NAc-(1-->O)-Thr/Ser. The different cores were substituted with varying numbers of disaccharide repeating units. By 300 MHz proton nuclear magnetic resonance spectroscopy the complete carbohydrate core structure of the fluorescently labeled glyco-peptide B was determined after Smith-degradation of its glycan chain. The NMR data confirmed and complemented the results of the mass spectroscopy experiments. Based on the S-layer glycopeptide structure, a pathway for its biosynthesis is suggested.

Structural analysis of O-linked oligosaccharide-alditols by electrospray-tandem mass spectrometry after mild periodate oxidation and derivatization with 2-aminopyridine

O-linked oligosaccharide-alditols were analyzed by a combination of high-performance liquid chromatography (HPLC) and electrospray-tandem mass spectrometry (ESI-MS/MS). First, oligosaccharide-alditols were treated with sodium meta-periodate under conditions where core N-acetylgalactosaminitol is specifically degraded. The resulting fragments were labeled with 2-aminopyridine and purified on a reversed-phase column. Pyridylamino oligosaccharides yielded protonated molecular ions in positive-ion ES-MS and gave Y-series sequence ions, arising from glycosidic cleavages, by ESI-tandem mass spectrometry. Information on sugar sequence and branching of oligosaccharides linked at C6 and C3 to the N-acetylgalactosaminitol can be obtained. A systematic study of various oligosaccharide-alditols demonstrated that this approach constitutes a powerful tool for the structural characterization of O-glycans available only in limited quantities.

Biochemical and immunological analyses of the flagellin of Clostridium tyrobutyricum ATCC 25755

The monoclonal antibody 21E7-B12 (IgG3) can be used in a direct method of Clostridium tyrobutyricum detection based on an immunoenzymatic assay. Immunoelectron microscopy demonstrated that the 21E7-B12 antibody recognized the surface-exposed epitopes on the flagellar filaments of C. tyrobutyricum. After flagellar extraction, the purified flagellin showed an apparent molecular mass of 46 kDa with an isoelectric point of 3.6. Sugar staining, mild periodate oxidation and beta-elimination experiments showed that the flagellin was glycosylated and that the 21E7-B12 epitope was located in the sugar moiety. Amino acid composition showed that the flagellar filament protein contained a high percentage of serine and threonine, while proline was absent. The first 23 residues of the N-terminal were determined and sequence homology with other flagellins was found.

Glycotargeting: the preparation of glyco-amino acids and derivatives from unprotected reducing sugars

Lectins are present on the surface of many cells. Many lectins actively recycle from membrane to endosomes and efficiently take up glycoconjugates in a sugar-dependent manner. On this basis, glycoconjugates, specially those obtained by chemical means, are good candidates as carriers of drugs, oligonucleotides or genes. In this paper, we present a panel of methods suitable to transform unprotected reducing oligosaccharides into glycosynthons designed to be easily linked to therapeutic agents. All the glycosynthons presented here are glycosylamines or derivatives, mainly glyco-amino acids or glycopeptides. Glycosylamines are easy to obtain, but they are very labile in slightly acidic or neutral medium; they must be stabilized, by acylation for instance. The coupling efficiency of a reducing sugar with ammonia as well as an alkylamine or an arylamine is higher at high temperature, however, because of the Amadori rearrangement, special conditions have to be selected to prepare the expected glycosylamine derivative with a high yield. Glycosylamines are easily acylated by N-protected amino acids, or by halogeno acids which can then be transformed into amino acids. Alternatively, unprotected reducing oligosaccharides may very efficiently be transformed into N-glycosyl-amino acids and then protected by N-acylation. With a glutamyl derivative having both the alpha-amino and the gamma-carboxylic groups free, the coupling and the acylation, which is intramolecular, are roughly quantitative. N-oligosaccharyl-amino acid derivatives are interesting glycosynthons, because their sugar moiety bears the specificity towards membrane lectins while the amino acid part has the capacity to easily substitute a therapeutic agent.