Liquid chromatography ion trap mass spectrometric analysis of oligosaccharides using permethylated derivatives

High-throughput and high-sensitivity N-Glycan profiling: A platform for biopharmaceutical development and disease biomarker discovery

Protein glycosylation contributes to critical biological function of glycoproteins. Glycan analysis is essential for the production of biopharmaceuticals as well as for the identification of disease biomarkers. However, glycans are highly heterogeneous, which has considerably hampered the progress of glycomics. Here, we present an improved 96-well plate format platform for streamlined glycan profiling that takes advantage of rapid glycoprotein denaturation, deglycosylation, fluorescent derivatization, and on-matrix glycan clean-up. This approach offers high sensitivity with consistent identification and quantification of diverse N-glycans across multiple samples on a high-throughput scale. We demonstrate its capability for N-glycan profiling of glycoproteins from various sources, including two recombinant monoclonal antibodies produced from Chinese Hamster Ovary cells, EG2-hFc and rituximab, polyclonal antibodies purified from human serum, and total glycoproteins from human serum. Combined with the complementary information obtained by sequential digestion from exoglycosidase arrays, this approach allows the detection and identification of multiple N-glycans in these complex biological samples. The reagents, workflow, and Hydrophilic interaction liquid chromatography with fluorescence detection (HILIC-FLD), are simple enough to be implemented into a straightforward user-friendly setup. This improved technology provides a powerful tool in support of rapid advancement of glycan analysis for biopharmaceutical development and biomarker discovery for clinical disease diagnosis.

Characterization of Isomeric Glycans by Reversed Phase Liquid Chromatography-Electronic Excitation Dissociation Tandem Mass Spectrometry

The occurrence of numerous structural isomers in glycans from biological sources presents a severe challenge for structural glycomics. The subtle differences among isomeric structures demand analytical methods that can provide structural details while working efficiently with on-line glycan separation methods. Although liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a powerful tool for mixture analysis, the commonly utilized collision-induced dissociation (CID) method often does not generate a sufficient number of fragments at the MS2 level for comprehensive structural characterization. Here, we studied the electronic excitation dissociation (EED) behaviors of metal-adducted, permethylated glycans, and identified key spectral features that could facilitate both topology and linkage determinations. We developed an EED-based, nanoscale, reversed phase (RP)LC-MS/MS platform, and demonstrated its ability to achieve complete structural elucidation of up to five structural isomers in a single LC-MS/MS analysis. Graphical Abstract.

Reversed-phase separation methods for glycan analysis

Reversed-phase chromatography is a method that is often used for glycan separation. For this, glycans are often derivatized with a hydrophobic tag to achieve retention on hydrophobic stationary phases. The separation and elution order of glycans in reversed-phase chromatography is highly dependent on the hydrophobicity of the tag and the contribution of the glycan itself to the retention. The contribution of the different monosaccharides to the retention strongly depends on the position and linkage, and isomer separation may be achieved. The influence of sialic acids and fucoses on the retention of glycans is still incompletely understood and deserves further study. Analysis of complex samples may come with incomplete separation of glycan species, thereby complicating reversed-phase chromatography with fluorescence or UV detection, whereas coupling with mass spectrometry detection allows the resolution of complex mixtures. Depending on the column properties, eluents, and run time, separation of isomeric and isobaric structures can be accomplished with reversed-phase chromatography. Alternatively, porous graphitized carbon chromatography and hydrophilic interaction liquid chromatography are also able to separate isomeric and isobaric structures, generally without the necessity of glycan labeling. Hydrophilic interaction liquid chromatography, porous graphitized carbon chromatography, and reversed-phase chromatography all serve different research purposes and thus can be used for different research questions. A great advantage of reversed-phase chromatography is its broad distribution as it is used in virtually every bioanalytical research laboratory, making it an attracting platform for glycan analysis.

Graphical Abstract

Separation and Identification of Isomeric Glycans by Selected Accumulation-Trapped Ion Mobility Spectrometry-Electron Activated Dissociation Tandem Mass Spectrometry

One of the major challenges in structural characterization of oligosaccharides is the presence of many structural isomers in most naturally occurring glycan mixtures. Although ion mobility spectrometry (IMS) has shown great promise in glycan isomer separation, conventional IMS separation occurs on the millisecond time scale, largely restricting its implementation to fast time-of-flight (TOF) analyzers which often lack the capability to perform electron activated dissociation (ExD) tandem MS analysis and the resolving power needed to resolve isobaric fragments. The recent development of trapped ion mobility spectrometry (TIMS) provides a promising new tool that offers high mobility resolution and compatibility with high-performance Fourier transform ion cyclotron resonance (FTICR) mass spectrometers when operated under the selected accumulation-TIMS (SA-TIMS) mode. Here, we present our initial results on the application of SA-TIMS-ExD-FTICR MS to the separation and identification of glycan linkage isomers.

Mass Spectrometry-Based N-Glycomics of Colorectal Cancer

Colorectal cancer (CRC) is one of the most prevalent cancers worldwide. An increased molecular understanding of the CRC pathology is warranted to gain insights into the underlying molecular and cellular mechanisms of the disease. Altered protein glycosylation patterns are associated with most diseases including malignant transformation. Recent advances in mass spectrometry and bioinformatics have accelerated glycomics research and present a new paradigm for cancer biomarker discovery. Mass spectrometry (MS)-based glycoproteomics and glycomics, therefore, hold considerable promise to improve the discovery of novel biomarkers with utility in disease diagnosis and therapy. This review focuses on the emerging field of glycomics to present a comprehensive review of advances in technologies and their application in studies aimed at discovering novel glycan-based biomarkers. We will also discuss some of the challenges associated with using glycans as biomarkers.

Targeted methods for quantitative analysis of protein glycosylation

Quantification of proteins by LC-MS/MS-MRM has become a standard method with broad projected clinical applicability. MRM quantification of protein modifications is, however, far less utilized, especially in the case of glycoproteins. This review summarizes current methods for quantitative analysis of protein glycosylation with a focus on MRM methods. We describe advantages of this quantitative approach, analytical parameters that need to be optimized to achieve reliable measurements, and point out the limitations. Differences between major classes of N- and O-glycopeptides are described and class-specific glycopeptide assays are demonstrated.

Mass spectrometry of glycans

Powerful new strategies based on mass spectrometry are revolutionizing the structural analysis and profiling of glycans and glycoconjugates. We survey here the major biosynthetic pathways that underlie the biological diversity in glycobiology, with emphasis on glycoproteins, and the approaches that can be used to address the resulting heterogeneity. Included among these are derivatizations, on- and off-line chromatography, electrospray and matrix-assisted laser desorption/ionization, and a variety of dissociation methods, the recently introduced electron-based techniques being of particular interest.

Nanoscale reversed-phase liquid chromatography-mass spectrometry of permethylated N-glycans

Reversed-phase liquid chromatography on the nanoscale coupled to electrospray tandem mass spectrometry was used to analyse a mixture of four commercial glycan standards, and the method was further adapted to N-glycans enzymatically released from alpha-1-acid glycoprotein and immunoglobulin gamma. Glycans were permethylated to enable their separation by reversed-phase chromatography and to facilitate interpretation of fragmentation data. Prior to derivatization of glycans by permethylation, they were reduced to cancel anomerism because, although feasible, it was not desired to separate α- and β-anomers. The effect of supplementing chromatographic solvent with sodium hydroxide to guide adduct formation was investigated. Raising the temperature in which the separation was performed improved chromatographic resolution and affected retention times as expected. It was shown by using the tetrasaccharides sialyl Lewis X and sialyl Lewis A that reversed-phase chromatography could achieve the separation of methylated isobaric glycan analytes. Isobaric glycans were detected among the N-glycans of immunoglobulin gamma and further analysed by tandem mass spectrometry.

In vitro selection of sialic acid specific RNA aptamer and its application to the rapid sensing of sialic acid modified sugars

Sialic acids (SAs) are located on the terminal positions of glycan on a cell surface, which play important role in the spread and metastasis of cancer cells and infection of pathogen. For their detection and diagnosis, the finding of SA specific ligand is an essential prerequisite. Here, RNA aptamer for N-acetylneuraminic acid (Neu5Ac), a representative of SAs, with the high affinity of 1.35 nM and the selectivity was screened by in vitro selection method. The strong binding of the screened aptamer was enough to protect the hydrolysis of Neu5Ac by neuraminidase with the stoichiometry of 1:1 molar ratio. For the rapid detection of SAs, the RNA aptamer was further engineered to the aptazyme sensor by conjugating with a ribozyme following the characterization of selected aptamer by RNase footprinting assay. Without additional desialylation, modification, or/and purification processes, the aptazyme indicated high catalytic activities in the presence of Neu5Ac over 20 µM in several minutes. Also, we observed that the aptazyme sensor shows high sensitivities to Neu5Ac-conjugated sugars as well as Neu5Ac monomer, but not in non-Neu5Ac modified sugars. The aptamer for Neu5Ac can support valuable tools in a wide range of bioanalytical applications as well as biosensors.

Comparing MALDI-MS, RP-LC-MALDI-MS and RP-LC-ESI-MS glycomic profiles of permethylated N-glycans derived from model glycoproteins and human blood serum

The glycomic profiling of purified glycoproteins and biological specimen is routinely achieved through different analytical methods, but mainly through MS and LC-MS. The enhanced ionization efficiency and improved tandem MS interpretation of permethylated glycans have prompted the popularity of this approach. This study focuses on comparing the glycomic profiling of permethylated N-glycans derived from model glycoproteins and human blood serum using MALDI-MS as well as RP-LC-MALDI-MS and RP-LC-ESI-MS. In the case of model glycoproteins, the glycomic profiles acquired using the three methods were very comparable. However, this was not completely true in the case of glycans derived from blood serum. RP-LC-ESI-MS analysis of reduced and permethylated N-glycans derived from 250 nl of blood serum allowed the confident detection of 73 glycans (the structures of which were confirmed by mass accuracy and tandem MS), while 53 and 43 structures were identified in the case of RP-LC-MALDI-MS and MALDI-MS analyses of the same sample, respectively. RP-LC-ESI-MS analysis facilitates automated and sensitive tandem MS acquisitions. The glycan structures that were detected only in the RP-LC-ESI-MS analysis were glycans existing at low abundances. This is suggesting the higher detection sensitivity of RP-LC-ESI-MS analysis, originating from both reduced competitive ionization and saturation of detectors, facilitated by the chromatographic separation. The latter also permitted the separation of several structural isomers; however, isomeric separations pertaining to linkages were not detected.

Systematic comparison of reverse phase and hydrophilic interaction liquid chromatography platforms for the analysis of N-linked glycans

Due to the hydrophilic nature of glycans, reverse phase chromatography has not been widely used as a glycomic separation technique coupled to mass spectrometry. Other approaches such as hydrophilic interaction chromatography and porous graphitized carbon chromatography are often employed, though these strategies frequently suffer from decreased chromatographic resolution, long equilibration times, indefinite retention, and column bleed. Herein, it is shown that, through an efficient hydrazone formation derivatization of N-linked glycans (~4 h of additional sample preparation time which is carried out in parallel), numerous experimental and practical advantages are gained when analyzing the glycans by online reverse phase chromatography. These benefits include an increased number of glycans detected, increased peak capacity of the separation, and the ability to analyze glycans on the identical liquid chromatography-mass spectrometry platform commonly used for proteomic analyses. The data presented show that separation of derivatized N-linked glycans by reverse phase chromatography significantly out-performs traditional separation of native or derivatized glycans by hydrophilic interaction chromatography. Furthermore, the movement to a more ubiquitous separation technique will afford numerous research groups the opportunity to analyze both proteomic and glycomic samples on the same platform with minimal time and physical change between experiments, increasing the efficiency of "multiomic" biological approaches.

Application of nano-LC-based glycomics towards biomarker discovery

The glycome, that is, the glycan components of a biological source, has been widely reported to change with disease states. However, mining the glycome for biomarkers is complicated by glycan structural heterogeneity. Nanoflow LC, or nano-LC, significantly addresses the problem by providing a highly sensitive and quantitative method of separating and profiling glycans. This review summarizes recent advances in analytical technology and methodology that enhance and augment the advantages offered by nano-LC. (e.g., reversed phase, hydrophilic interaction and porous graphitized carbon chromatography, as well as associated derivatization strategies), detectors (e.g., fluorescence and MS), and technology platforms (particularly chip-based nano-LC) are examined in detail, along with their application to biomarker discovery. Particular emphasis is placed on methods and technologies that allow structure-specific glycan profiling.

Annotation and structural analysis of sialylated human milk oligosaccharides

Sialylated human milk oligosaccharides (SHMOs) are important components of human milk oligosaccharides. Sialic acids are typically found on the nonreducing end and are known binding sites for pathogens and aid in neonates' brain development. Due to their negative charge and hydrophilic nature, they also help modulate cell-cell interactions. It has also been shown that sialic acids are involved in regulating the immune response and aid in brain development. In this study, the enriched SHMOs from pooled milk sample were analyzed by HPLC-Chip/QTOF MS. The instrument employs a microchip-based nano-LC column packed with porous graphitized carbon (PGC) to provide excellent isomer separation for SHMOs with highly reproducible retention time. The precursor ions were further examined with collision-induced dissociation (CID). By applying the proper collision energy, isomers can be readily differentiated by diagnostic peaks and characteristic fragmentation patterns. A set of 30 SHMO structures with retention times, accurate masses, and MS/MS spectra was deduced and incorporated into an HMO library. When combined with previously determined neutral components, a library with over 70 structures is obtained allowing high-throughput oligosaccharide structure identification.

Advances in structure elucidation of small molecules using mass spectrometry

The structural elucidation of small molecules using mass spectrometry plays an important role in modern life sciences and bioanalytical approaches. This review covers different soft and hard ionization techniques and figures of merit for modern mass spectrometers, such as mass resolving power, mass accuracy, isotopic abundance accuracy, accurate mass multiple-stage MS(n) capability, as well as hybrid mass spectrometric and orthogonal chromatographic approaches. The latter part discusses mass spectral data handling strategies, which includes background and noise subtraction, adduct formation and detection, charge state determination, accurate mass measurements, elemental composition determinations, and complex data-dependent setups with ion maps and ion trees. The importance of mass spectral library search algorithms for tandem mass spectra and multiple-stage MS(n) mass spectra as well as mass spectral tree libraries that combine multiple-stage mass spectra are outlined. The successive chapter discusses mass spectral fragmentation pathways, biotransformation reactions and drug metabolism studies, the mass spectral simulation and generation of in silico mass spectra, expert systems for mass spectral interpretation, and the use of computational chemistry to explain gas-phase phenomena. A single chapter discusses data handling for hyphenated approaches including mass spectral deconvolution for clean mass spectra, cheminformatics approaches and structure retention relationships, and retention index predictions for gas and liquid chromatography. The last section reviews the current state of electronic data sharing of mass spectra and discusses the importance of software development for the advancement of structure elucidation of small molecules. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s12566-010-0015-9) contains supplementary material, which is available to authorized users.

Chip-based reversed-phase liquid chromatography-mass spectrometry of permethylated N-linked glycans: a potential methodology for cancer-biomarker discovery

The study of protein glycosylation in biological fluids and tissues has substantial medical importance, as changes in glycan structures have now been associated with a number of diseases. Quantification of glycomic-profile changes is becoming increasingly important in the search for disease biomarkers. Here, we report a highly reproducible combination of a glycomic sample preparation/solid-phase derivatization of glycoprotein-derived N-linked glycans with their subsequent microchip-based separation and mass-spectrometric (MS) measurements. Following our previously described reductive beta-elimination for O-linked glycans with ammonia-borane complex to reduce N-linked structures, the N-linked alditol structures are effectively methylated in dimethylformamide medium to avoid artefacts in MS measurements. Reversed-phase microfluidic liquid chromatography (LC) of methylated N-linked oligosaccharide alditols resolved some closely related structures into regular retention increments, aiding in their structural assignments. Optimized LC gradients, together with nanospray MS, have been applied here in the quantitative measurements of N-linked glycans in blood serum, distinguishing breast cancer patients from control individuals.

A multi-method approach toward de novo glycan characterization: a Man-5 case study

Regulatory agencies' expectations for biotherapeutic approval are becoming more stringent with regard to product characterization, where minor species as low as 0.1% of a given profile are typically identified. The mission of this manuscript is to demonstrate a multi-method approach toward de novo glycan characterization and quantitation, including minor species at or approaching the 0.1% benchmark. Recently, unexpected isomers of the Man(5)GlcNAc(2) (M(5)) were reported (Prien JM, Ashline DJ, Lapadula AJ, Zhang H, Reinhold VN. 2009. The high mannose glycans from bovine ribonuclease B isomer characterization by ion trap mass spectrometry (MS). J Am Soc Mass Spectrom. 20:539-556). In the current study, quantitative analysis of these isomers found in commercial M(5) standard demonstrated that they are in low abundance (<1% of the total) and therefore an exemplary "litmus test" for minor species characterization. A simple workflow devised around three core well-established analytical procedures: (1) fluorescence derivatization; (2) online rapid resolution reversed-phase separation coupled with negative-mode sequential mass spectrometry (RRRP-(-)-MS(n)); and (3) permethylation derivatization with nanospray sequential mass spectrometry (NSI-MS(n)) provides comprehensive glycan structural determination. All methods have limitations; however, a multi-method workflow is an at-line stopgap/solution which mitigates each method's individual shortcoming(s) providing greater opportunity for more comprehensive characterization. This manuscript is the first to demonstrate quantitative chromatographic separation of the M(5) isomers and the use of a commercially available stable isotope variant of 2-aminobenzoic acid to detect and chromatographically resolve multiple M(5) isomers in bovine ribonuclease B. With this multi-method approach, we have the capabilities to comprehensively characterize a biotherapeutic's glycan array in a de novo manner, including structural isomers at >/=0.1% of the total chromatographic peak area.

Gas-phase oligosaccharide nonreducing end (GONE) sequencing and structural analysis by reversed phase HPLC/mass spectrometry with polarity switching

Here we describe a technique to obtain all the N-linked oligosaccharide structures from a single reversed-phase (RP) HPLC run using on-line tandem MS in both positive and negative ion modes with polarity switching. Oligosaccharides labeled with 2-aminobenzamide (2AB) were used because they generated good ionization efficiency in both ion polarities. In the positive ion mode, protonated oligosaccharide ions lose sugar residues sequentially from the nonreducing end with each round of MS fragmentation, revealing the oligosaccharide sequence from greatly simplified tandem MS spectra. In the negative ion mode, diagnostic ions, including those from cross-ring cleavages, are readily observed in the MS2 spectra of deprotonated oligosaccharide ions, providing detailed structural information, such as branch composition and linkage positions. Both positive and negative ion modes can be programmed into the same LC/MS experiment through polarity switching of the MS instrument. The gas-phase oligosaccharide nonreducing end (GONE) sequencing data, in combination with the diagnostic ions generated in negative ion tandem MS, allow both sequence and structural information to be obtained for all eluting species during a single RP-HPLC chromatographic run. This technique generates oligosaccharide analyses at high speed and sensitivity, and reveals structural features that can be difficult to obtain by traditional methods.

On the use of DHB/aniline and DHB/N,N-dimethylaniline matrices for improved detection of carbohydrates: automated identification of oligosaccharides and quantitative analysis of sialylated glycans by MALDI-TOF mass spectrometry

This study demonstrates the application of 2,5-dihydrohybenzoic acid/aniline (DHB/An) and 2,5-dihydroxybenzoic acid/N,N-dimethylaniline (DHB/DMA) matrices for automated identification and quantitative analysis of native oligosaccharides by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Both matrices are shown to be superior to pure DHB for native glycans in terms of signal intensities of analytes and homogeneity of sample distribution throughout the crystal layer. On-target formation of stable aniline Schiff base derivatives of glycans in DHB/An and the complete absence of such products in the mass spectra acquired in DHB/DMA matrix provide a platform for automated identification of reducing oligosaccharides in the MALDI mass spectra of complex samples. The study also shows how enhanced sensitivity is achieved with the use of these matrices and how the homogeneity of deposited sample material may be exploited for quick and accurate quantitative analysis of native glycan mixtures containing neutral and sialylated oligosaccharides in the low-nanogram to mid-picogram range.

Ultraviolet photodissociation at 355 nm of fluorescently labeled oligosaccharides

Ultraviolet photodissociation (UVPD) produces complementary fragmentation to collision-induced dissociation (CID) when implemented for activation of fluorescently labeled oligosaccharide and glycan ions. Reductive amination of oligosaccharides with fluorophore reagents results in efficient photon absorption at 355 nm, producing fragment ions from the nonreducing end that do not contain the appended fluorophore. In contrast to the fragment ions observed upon UVPD (A- and C-type ions), CID produces mainly reducing end fragments retaining the fluorophore (Y-type ions). UVPD affords better isomeric differentiation of both the lacto-N-fucopentaoses series and the lacto-N-difucohexaoses series, but in general, the combination of UVPD and CID offers the most diagnostic elucidation of complex branched oligosaccharides. Four fluorophores yielded similar MS/MS results; however, 6-aminoquinoline (6-AQ), 2-amino-9(10 H)-acridone (AMAC) and 7-aminomethylcoumarin (AMC) afforded more efficient photon absorption and subsequent dissociation than 2-aminobenzamide (2-AB). UVPD also was useful for characterization of glycans released from ribonuclease B and derivatized with 6-AQ. Lastly, electron photodetachment dissociation of oligosaccharides derivatized with 7-amino-1,3-naphthalenedisulfonic acid (AGA) yielded unique cross-ring cleavages similar to those obtained by electron detachment dissociation.

A glycomics platform for the analysis of permethylated oligosaccharide alditols

This communication reports the development of an LC/MS platform for the analysis of permethylated oligosaccharide alditols that, for the first time, demonstrates routine online oligosaccharide isomer separation of these compounds before introduction into the mass spectrometer. The method leverages a high-resolution liquid chromatography system with the superior fragmentation pattern characteristics of permethylated oligosaccharide alditols that are dissociated under low-energy collision conditions using quadrupole orthogonal time-of-flight (QoTOF) instrumentation and up to pseudo MS(3) mass spectrometry. Glycoforms, including isomers, are readily identified and their structures assigned. The isomer-specific spectra include highly informative cross-ring and elimination fragments, branch position specific signatures, and glycosidic bond fragments, thus facilitating linkage, branch, and sequence assignment. The method is sensitive and can be applied using as little as 40 fmol of derivatized oligosaccharide. Because permethylation renders oligosaccharides nearly chemically equivalent in the mass spectrometer, the method is semiquantitative and, in this regard, is comparable to methods reported using high field NMR and capillary electrophoresis. In this postgenomic age, the importance of glycosylation in biological processes has become clear. The nature of many of the important questions in glycomics is such that sample material is often extremely limited, thus necessitating the development of highly sensitive methods for rigorous structural assignment of the oligosaccharides in complex mixtures. The glycomics platform presented here fulfills these criteria and should lead to more facile glycomics analyses.

Tools for glycomics: relative quantitation of glycans by isotopic permethylation using 13CH3I

Analysis of oligosaccharides by mass spectrometry (MS) has enabled the investigation of the glycan repertoire of organisms with high resolution and sensitivity. It is difficult, however, to correlate the expression of glycosyltransferases with the glycan structures present in a particular cell type or tissue because the use of MS for quantitative purposes has significant limitations. For this reason, in order to develop a technique that would allow relative glycan quantification by MS analysis between two samples, a procedure was developed for the isotopic labeling of oligosaccharides with (13)C-labeled methyl iodide using standard permethylation conditions. Separate aliquots of oligosaccharides from human milk were labeled with (12)C or (13)C methyl iodide; the labeled and non-labeled glycans were mixed in known proportions, and the mixtures analyzed by MS. Results indicated that the isotopic labeling described here was capable of providing relative quantitative data with a dynamic range of at least two orders of magnitude, adequate linearity, and reproducibility with a coefficient of variation that was 13% on average. This procedure was used to analyze N-linked glycans released from various mixtures of glycoproteins, such as alpha-1 acid glycoprotein, human transferrin, and bovine fetuin, using MS techniques that included matrix assisted laser desorption ionization-time of flight MS and electrospray ionization with ion cyclotron resonance-Fourier transformation MS. The measured (12)C:(13)C ratios from mixtures of glycans permethylated with either (12)CH(3)I or (13)CH(3)I were consistent with the theoretical proportions. This technique is an effective procedure for relative quantitative glycan analysis by MS.

Strategies for analysis of glycoprotein glycosylation

Glycoproteins are known to exhibit multiple biological functions. In order to assign distinct functional properties to defined structural features, detailed information on the respective carbohydrate moieties is required. Chemical and biochemical analyses, however, are often impeded by the small amounts of sample available and the vast structural heterogeneity of these glycans, thus necessitating highly sensitive and efficient methods for detection, separation and structural investigation. The aim of this article is to briefly review suitable strategies for characterization of glycosylation at the levels of intact proteins, glycopeptides and free oligosaccharides. Furthermore, methods commonly used for isolation, fractionation and carbohydrate structure analysis of liberated glycoprotein glycans are discussed in the context of potential applications in glycoproteomics.

The use of mass spectrometry for the proteomic analysis of glycosylation

Of all protein PTMs, glycosylation is by far the most common, and is a target for proteomic research. Glycosylation plays key roles in controlling various cellular processes and the modifications of the glycan structures in diseases highlight the clinical importance of this PTM. Glycosylation analysis remains a difficult task. MS, in combination with modern separation methodologies, is one of the most powerful and versatile techniques for the structural analysis of glycoconjugates. This review describes methodologies based on MS for detailed characterization of glycoconjugates in complex biological samples at the sensitivity required for proteomic work.

Hydrophilic interaction chromatography

Separation of polar compounds on polar stationary phases with partly aqueous eluents is by no means a new separation mode in LC. The first HPLC applications were published more than 30 years ago, and were for a long time mostly confined to carbohydrate analysis. In the early 1990s new phases started to emerge, and the practice was given a name, hydrophilic interaction chromatography (HILIC). Although the use of this separation mode has been relatively limited, we have seen a sudden increase in popularity over the last few years, promoted by the need to analyze polar compounds in increasingly complex mixtures. Another reason for the increase in popularity is the widespread use of MS coupled to LC. The partly aqueous eluents high in ACN with a limited need of adding salt is almost ideal for ESI. The applications now encompass most categories of polar compounds, charged as well as uncharged, although HILIC is particularly well suited for solutes lacking charge where coulombic interactions cannot be used to mediate retention. The review attempts to summarize the ongoing discussion on the separation mechanism and gives an overview of the stationary phases used and the applications addressed with this separation mode in LC.

Miniaturized separation techniques in glycomic investigations

High-sensitivity glycomic analyses are becoming of a great interest in modern biomedical and clinical research, as well as in the development of recombinant protein products. The evolution of separation techniques for glycomic analysis at high sensitivity is highlighted in this review. These methodologies include capillary liquid chromatography, capillary electrophoresis (CE) and capillary electrochromatography (CEC). The potential of such methodologies in glycomic analysis is demonstrated for model glycoproteins as well as total glycomes derived from biological samples.

Differentiating structural isomers of sialylated glycans by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry

Using model acidic glycans, we demonstrate the benefits of permethylation for matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI/TOF-TOF) tandem mass spectrometry. With both the linear and branched structures, extensive cross-ring fragmentation product ions were generated, yielding valuable information on sugar linkages. Elimination of the negative charges commonly associated with sialylated structures through permethylation allowed their structural analysis in the positive ion mode. Extensive A- and X-type ions were observed for the linear structures, and slightly weaker signals for the branched sialylated structures. The diagnostic cross-ring fragments, permitting a distinction between alpha2-3 and alpha2-6 linkages of the sialic acid residues, were seen in abundance. Importantly, the cross-ring fragmentation with the branched structures provides adequate information to assign sialic acid residues, with a specific linkage, to a particular antenna.

Isotope tag method for quantitative analysis of carbohydrates by liquid chromatography-mass spectrometry

We have previously demonstrated that liquid chromatography/mass spectrometry equipped with a graphitized carbon column (GCC-LC/MS) is useful for the structural analysis of carbohydrates in a glycoprotein. Here, we studied the monosaccharide composition analysis and quantitative oligosaccharide profiling by GCC-LC/MS. Monosaccharides were labeled with 2-aminopyridine and then separated and monitored by GCC-LC/MS in the selective ion mode. The use of tetradeuterium-labeled pyridylamino (d4-PA) monosaccharides as internal standards, which were prepared by the tagging of standard monosaccharides with hexadeuterium-labeled 2-aminopyridine (d6-AP), afforded a good linearity and reproducibility in ESIMS analysis. This method was successfully applied to the monosaccharide composition analysis of model glycoproteins, fetuin, and erythropoietin. For quantitative oligosaccharide profiling, oligosaccharides released from an analyte and a standard glycoprotein were tagged with d0- and d6-AP, respectively, and an equal amount of d0- and d4-PA oligosaccharides were coinjected into GCC-LC/MS. In this procedure, the oligosaccharides that existed in either analyte or a standard glycoprotein appeared as single ions, and the oligosaccharides that existed in both analyte and a standard glycoprotein were detected as paired ions. The relative amount of analyte oligosaccharides could be determined on the basis of the analyte/internal standard ion-pair intensity ratio. The quantitative oligosaccharide profiling enabled us to make a quantitative and qualitative comparison of glycosylation between the analyte and standard glycoproteins. The isotope tag method can be applicable for quality control and comparability assessment of glycoprotein products as well as the analysis of glycan alteration in some diseases.

New hyphenated methodologies in high-sensitivity glycoprotein analysis

High-sensitivity glycoprotein analyses are of particular interest in modern biomedical and clinical research, as well as in the development of recombinant protein products. The evolution of new hyphenated methodologies in high-sensitivity glycoprotein analysis is highlighted in this thematic review. These methodologies include, in particular, capillary LC/MALDI/TOF/TOF MS in conjunction with online permethylation platform, and silica-based lectin microcolumns interfaced to MS. The potential of these methodologies in glycomic and glycoproteomic analysis is demonstrated for model glycoproteins as well as total glycomes and glycoproteomes derived from biological samples. Additionally, the applications of CE-MS, CEC, and nanoLC with graphitized carbon in the areas of glycomics and glycoproteomics are described.

Simultaneous Analysis of 2-Aminopyridine-Derivatized Neutral and Sialylated Oligosaccharides from Human Serum in the Negative-Ion Mode by Sonic Spray Ionization Ion Trap Mass Spectrometry

Neutral and acidic (sialylated) 2-aminopyridine-derivatized (PA) oligosaccharides were analyzed by using reversed-phase high-performance liquid chromatography/ion trap mass spectrometry (RP-HPLC/IT MS) with a sonic spray ionization (SSI) source. Under the RP-HPLC separation using a buffer of 1 mM ammonium acetate (pH4.3) at a flow rate of 0.2 mL/min, both PA-oligosaccharides in the negative-ion mode showed a comparable degree of ionization efficiency, differing from that of the positive-ion mode, which exhibits a wide gap between their ionization efficiencies. In addition, the ion intensities of both PA-oligosaccharides were higher in the negative-ion mode than in the positive-ion mode. These results strongly suggest that the negative-ion mode of SSI-MS is suitable for simultaneous analysis of neutral and acidic (sialylated) oligosaccharides in RP-HPLC/MS. In the present study, RP-HPLC/SSI-IT MS in the negative-ion mode was used in the analysis of PA-oligosaccharides from human serum and its usefulness was investigated. As a result, 32 neutral and sialylated PA-oligosaccharides from human serum were identified with differentiating isomeric oligosaccharides and relatively quantified by a single HPLC/MS run. This method is useful for simple and rapid analysis of the overall distribution of neutral and sialylated oligosaccharides in a complex sample such as serum.

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.

Characterization of the N-linked glycans of Giardia intestinalis

This article reports the first rigorous evidence for the existence of N-glycans in Giardia intestinalis, a parasite that is a widespread human pathogen, being a major cause of enteric disease in the world. Excreted/secreted molecules of G. intestinalis are known to stimulate the immune system. Structural strategies based on MALDI and electrospray mass spectrometry were employed to examine the excreted/secreted molecules for their N-glycan content. These revealed that the major oligosaccharides released by peptide N-glycosidase F are complex-type structures and correspond to bi-, and triantennary structures without core (alpha1,6) fucosylation. The major nonreducing epitopes in these complex-type glycans are: Galbeta1-4GlcNAc (LacNAc) and NeuAc alpha2-6Galbeta1-4GlcNAc (sialylated LacNAc).

Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry of Human α-1-Acid Glycoprotein

The ultrahigh resolution and sensitivity of electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry have for the first time been exploited for the characterization of highly sialylated glycoproteins, using human alpha-1-acid glycoprotein as the model compound. An alternative approach to the widely used high-performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization (MALDI) assays is described. This new method does not require any enzymatic or chemical digestion (removal of sialyl groups or deglycosylation), chemical derivatization (introduction of chromophore groups), or preliminary chromatographic separation (HPLC or electrophoresis). Following ESI and accumulation of ions in a hexapole ion guide, ions are injected into the ICR cell. A selected mass window from the overall ion population is isolated and axialized prior to detection. After acquisition and Fourier transform of the transient signal the resulted spectrum is evaluated in order to determine the charge state of the detected ions and the isotope pattern of the measured protein glycoform. The presence of ions from the same glycoform with different charge states was confirmed. The advantages and limitations of the technique are discussed. Future prospects and possible applications are indicated.

Mass spectrometry of oligosaccharides

Glycosylation is a common post-translational modification to cell surface and extracellular matrix (ECM) proteins as well as to lipids. As a result, cells carry a dense coat of carbohydrates on their surfaces that mediates a wide variety of cell-cell and cell-matrix interactions that are crucial to development and function. Because of the historical difficulties with the analysis of complex carbohydrate structures, a detailed understanding of their roles in biology has been slow to develop. Just as mass spectrometry has proven to be the core technology behind proteomics, it stands to play a similar role in the study of functional implications of carbohydrate expression, known as glycomics. This review summarizes the state of knowledge for the mass spectrometric analysis of oligosaccharides with regard to neutral, sialylated, and sulfated compound classes. Mass spectrometric techniques for the ionization and fragmentation of oligosaccharides are discussed so as to give the reader the background to make informed decisions to solve structure-activity relations in glycomics.

Structural analysis of permethylated oligosaccharides using electrospray ionization quadrupole time-of-flight tandem mass spectrometry and deutero-reduction

Deutero-reduced permethylated oligosaccharides were analyzed by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) using a hybrid quadrupole orthogonal acceleration time-of-flight mass spectrometer, fitted with a nanoflow ESI source. Under these ionization conditions such derivatives preferentially form sodiated molecular species in addition to protonated molecular species. Under collision-induced dissociation, protonated and sodiated molecular species yield simple and predictable fragment mass spectra. A systematic study was conducted on a series of deutero-reduced permethylated glycans to allow rationalization of the fragmentation processes. MS/MS spectra were characterized by fragments resulting from the cleavage of glycosidic bonds. These fragments originating from both the reducing and the non-reducing ends of the glycan yield information on sequence and branching. Furthermore, the substituent 3-linked to a HexNAc unit was readily eliminated. Special attention was devoted to a systematic study of fucosylated glycans. The fucosylated deutero-reduced permethylated glycans were submitted to an acidic hydrolysis, releasing specifically the fucosyl residues. The nascent free hydroxyl groups were subsequently CD3-labelled in order to determine the positions initially bearing the fucosyl residues along the oligosaccharide backbone. This methodology was finally applied to characterize a glycan pool enzymatically released from glycoproteins. The present data show that structural elucidation can be achieved at the 50 fmol level.

Microanalysis of N-linked oligosaccharides in a glycoprotein by capillary liquid chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry

We have studied rapid and simple sugar mapping using liquid chromatography/electrospray ionization mass spectrometry (LC/MS) equipped with a graphitized carbon column. The oligosaccharide mixture was separated on the basis of the sequence, branching structure, and linkage, and each oligosaccharide was characterized based on its molecular mass. In this study we demonstrated the usefulness of capillary LC/MS (CapLC/MS) and capillary liquid chromatography/tandem mass spectrometry (CapLC/MS/MS) as sensitive means for accomplishing the structural analysis of oligosaccharides in a low-abundance glycoprotein. The carbohydrate heterogeneity and molecular mass information of each oligosaccharide can be readily obtained from CapLC/MS of a small amount of glycoprotein. CapLC/MS/MS provided b-ion series, which is informative with regard to monosaccharide sequence. Exoglycosidase digestion followed by CapLC/MS elucidated a carbohydrate residue linkage. Using this method, we characterized N-linked oligosaccharides in hepatocyte growth factor produced in mouse myeloma NS0 cells as the complex-type bi-, tri-, and tetraantennary terminated with N-glycolylneuraminic acids and alpha-linked galactose residues. Sugar mapping with CapLC/MS and CapLC/MS/MS is useful for monitoring glycosylation patterns and for structural analysis of carbohydrates in a low-abundance glycoprotein and thus will become a powerful tool in biological, pharmaceutical, and clinical studies.

Reversed-phase ion-pairing liquid chromatography/ion trap mass spectrometry for the analysis of negatively charged, derivatized glycans

The significant complexity, similar polarity and lack of ionizable sites make the analysis of glycans an analytical challenge. These compounds are often derivatized and separated by normal-phase high-performance liquid chromatography (HPLC) or capillary electrophoresis (CE) followed by UV or fluorescence detection. Due to widespread use of reversed-phase chromatography coupled to electrospray mass spectrometry as an analytical tool, our laboratory has developed this methodology for the analysis of glycans derivatized with a negatively charged tag, 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS). It is possible to exploit the ability of this negatively charged tag to interact with a mobile phase ion-pairing reagent, allowing retention on a reversed-phase C(18) column for subsequent on-line UV or MS analysis. ANTS-derivatized samples, including a maltooligosaccharide ladder and glycans released from bovine ribonuclease B, bovine fetuin, and chicken ovalbumin, were analyzed using this method. In addition to reversed-phase retention, ribonuclease B and ovalbumin derivatives displayed highly desirable isomeric separation. With the use of mass spectrometric detection for glycan identity, this allowed relative quantitation of individual components.

Capillary electrophoresis/electrospray ion trap mass spectrometry for the analysis of negatively charged derivatized and underivatized glycans

The increasing interest in the development of glycoproteins for therapeutic purposes has created a greater demand for methods to characterize the sugar moieties bound to them. Traditionally, released carbohydrates are derivatized using such methods as permethylation or fluorescent tagging prior to analysis by high performance liquid chromatography (HPLC), capillary electrophoresis (CE), or direct infusion mass spectrometry. However, little research has been performed using CE with on-line mass spectrometry (MS) detection. The CE separation of neutral oligosaccharides requires the covalent attachment of a charged species for electrophoretic migration. Among charged labels which have shown promise in assisting CE and HPLC separation is the fluorophore 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS). This report describes the qualitative profiling of charged ANTS-derivatized and underivatized complex glycans by CE with on-line electrospray ion trap mass spectrometry. Several neutral standard glycans including a maltooligosaccharide ladder were derivatized with ANTS and subjected to CE/UV and CE/MS using low pH buffers consisting of citric and 6-aminocaproic acid salts. The ANTS-derivatized species were detected as negative ions, and multiple stage MS analysis provided valuable structural information. Fragment ions were easily identified, showing promise for the identification of unknowns. N-Linked glycans released from bovine fetuin were used to demonstrate the applicability of ANTS derivatization followed by CE/MS for the analysis of negatively charged glycans. Analyses were performed on both underivatized and ANTS-derivatized species, and sialylated glycans were separated and detected in both forms. The ability of the ion trap mass spectrometer to perform multiple stage analysis was exploited, with MS5 information obtained on selected glycans. This technique presents a complementary method to existing methodologies for the profiling of glycan mixtures.

Current awareness. Current literature in mass spectrometry

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (3 Weeks journals - Search completed at 7th. Mar. 2001)