Recovery of intact 2-aminobenzamide-labeled O-glycans released from glycoproteins by hydrazinolysis

Engineering, conjugation, and immunogenicity assessment of Escherichia coli O121 O antigen for its potential use as a typhoid vaccine component

State-of-the-art production technologies for conjugate vaccines are complex, multi-step processes. An alternative approach to produce glycoconjugates is based on the bacterial N-linked protein glycosylation system first described in Campylobacter jejuni. The C. jejuni N-glycosylation system has been successfully transferred into Escherichia coli, enabling in vivo production of customized recombinant glycoproteins. However, some antigenic bacterial cell surface polysaccharides, like the Vi antigen of Salmonella enterica serovar Typhi, have not been reported to be accessible to the bacterial oligosaccharyltransferase PglB, hence hamper development of novel conjugate vaccines against typhoid fever. In this report, Vi-like polysaccharide structures that can be transferred by PglB were evaluated as typhoid vaccine components. A polysaccharide fulfilling these requirements was found in Escherichia coli serovar O121. Inactivation of the E. coli O121 O antigen cluster encoded gene wbqG resulted in expression of O polysaccharides reactive with antibodies raised against the Vi antigen. The structure of the recombinantly expressed mutant O polysaccharide was elucidated using a novel HPLC and mass spectrometry based method for purified undecaprenyl pyrophosphate (Und-PP) linked glycans, and the presence of epitopes also found in the Vi antigen was confirmed. The mutant O antigen structure was transferred to acceptor proteins using the bacterial N-glycosylation system, and immunogenicity of the resulting conjugates was evaluated in mice. The conjugate-induced antibodies reacted in an enzyme-linked immunosorbent assay with E. coli O121 LPS. One animal developed a significant rise in serum immunoglobulin anti-Vi titer upon immunization.

Suppression of peeling during the release of O-glycans by hydrazinolysis

The analysis of O-glycans is essential for better understanding their functions in biological processes. Although many techniques for O-glycan release have been developed, the hydrazinolysis release method is the best for producing O-glycans with free reducing termini in high yield. This release technique allows the glycans to be labeled with a fluorophore and analyzed by fluorescence detection. Under the hydrazinolysis release conditions, a side reaction is observed and causes the loss of monosaccharides from the reducing terminus of the glycans (known as peeling). Using bovine fetuin (because it contains the sialylated O-glycans most commonly found on biopharmaceuticals) and bovine submaxillary gland mucin (BSM), here we demonstrate that peeling can be greatly reduced when the sample is buffer exchanged prior to hydrazinolysis with solutions of either 0.1% trifluoroacetic acid (TFA) or low-molarity (100, 50, 20, and 5 mM) ethylenediaminetetraacetic acid (EDTA). The addition of calcium chloride to fetuin resulted in an increase in peeling, whereas subsequent washing with EDTA abolished this effect, suggesting a role of calcium and possibly other cations in causing peeling. The presented technique for sample preparation prior to hydrazinolysis greatly reduces the level of undesirable cleavage products in O-glycan analysis and increases the robustness of the method.

Mass spectrometric O-glycan analysis after combined O-glycan release by beta-elimination and 1-phenyl-3-methyl-5-pyrazolone labeling

BACKGROUND

Analysis of protein glycosylation is an important first step towards establishing the functions of glycans in health and disease. In contrast to N-glycans which are generally enzymatically released for analysis, there is no corresponding enzyme for O-glycan liberation. Therefore, O-glycans are generally released by chemical methods involving tedious procedures.

METHODS

Here, a straightforward method for the combined release and labeling of O-linked glycans from glycoproteins is described. Dimethylamine serves as the releasing agent, and 1-phenyl-3-methyl-5-pyrazolone (PMP) is employed for a prompt reaction with the reducing end of the freshly released O-glycan structures via an aldol condensation followed by a Michael-type addition resulting in a 2:1 stoichiometry of PMP per glycan. Samples are analyzed by nanoLC coupled to mass spectrometry.

RESULTS

Mucin from bovine submaxillary gland was used as a model protein to evaluate and optimize the approach that was further applied to bile salt stimulated lipase (BSSL) isolated from human milk. Next to previously reported O-glycan structures two additional oligosaccharides could be detected for BSSL.

GENERAL SIGNIFICANCE

In conclusion, the facile protocol established is suitable for the analysis of complex O-linked oligosaccharides from various biological samples. This article is part of a Special Issue entitled Glycoproteomics.

Hyphenated technique for releasing and MALDI MS analysis of O-glycans in mucin-type glycoprotein samples

We developed an automatic apparatus for the release of O-glycans from mucin-type glycoproteins and proteoglycans (Matsuno, Y.-k.; Yamada, K.; Tanabe, A.; Kinoshita, M.; Maruyama, S.-z.; Osaka, Y.-s.; Masuko, T.; Kakehi, K. Anal. Biochem. 2007, 363, 245-257. Yamada, K.; Hyodo, S.; Matsuno, Y. K.; Kinoshita, M.; Maruyama, S. Z.; Osaka, Y. S.; Casal, E.; Lee, Y. C.; Kakehi, K. Anal. Biochem. 2007, 371, 52-61). The method allows rapid release of O-glycans as the reducing form within 10 min. In the present study, we connected the device to a MALDI-TOF MS spotter and achieved routine analysis of O-glycans in biological samples for clinical use after in situ derivatization of the released O-glycans with phenylhydrazine. We applied the method to the analysis of O-glycans expressed on MKN45 cells derived from human stomach cancer cells and found that MKN45 cells expressed characteristic trisialo-polylactosamine-type glycans as reported previously (Yamada, K.; Kinoshita, M.; Hayakawa, T.; Nakaya, S.; Kakehi, K. J. Proteome Res. 2009, 8, 521-537). We also applied the method to the analysis of O-glycans in serum samples. The present technique is the first attempt to use MS measurement for routine clinical diagnostic works.

A systematic approach to protein glycosylation analysis: a path through the maze

Protein glycosylation is an important post-translational modification. It is a feature that enhances the functional diversity of proteins and influences their biological activity. A wide range of functions for glycans have been described, from structural roles to participation in molecular trafficking, self-recognition and clearance. Understanding the basis of these functions is challenging because the biosynthetic machinery that constructs glycans executes sequential and competitive steps that result in a mixture of glycosylated variants (glycoforms) for each glycoprotein. Additionally, naturally occurring glycoproteins are often present at low levels, putting pressure on the sensitivity of the analytical technologies. No universal method for the rapid and reliable identification of glycan structure is currently available; hence, research goals must dictate the best method or combination of methods. To this end, we introduce some of the major technologies routinely used for structural N- and O-glycan analysis, describing the complementary information that each provides.

Glycan labeling strategies and their use in identification and quantification

Most methods for the analysis of oligosaccharides from biological sources require a glycan derivatization step: glycans may be derivatized to introduce a chromophore or fluorophore, facilitating detection after chromatographic or electrophoretic separation. Derivatization can also be applied to link charged or hydrophobic groups at the reducing end to enhance glycan separation and mass-spectrometric detection. Moreover, derivatization steps such as permethylation aim at stabilizing sialic acid residues, enhancing mass-spectrometric sensitivity, and supporting detailed structural characterization by (tandem) mass spectrometry. Finally, many glycan labels serve as a linker for oligosaccharide attachment to surfaces or carrier proteins, thereby allowing interaction studies with carbohydrate-binding proteins. In this review, various aspects of glycan labeling, separation, and detection strategies are discussed.

Identification of N-glycans from Ebola virus glycoproteins by matrix-assisted laser desorption/ionisation time-of-flight and negative ion electrospray tandem mass spectrometry

The larger fragment of the transmembrane glycoprotein (GP1) and the soluble glycoprotein (sGP) of Ebola virus were expressed in human embryonic kidney cells and the secreted products were purified from the supernatant for carbohydrate analysis. The N-glycans were released with PNGase F from within sodium dodecyl sulphate/polyacrylamide gel electrophoresis (SDS-PAGE) gels. Identification of the glycans was made with normal-phase high-performance liquid chromatography (HPLC), matrix-assisted laser desorption/ionisation mass spectrometry, negative ion electrospray ionisation fragmentation mass spectrometry and exoglycosidase digestion. Most glycans were complex bi-, tri- and tetra-antennary compounds with reduced amounts of galactose. No bisected compounds were detected. Triantennary glycans were branched on the 6-antenna; fucose was attached to the core GlcNAc residue. Sialylated glycans were present on sGP but were largely absent from GP1, the larger fragment of the transmembrane glycoprotein. Consistent with this was the generally higher level of processing of carbohydrates found on sGP as evidenced by a higher percentage of galactose and lower levels of high-mannose glycans than were found on GP1. These results confirm and expand previous findings on partial characterisation of the Ebola virus transmembrane glycoprotein. They represent the first detailed data on carbohydrate structures of the Ebola virus sGP.

Glycomics and disease markers

Glycomics is the comprehensive study of all glycans expressed in biological systems. The biosynthesis of glycan relies on a number of highly competitive processes involving glycosyl transferases. Glycosylation is therefore highly sensitive to the biochemical environment and has been implicated in many diseases including cancer. Recently, interest in profiling the glycome has increased owing to the potential of glycans for disease markers. In this regard, mass spectrometry is emerging as a powerful technique for profiling the glycome. Global glycan profiling of human serum based on mass spectrometry has already led to several potentially promising markers for several types of cancer and diseases.

Carbohydrate analysis throughout the development of a protein therapeutic

This review discusses the challenges involved in the characterization of the glycosylation of therapeutic glycoproteins. The focus is on methods that are most commonly used in regulatory filings and lot release testing of therapeutic glycoproteins. The different types of assays for carbohydrate analysis are reviewed, including the distinction between assays appropriate for lot release or better suited to testing during early drug development or in-depth characterization of the glycosylation. Characteristics of the glycoprotein and production process that should be considered when determining the amount of testing, the number of different methods to employ and when the testing should be performed during development of protein therapeutics is also discussed.

Development of a single column method for the separation of lipid- and protein-derived oligosaccharides

Fluorescent labeling of oligosaccharides with anthranilic acid (2-aminobenzoic acid; 2AA), or 2-aminobenzamide (2AB) permits the rapid, sensitive analysis of structures present in cells and tissues. Normal-phase (NP)/hydrophilic interaction chromatography (HILIC) is commonly used to separate fluorophore-derivatized oligosaccharides. Column elution is expressed as glucose units (GU) following calculation of relative retention when compared to an external glucose oligomer standard. However, there is significant overlap between sialylated and neutral oligosaccharides. Normal-phase anion-exchange (NP-AE) HPLC can separate differing classes of oligosaccharides according to the number of charged residues, but relative retention times in GU cannot be calculated across the entire gradient. We have overcome this difficulty by use of a Dionex AS11 column that combines both hydrophilic interaction and anion-exchange chromatographies, termed HIAX, which enables the calculation of GU values for oligosaccharides that carry sialylated or other negatively charged groups. The same method may also be employed for 2AB and other fluorophore-labeled oligosaccharides. Additionally, the same HPLC eluants are used for the differing HPLC columns. Therefore, analysis of HILIC- or HIAX-separated fluorophore-labeled oligosaccharides can be performed using a single HPLC system with a single set of eluents following a simple column change.

Glycan reductive isotope labeling for quantitative glycomics

Many diseases and disorders are characterized by quantitative and/or qualitative changes in complex carbohydrates. Mass spectrometry methods show promise in monitoring and detecting these important biological changes. Here we report a new glycomics method, termed glycan reductive isotope labeling (GRIL), where free glycans are derivatized by reductive amination with the differentially coded stable isotope tags [(12)C(6)]aniline and [(13)C(6)]aniline. These dual-labeled aniline-tagged glycans can be recovered by reverse-phase chromatography and can be quantified based on ultraviolet (UV) absorbance and relative ion abundances. Unlike previously reported isotopically coded reagents for glycans, GRIL does not contain deuterium, which can be chromatographically resolved. Our method shows no chromatographic resolution of differentially labeled glycans. Mixtures of differentially tagged glycans can be directly compared and quantified using mass spectrometric techniques. We demonstrate the use of GRIL to determine relative differences in glycan amount and composition. We analyze free glycans and glycans enzymatically or chemically released from a variety of standard glycoproteins, as well as human and mouse serum glycoproteins, using this method. This technique allows linear relative quantitation of glycans over a 10-fold concentration range and can accurately quantify sub-picomole levels of released glycans, providing a needed advancement in the field of glycomics.

Analysis of protein glycosylation and phosphorylation using liquid phase separation, protein microarray technology, and mass spectrometry

Protein glycosylation and phosphorylation are very common posttranslational modifications. The alteration of these modifications in cancer cells is closely related to the onset and progression of cancer and other disease states. In this protocol, strategies for monitoring the changes in protein glycosylation and phosphorylation in serum or tissue cells on a global scale and specifically characterizing these alterations are included. The technique is based on lectin affinity enrichment for glycoproteins, all liquid-phase two-dimensional fractionation, protein microarray, and mass spectrometry technology. Proteins are separated based on pI in the first dimension using chromatofocusing (CF) or liquid isoelectric focusing (IEF) followed by the second-dimension separation using nonporous silica RP-HPLC. Five lectins with different binding specificities to glycan structures are used for screening glycosylation patterns in human serum through a biotin streptavidin system. Fluorescent phosphodyes and phosphospecific antibodies are employed to detect specific phosphorylated proteins in cell lines or human tissues. The purified proteins of interest are identified by peptide sequencing. Their modifications including glycosylation and phosphorylation could be further characterized by mass-spectrometry-based approaches. These strategies can be used in biological samples for large-scale glycoproteome/phosphoproteome screening as well as for individual protein modification analysis.

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.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update covering the period 2001-2002

This review is the second update of the original review on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates that was published in 1999. It covers fundamental aspects of the technique as applied to carbohydrates, fragmentation of carbohydrates, studies of specific carbohydrate types such as those from plant cell walls and those attached to proteins and lipids, studies of glycosyl-transferases and glycosidases, and studies where MALDI has been used to monitor products of chemical synthesis. Use of the technique shows a steady annual increase at the expense of older techniques such as FAB. There is an increasing emphasis on its use for examination of biological systems rather than on studies of fundamental aspects and method development and this is reflected by much of the work on applications appearing in tabular form.

HPLC-based analysis of serum N-glycans on a 96-well plate platform with dedicated database software

We present a robust, fully automatable technology platform that includes computer software for the detailed analysis of low femtomoles of N-linked sugars released from glycoproteins. Features include (i) sample immobilization in 96-well plates, glycan release, and fluorescent labeling; (ii) quantitative HPLC analysis, including monosaccharide sequence, linkage, and arm-specific information for charged and neutral glycans; (iii) automatic structural assignment of peaks from HPLC profiles via web-based software that accesses our database (GlycoBase) of more than 350 N-glycan structures, including 117 present in the human serum glycome; and (iv) software (autoGU) that progressively analyzes data from exoglycosidase digestions to produce a refined list of final structures. The N-glycans from a plate of 96 samples can be released and purified in 2 or 3 days and profiled in 2 days. This strategy can be used for (i) identification and screening of disease biomarkers and (ii) monitoring the production of therapeutic glycoproteins, allowing optimization of production conditions. This technology is also suitable for preparing released glycans for other analytical techniques. Here we demonstrate its application to rheumatoid arthritis using 5 microl of patient serum.

N-linked Glycosylation Profiling of Pancreatic Cancer Serum Using Capillary Liquid Phase Separation Coupled with Mass Spectrometric Analysis

Glycoproteins play important roles in various biological processes including intracellular transport, cell recognition, and cell-cell interactions. The change of the cellular glycosylation profile may have profound effects on cellular homeostasis and malignancy. Therefore, we have developed a sensitive screening approach for the comprehensive analysis of N-glycans and glycosylation sites on human serum proteins. Using this approach, N-linked glycopeptides were extracted by double lectin affinity chromatography. The glycans were enzymatically cleaved from the peptides and then profiled using capillary hydrophilic interaction liquid chromatography coupled online with ESI-TOF MS. The structures of the separated glycans were determined by MALDI quadrupole ion-trap TOF mass spectrometry in both positive and negative modes. The glycosylation sites were elucidated by sequencing of PNGase F modified glycopeptides using nanoRP-LC-ESI-MS/MS. Alterations of glycosylation were analyzed by comparing oligosaccharide expression of serum glycoproteins at different disease stages. The efficiency of this method was demonstrated by the analysis of pancreatic cancer serum compared to normal serum. Ninety-two individual glycosylation sites and 202 glycan peaks with 105 unique carbohydrate structures were identified from approximately 25 mug glycopeptides. Forty-four oligosaccharides were found to be distinct in the pancreatic cancer serum. Increased branching of N-linked oligosaccharides and increased fucosylation and sialylation were observed in samples from patients with pancreatic cancer. The methodology described in this study may elucidate novel, cancer-specific oligosaccharides and glycosylation sites, some of which may have utility as useful biomarkers of cancer.

Proteomic analysis of glycosylation: structural determination of N- and O-linked glycans by mass spectrometry

This review summarizes the methods, mainly based on mass spectrometry, for the structural determination of N- and O-linked carbohydrates that are post-translationally attached to a large number of proteins and which play a key role in determining the function and biophysical properties of these compounds. Analysis of these carbohydrates has proved difficult in the past due to their structural complexity. However, modern analytical methods such as mass spectrometry have the ability to elucidate most structural details at the concentration levels required for proteomics. This review describes methods for direct examination of glycoproteins by mass spectrometry, the release of N- and O-linked glycans from glycoproteins separated in sodium dodecyl sulfate polyacrylamide electrophoresis gels, and the analysis of these compounds by techniques such as matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry. Matrix-assisted laser desorption/ionization mass spectrometry provides the most rapid method for comparing glycan profiles and is probably most appropriate for clinical studies. One of the most promising techniques for determining the structures of N-glycans in proteomic studies is negative ion fragmentation of electrosprayed ions. This technique combines high throughput with ease of structural interpretation and provides structural details that are difficult to obtain by classical methods.

Determining the structure of oligosaccharides N- and O-linked to glycoproteins

Many proteins involved in biological events are glycosylated. A glycoprotein consists of a mixture of glycosylation variants of a single polypeptide chain, known as glycoforms. It has become clear that a detailed understanding of the roles which glycosylation plays in the biosynthesis, transport, biological function, and degradation of a glycoprotein can only be achieved when the protein and sugar(s) are viewed as an entity. Many glycoproteins can now be modeled by combining glycan sequencing data and oligosaccharide structural information with protein structural data. Pivotal to this approach is sensitive, state-of-the-art oligosaccharide sequencing technology which can give a rapid insight into the glycosylation of a glycoprotein without the need for sophisticated equipment and expertise. This unit gives a detailed introduction into the analysis of glycans, and the many figures will help the user identify which type of experiment needs to be undertaken. Methods for releasing glycans from glycoproteins are followed by protocols for labeling and purifying (by HPLC) the glycans from the rest of the components. Strategies for N- and O-glycan analysis are also included.

Versatile fluorescent derivatization of glycans for glycomic analysis

The new field of functional glycomics encompasses information about both glycan structure and recognition by carbohydrate-binding proteins (CBPs) and is now being explored through glycan array technology. Glycan array construction, however, is limited by the complexity of efficiently generating derivatives of free, reducing glycans with primary amines for conjugation. Here we describe a straightforward method to derivatize glycans with 2,6-diaminopyridine (DAP) to generate fluorescently labeled glycans (glycan-DAP conjugates or GDAPs) that contain a primary amine for further conjugation. We converted a wide variety of glycans, including milk sugars, N-glycans, glycosaminoglycans and chitin-derived glycans, to GDAPs, as verified by HPLC and mass spectrometry. We covalently conjugated GDAPs to N-hydroxysuccinimide (NHS)-activated glass slides, maleimide-activated protein, carboxylated microspheres and NHS-biotin to provide quantifiable fluorescent derivatives. All types of conjugated glycans were well-recognized by appropriate CBPs. Thus, GDAP derivatives provide versatile new tools for biologists to quantify and covalently capture minute quantities of glycans for exploring their structures and functions and generating new glycan arrays from naturally occurring glycans.

Analysis of fluorescently labeled glycosphingolipid-derived oligosaccharides following ceramide glycanase digestion and anthranilic acid labeling

Interest in cellular glycosphingolipid (GSL) function has necessitated the development of a rapid and sensitive method to both analyze and characterize the full complement of structures present in various cells and tissues. An optimized method to characterize oligosaccharides released from glycosphingolipids following ceramide glycanase digestion has been developed. The procedure uses the fluorescent compound anthranilic acid (2-aminobenzoic acid; 2-AA) to label oligosaccharides prior to analysis using normal-phase high-performance liquid chromatography. The labeling procedure is rapid, selective, and easy to perform and is based on the published method of Anumula and Dhume [Glycobiology 8 (1998) 685], originally used to analyze N-linked oligosaccharides. It is less time consuming than a previously published 2-aminobenzamide labeling method [Anal. Biochem. 298 (2001) 207] for analyzing GSL-derived oligosaccharides, as the fluorescent labeling is performed on the enzyme reaction mixture. The purification of 2-AA-labeled products has been improved to ensure recovery of oligosaccharides containing one to four monosaccharide units, which was not previously possible using the Anumula and Dhume post-derivatization purification procedure. This new approach may also be used to analyze both N- and O-linked oligosaccharides.

Fragmentation of oligosaccharide ions with 157 nm vacuum ultraviolet light

The 157 nm photofragmentation of native and derivatized oligosaccharides was studied in a linear ion trap and in a home-built matrix-assisted laser desorption/ionization (MALDI) tandem time-of-flight (TOF/TOF) mass spectrometer, and the results were compared with collision-induced dissociation (CID) experiments. Photodissociation produces product ions corresponding to high-energy fragmentation pathways; for cation-derivatized oligosaccharides, it yields strong cross-ring fragment ions and provides better sequence coverage than low- and high-energy CID experiments. On the other hand, for native oligosaccharides, CID yielded somewhat better sequence coverage than photodissociation. The ion trap enables CID hybrid MS3 experiments on the high-energy fragment ions obtained from photodissociation.

Structural characterization of 2-aminobenzamide-derivatized oligosaccharides using a matrix-assisted laser desorption/ionization two-stage time-of-flight tandem mass spectrometer

Oligosaccharides were derivatized by reductive amination using 2-aminobenzamide (2-AB) and analyzed by matrix-assisted laser desorption/ionization two-stage time-of-flight (MALDI-TOF/TOF) tandem mass spectrometry (MS/MS) in the positive ion mode. The major signals were obtained under these conditions from the [M+Na]+ ions for all 2-AB-derivatized oligosaccharides. A systematic study was conducted on a series of 2-AB-derivatized oligosaccharides to allow rationalization of the fragmentation processes. The MALDI-TOF/TOF-MS/MS spectra of the [M+Na]+ ions of 2-AB-derivatized oligosaccharides were dominated by glycosidic cleavages. These fragments originating both from the reducing and the non-reducing ends of the oligosaccharide yield information on sequence and branching. Moreover, the MALDI-TOF/TOF-MS/MS spectra were also characterized by abundant cross-ring fragments which are very informative on the linkages of the monosaccharide residues constituting these oligosaccharides. MALDI-TOF/TOF-MS/MS analysis of 2-AB-derivatized oligosaccharides, by providing structural information at the low-picomole level, appears to be a powerful tool for carbohydrate structural analysis.

Electrospray ionization ion trap mass spectrometry for structural characterization of oligosaccharides derivatized with 2-aminobenzamide

The use of electrospray ionization (ESI) quadrupole ion trap mass spectrometry and reversed-phase high-performance liquid chromatography (HPLC) for the characterization of 2-aminobenzamide (2AB)-labeled oligosaccharides and N-linked protein oligosaccharide mixtures is described. The major signals were obtained under these conditions from the [M+Na]+ ions for all 2AB-derivatized oligosaccharides. Under collision-induced dissociation, sodiated molecular species generated in the ESI mode yield simple and predictable mass spectra. Tandem mass spectrometry (MS/MS) experiments with orders higher than two offer a number of ways to enhance MS/MS spectra and to derive information not present in MS and MS2 spectra. Information on composition, sequence, branching and, to some extent, interglycosidic linkages can be deduced from fragments resulting from the cleavage of glycosidic bonds and from weak cross-ring cleavage products. Reversed-phase HPLC and derivatization by reductive amination using 2-aminobenzamide were finally applied to characterize a glycan pool enzymatically released from glycoproteins.

Glycoform composition profiling of O-glycopeptides derived from human serum IgA1 by matrix-assisted laser desorption ionization-time of flight-mass spectrometry

Pools of O-glycopeptides prepared from trypsin-digested reduced and alkylated human serum IgA1 have been analyzed using matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-ToF-MS) in the positive-ion mode, using 2,4,6-trihydroxy acetophenone-ammonium citrate matrix. Dozens of such pools prepared from normal serum IgA1 and from serum of patients with a number of different medical conditions have been routinely analyzed in this manner. The glycopeptides present in these pools possess identical amino acid sequences but are substituted with a variety of neutral and sialylated glycans and the spectra obtained were such that individual compositional glycoforms were baseline resolved. In addition, the spectra were reproducible, exhibiting a relative peak intensity and area variation of around 11-16%, enabling the technique to be used for the relative quantitation of the different compositional glycoforms present. This could be achieved manually or by applying a Java program especially developed for this purpose. The MS analysis described here is a major improvement over present MALDI methods used for profiling the O-glycosylation of IgA1. The MS methodology together with the Java data analysis are expected to be generally applicable for profiling O-linked glycopeptides derived from other glycoproteins and probably for N-linked glycopeptide pools.

Normal-Phase Nanoscale Liquid Chromatography−Mass Spectrometry of Underivatized Oligosaccharides at Low-Femtomole Sensitivity

We here describe the online liquid chromatography (LC) electrospray ionization mass spectrometry (MS) of underivatized glycans using a nanoscale normal-phase amide column at a flow rate of 300 nL/min. Retention on the amide column is based on polar interactions of the oligosaccharide hydroxyl groups with the stationary phase, and thus, the retention time predictably increases with elongation of the oligosaccharide chain. The system is characterized by its high chromatographic resolution, which routinely allows the separation of isobaric structures. Separation of oligosaccharide mixtures over a 1-h range permits the detailed characterization of the different species by multiple ion selection and fragmentation steps using ion trap MS. The here presented miniaturization of the online-LC system to the nanoscale in combination with ion trap MS allows the detection of oligosaccharide species in a mixture at low-femtomole sensitivity. Online normal-phase nano-LC-MS of complex oligosaccharide mixtures further facilitates the sensitive and detailed structural analysis of oligosaccharides by overcoming the need for cumbersome and time-consuming derivatization procedures such as reductive amination for labeling with hydrophobic fluorophores or labeling with tritium. The method should be useful for the sensitive and quick analysis of glycosylation patterns and individual oligosaccharides from biotechnologically produced glycoproteins as well as scarcely available biological samples.

An analytical and structural database provides a strategy for sequencing O-glycans from microgram quantities of glycoproteins

A sensitive, rapid, quantitative strategy has been developed for O-glycan analysis. A structural database has been constructed that currently contains analytical parameters for more than 50 glycans, enabling identification of O-glycans at the subpicomole level. The database contains the structure, molecular weight, and both normal and reversed-phase HPLC elution positions for each glycan. These observed parameters reflect the mass, three-dimensional shape, and hydrophobicity of the glycans and, therefore, provide information relating to linkage and arm specificity as well as monosaccharide composition. Initially the database was constructed by analyzing glycans released by mild hydrazinolysis from bovine serum fetuin, synthetic glycopeptides, human glycophorin A, and serum IgA1. The structures of the fluorescently labeled sugars were determined from a combination of HPLC data, mass spectrometric composition and mass fragmentation data, and exoglycosidase digestions. This approach was then applied to human neutrophil gelatinase B and secretory IgA, where 18 and 25 O-glycans were identified, respectively, and the parameters of these glycans were added to the database. This approach provides a basis for the analysis of subpicomole quantities of O-glycans from normal levels of natural glycoproteins.