Capillary electrophoresis of sialic acid-containing glycoprotein. Effect of the heterogeneity of carbohydrate chains on glycoform separation using an alpha1-acid glycoprotein as a model

Recent advances in top-down proteome sample processing ahead of MS analysis

Top-down proteomics is emerging as a preferred approach to investigate biological systems, with objectives ranging from the detailed assessment of a single protein therapeutic, to the complete characterization of every possible protein including their modifications, which define the human proteoform. Given the controlling influence of protein modifications on their biological function, understanding how gene products manifest or respond to disease is most precisely achieved by characterization at the intact protein level. Top-down mass spectrometry (MS) analysis of proteins entails unique challenges associated with processing whole proteins while maintaining their integrity throughout the processes of extraction, enrichment, purification, and fractionation. Recent advances in each of these critical front-end preparation processes, including minimalistic workflows, have greatly expanded the capacity of MS for top-down proteome analysis. Acknowledging the many contributions in MS technology and sample processing, the present review aims to highlight the diverse strategies that have forged a pathway for top-down proteomics. We comprehensively discuss the evolution of front-end workflows that today facilitate optimal characterization of proteoform-driven biology, including a brief description of the clinical applications that have motivated these impactful contributions.

Glycoprotein analysis using lectin microcolumns and capillary electrophoresis: Characterization of alpha1-acid glycoprotein by combined separation methods

Separations based on combinations of 2.1 mm I.D. high-performance affinity microcolumns and capillary electrophoresis were developed and used to characterize the glycoforms of an intact glycoprotein. Human alpha₁-acid glycoprotein (AGP) was used as a model analyte due to its heterogeneous glycosylation resulting from variations in its degree of branching, fucosylation, and number of sialic acids. Three separation formats were examined based on microcolumns that contained the lectins concanavalin A (Con A) or Aleuria aurantia lectin (AAL). These microcolumns were used with one another or in combination with capillary electrophoresis. N-Glycan analysis of the non-retained and retained AGP fractions was carried out by using PNGase F digestion and nanoflow electrospray ionization mass spectrometry. Con A microcolumns were found to selectively enrich AGP that contained bi-antennary N-glycans, while AAL microcolumns retained AGP with fucose-containing N-glycans. Results from these separation methods indicated that fucosylation of the N-linked glycans was more abundant when a high degree of branching was present in AGP. Sialic acid residues were more abundant when higher degrees of branching and more fucose residues were present in AGP. The separation and analysis methods that were developed could be used with relatively small amounts of AGP and can be adapted for use with other intact glycoproteins.

NEGATIVE ION MASS SPECTROMETRY FOR THE ANALYSIS OF N-LINKED GLYCANS

N-glycans from glycoproteins are complex, branched structures whose structural determination presents many analytical problems. Mass spectrometry, usually conducted in positive ion mode, often requires extensive sample manipulation, usually by derivatization such as permethylation, to provide the necessary structure-revealing fragment ions. The newer but, so far, lesser used negative ion techniques, on the contrary, provide a wealth of structural information not present in positive ion spectra that greatly simplify the analysis of these compounds and can usually be conducted without the need for derivatization. This review describes the use of negative ion mass spectrometry for the structural analysis of N-linked glycans and emphasises the many advantages that can be gained by this mode of operation. Biosynthesis and structures of the compounds are described followed by methods for release of the glycans from the protein. Methods for ionization are discussed with emphasis on matrix-assisted laser desorption/ionization (MALDI) and methods for producing negative ions from neutral compounds. Acidic glycans naturally give deprotonated species under most ionization conditions. Fragmentation of negative ions is discussed next with particular reference to those ions that are diagnostic for specific features such as the branching topology of the glycans and substitution positions of moieties such as fucose and sulfate, features that are often difficult to identify easily by conventional techniques such as positive ion fragmentation and exoglycosidase digestions. The advantages of negative over positive ions for this structural work are emphasised with an example of a series of glycans where all other methods failed to produce a structure. Fragmentation of derivatized glycans is discussed next, both with respect to derivatives at the reducing terminus of the molecules, and to methods for neutralization of the acidic groups on sialic acids to both stabilize them for MALDI analysis and to produce the diagnostic fragments seen with the neutral glycans. The use of ion mobility, combined with conventional mass spectrometry is described with emphasis on its use to extract clean glycan spectra both before and after fragmentation, to separate isomers and its use to extract additional information from separated fragment ions. A section on applications follows with examples of the identification of novel structures from lower organisms and tables listing the use of negative ions for structural identification of specific glycoproteins, glycans from viruses and uses in the biopharmaceutical industry and in medicine. The review concludes with a summary of the advantages and disadvantages of the technique. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Development and evaluation of silica-based lectin microcolumns for glycoform analysis of alpha1-acid glycoprotein

Silica-based lectin microcolumns were developed and optimized for the separation and analysis of glycoform fractions in alpha₁-acid glycoprotein (AGP) based on both the degree of branching and level of fucosylation. Concanavalin A (Con A) and Aleuria Aurantia lectin (AAL) were immobilized onto HPLC-grade silica by reductive amination and packed into 2.1 mm i.d. × 5.0 cm microcolumns. Factors examined for these microcolumns include their protein content, binding capacity, binding strength and band-broadening under isocratic conditions (Con A) or step elution conditions (AAL) and in the presence of various flow rates or temperatures. These factors were examined by using experiments based on frontal analysis, zonal elution, peak profiling and peak decay analysis. Up to 200 μg AGP could be loaded onto a Con A microcolumn and provide linear elution conditions, and 100 μg AGP could be applied to an AAL microcolumn. The final conditions for separating retained and non-retained AGP glycoform fractions on a Con A microcolumn used a flow rate of 50 μL min^-1 and a temperature of 50 °C, which gave a separation of these fractions within 20 min or less. The final conditions for an AAL microcolumn included a flow rate of 0.75 mL min^-1, a temperature of 50 °C, and the use of 2.0 mM l-fucose as a competing agent for elution, giving a separation of non-retained and retained AGP glycoforms in 6 min or less. The inter-day precisions were ±0.7-4.0% or less for the retention times of the AGP glycoforms and ±2.2-3.0% or less for their peak areas.

Glycoform analysis of alpha1-acid glycoprotein based on capillary electrophoresis and electrophoretic injection

A method based on capillary electrophoresis (CE) with electrophoretic injection and absorbance detection was developed for the direct analysis of AGP glycoforms in human serum. Electrophoretic injection of AGP was performed in the reversed-polarity mode of CE with a capillary coated with poly(ethylene oxide) and that had minimal electroosmotic flow. This situation created an essentially stationary interface between the sample and running buffer during injection and sample stacking. This approach allowed an 11,000-fold increase in sample loading for a 5min injection versus hydrodynamic injection and without introducing any significant levels of extra band-broadening. This method was used with sample pretreatment methods based on acid precipitation and desalting to examine AGP glycoforms in only 65μL of serum. A limit of detection of 2.1-11.3nM was obtained for the major AGP glycoform bands in serum, and the sample pretreatment method gave a recovery of 72.3-80.9% for these glycoforms. The precision for the migration times was ±0.08-0.13% and the precision for the peak areas was ±0.34-1.18% when using serum samples and an internal standard. This method was used for both normal pooled serum and serum from individuals with systemic lupus erythematosus. Results were obtained in a separation time of 25min and allowed the comparison of up to eleven glycoform bands in these samples. A similar approach may be useful in examining additional glycoproteins in serum or other types of biological samples.

Glycoform analysis of alpha1-acid glycoprotein by capillary electrophoresis

A relatively fast and reproducible CE separation was developed for the glycoform analysis of α1-acid glycoprotein (AGP). Factors that were considered included the pH for this separation and various techniques for coating the capillary and/or to minimize electroosmotic flow and protein adsorption. Optimum resolution of the AGP glycoforms was obtained at pH 4.2 with a running buffer containing 0.1% Brij 35 and by using static and dynamic coatings of PEO on the capillary. These conditions made it possible to separate nine AGP glycoform bands in about 20min. The limit of detection (based on absorbance measurements) ranged from 0.09 to 0.38μM for these AGP glycoform bands, and the linear range extended up to a total AGP concentration of at least 240μM. The migration times for the glycoform bands had typical within-day and day-to-day precisions of ±0.16-0.23% or less, respectively, on a single treated capillary and the variation between capillaries was ±0.56% or less. A charge ladder approach was employed to examine the mass or charge differences in the glycoforms that made up these bands, giving a good fit to a model in which the neighboring bands differed by one charge (e.g., from a sialic acid residue) and had an average mass difference of approximately 0.7-0.9kDa. The approaches used to develop this separation method are not limited to AGP but could be extended to the analysis of other glycoproteins by CE.

Discrimination of Glycoproteins from Unglycosylated Proteins in Capillary Electrophoresis: Two-Color LIF Detection Coupled with Post-column Derivatization

Glycosylation is one of the most important posttranslational modifications (PTMs) which lead to the functionalization of proteins. Here, we describe one method for discriminating glycosylated proteins from unglycosylated ones in their mixture sample by capillary electrophoretic separation and two-color laser-induced fluorescence detection coupled with post-column derivatization. Two lasers emitting at 450 and 532 nm permit the detection of amino groups of proteins derivatized by naphthalene-2,3-dicarboxaldehyde and a fluorescently labeled lectin, tetramethylrhodamine-labeled concanavalin A (Rh-Con A), respectively. When a protein mixture react with Rh-Con A, the glycoproteins bound with Rh-Con A exhibit signals at the same migration time in two electropherograms obtained by 450- and 532-nm lasers whereas unbound proteins show a signal only in the electropherogram of the 450-nm laser. So, when one protein is glycosylated it is detected at the same migration time in the electropherograms obtained by two lasers.

Capillary-based lectin affinity electrophoresis for interaction analysis between lectins and glycans

Capillary affinity electrophoresis (CAE) is a powerful technique for glycan analysis, and one of the analytical approaches for analyzing the interaction between lectins and glycans. The method is based on the high-resolution separation of fluorescently labeled glycans by capillary electrophoresis (CE) with laser-induced fluorescence detection (LIF) in the presence of lectins (or glycan binding proteins). CAE allows simultaneous determination of glycan structures in a complex mixture of glycans. In addition, we can calculate the binding kinetics on a specific glycan in the complex mixture of glycans with a lectin. Here, we show detailed procedures for capillary affinity electrophoresis of fluorescently labeled glycans with lectins using CE-LIF apparatus. Its application to screening a sialic acid binding protein in plant barks is also shown.

Contribution of CE to the analysis of protein or peptide biomarkers

Biomarker analysis is pivotal for disease diagnosis and one important class of biomarkers is constituted by proteins and peptides. This review focuses on protein and peptide analyses from biological fluids performed by capillary electrophoresis. The various strategies that have been reported to prevent difficulties due to the handling of real samples are described. Innovative techniques to overcome the complexity of the sample, to prevent the adsorption of the analytes on the inner capillary wall, and to increase the sensibility of the analysis are summarized and illustrated by different applications. To fully illustrate the contribution of CE to the analysis of biomarkers from human sample, two detailed protocols are given: the analysis from CSF of five amyloid peptide, biomarkers of the Alzheimer disease, and the analysis of sialoforms of transferrin from human serum.

An efficient method for selectively imaging and quantifying in situ the expression of sialylated glycoproteins on living cells

A simple and efficient method for selectively imaging and monitoring in situ the expression of sialylated glycoproteins on living cells has been developed. Treating living cells by mild periodate oxidation to selectively generate aldehydes on sialylated glycoproteins, followed by direct labeling of aldehydes with a commercially available fluorescent tag, fluorescein-5-thiosemicarbazide (FTSC), allows in situ imaging and quantification of sialylated glycoproteins on living cells. Under optimum reaction conditions, the periodate oxidation-based FTSC ligation (PF) strategy could be completed within 40 min. The cells undergoing the PF assay revealed a 91% viability and a fairly high-level of metabolic activity. Compared with current labeling methods, the PF assay proved to be a simpler and faster means of imaging sialylated glycoproteins on living cells. The PF assay has been successfully applied to imaging the location and quantification of the abundance of sialylated glycoproteins on tumor and normal cells. Our results demonstrated the methodological significance in clinical diagnosis and functional elucidation studies.

Post-translation modification of proteins; methodologies and applications in plant sciences

Proteins have the potential to undergo a variety of post-translational modifications and the different methods available to study these cellular processes has advanced rapidly with the continuing development of proteomic technologies. In this review we aim to detail five major post-translational modifications (phosphorylation, glycosylaion, lipid modification, ubiquitination and redox-related modifications), elaborate on the techniques that have been developed for their analysis and briefly discuss the study of these modifications in selected areas of plant science.

Charge heterogeneity of a therapeutic monoclonal antibody conjugated with a cytotoxic antitumor antibiotic, calicheamicin

A robust and highly reproducible capillary isoelectric focusing (cIEF) method for the evaluation of charge heterogeneity of monoclonal antibody (mAb) pharmaceutical which contains covalently bound antitumor compounds was developed using a combination of commercially available dimethylpolysiloxane-coated capillary and carrier ampholyte. In order to optimize major analytical parameters for robust mobilization, experimental responses from three pI markers were selected. The optimized method gave excellent repeatability and intermediate precision in estimated pI values of charge variants with relative standard deviations (RSDs) of not more than 0.06% and 0.95%, respectively, when using IgG(4) as a model. Furthermore, RSDs of charge variant compositions were less than 5.0%. These results suggest that the proposed method can be a powerful tool for reproducible evaluation of charge variants of both naked mAbs and their conjugates with high resolution, and it is applicable to quality testing and detailed characterization in the pharmaceutical industry. In addition, it should be noticed that the method provided non-linear pH gradient within the tested ranges, from pI 9.50 to 3.78, and the pH gradient caused the inconsistency of estimated pI ranges between cIEF and gel IEF. This result indicates that selecting appropriate pI markers based on the target pI ranges of charge variants for each mAb related pharmaceutical is highly recommended for the precise determination of pI values.

Capillary lectin-affinity electrophoresis for glycan analysis

Glycosylation is one of the most important post-translational events for proteins, affecting their functions in health and disease, and plays significant roles in various information trafficking for intercellular and intracellular biological events. The glycans which show such important effects are generally present as quite complex mixtures in minute amounts. The approach described here makes it possible to profile glycans for the analysis of post-translational modification of proteins with carbohydrates. The method is based on high-resolution separation of fluorescent-labeled carbohydrates by capillary electrophoresis with laser-induced fluorescent detection in the presence of carbohydrate-binding proteins at different concentrations. The technique affords simultaneous determination of glycans having similar structures even in complex mixtures.

Enrichment strategies for glycopeptides

In order to understand glycoprotein functionality, information on the structure of both the core proteins and the glycan moieties is necessary. From a practical viewpoint, glycopeptides rather than whole glycoproteins are the general targets for structural analysis, which is primarily carried out by employing mass spectrometry (MS). Using the "glycoproteomics" concept, several techniques have recently been developed to allow the preparation of a series of reference glycopeptides. In this chapter, we describe two selective capturing methods for glycopeptides, i.e., lectin-affinity chromatography and polysaccharide hydrophilic affinity physicochemical chromatography. The combined use of these methods effectively removes non-glycosylated peptides, the inclusion of which substantially interferes with glycopeptide ionization in MS analysis.

Capillary electrophoresis-electrospray ionization mass spectrometry for rapid and sensitive N-glycan analysis of glycoproteins as 9-fluorenylmethyl derivatives

It is well known that most protein therapeutics such as monoclonal antibody pharmaceuticals and other biopharmaceuticals including cancer biomarkers are glycoproteins, and thus the development of high-throughput and sensitive analytical methods for glycans is essential in terms of their determination and quality control. We previously reported a novel alternative labeling method for glycans involving 9-fluorenylmethyl chloroformate (Fmoc-Cl) instead of the conventional reductive amination procedure. The derivatives were analyzed by high-performance liquid chromatography (HPLC) (Kamoda S, Nakano M, Ishikawa R, Suzuki S, Kakehi K. 2005. Rapid and sensitive screening of N-glycans as 9-fluorenylmethyl derivatives by high-performance liquid chromatography: A method which can recover free oligosaccharides after analysis. J Proteome Res. 4:146-152). This method was rapid and simple; however, it was time-consuming in terms of analysis by HPLC and did not provide so much information such as the detailed structures and mass numbers of glycans. Here we have developed a high-throughput and highly sensitive method. It comprises three steps, i.e., release of glycans, derivatization with Fmoc, and capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI MS) analysis. We analyzed several glycoproteins such as fetuin, alpha1 acid glycoprotein, IgG, and transferrin in order to validate this method. We were able to analyze the above glycoproteins with the three-step procedure within only 5 h, which provided detailed N-glycan patterns. Moreover, the MS/MS analysis allowed identification of the N-glycan structures. As novel applications, the method was employed for the analysis of N-glycans derived from monoclonal antibody pharmaceuticals and also from alpha-fetoprotein; the latter is known as one of the tumor markers of hepatocellular carcinomas. We were able to easily and rapidly determine the detailed structures of the N-glycans. The present method is very useful for the analysis of large numbers of samples such as a routine analysis.

Mass spectrometric and linear discriminant analysis of N-glycans of human serum alpha-1-acid glycoprotein in cancer patients and healthy individuals

N-glycan oligosaccharides of human serum alpha(1)-acid glycoprotein (AGP) samples isolated from 43 individuals (healthy individuals and patients with lymphoma and with ovarian tumor) were analyzed by MALDI-TOF mass spectrometry and a multivariate statistical method (linear discriminant analysis, LDA). 34 different glycan structures have been identified. From the glycosylation pattern determined by mass spectrometry fucosylation and branching indices have been calculated. These parameters show only small differences between the patient groups studied, but these differences are not sufficiently large to use as a potential biomarker. LDA analysis, on the other hand shows a very good separation between the three groups (with a classification of 88%). Cross-validation indicates that the method has predictive power: Identifying cancerous vs. healthy individuals shows 96% selectivity and 93% specificity; identification of lymphoma vs. the mixed group of healthy and ovarian tumor cases is also promising (72% selectivity and 84% specificity). The pilot study presented here demonstrates that mass spectrometry combined with linear discriminant analysis (LDA) may provide valuable data for identifying and studying the pathophysiology of malignant diseases.

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.

Selective sampling of multiply phosphorylated peptides by capillary electrophoresis for electrospray ionization mass spectrometry analysis

The ionization of phosphorylated peptides in positive ion mode mass spectrometry is generally less efficient compared with the ionization of their non-phosphorylated counterparts. This can make phosphopeptides much more difficult to detect. One way to enhance the detection of phosphorylated proteins and peptides is by selectively isolating these species. Current approaches of phosphopeptide isolation are based on the favorable interactions of phosphate groups with immobilized metals. While these methods can be effective in the extraction, they can lead to incomplete sample recovery, particularly for the most strongly bound multiply phosphorylated components. A non-sorptive method of phosphopeptide isolation using capillary electrophoresis (CE) was recently reported [Zhang et al., Anal. Chem. 77 (2005) 6078]. The relatively low isoelectric points of phosphopeptides cause them to remain anionic at acidic sample pH. Hence, they can be selectively injected into the capillary by an applied field after the electroosmotic flow (EOF) is suppressed. The technique was previously coupled with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). In this work, the exploitation of selective sampling in conjugation with electrospray ionization mass spectrometry (ESI-MS) is presented. The transition was not immediately straightforward. A number of major alterations were necessary for ESI interfacing. These adaptations include the choice of a suitable capillary coating for EOF control and the incorporation of organic solvent for efficient ESI. As expected, selective injection of phosphopeptides greatly enhanced the sensitivity of their detection in ESI-MS, particularly for the multiply phosphorylated species that were traditionally most problematic. Furthermore, an electrophoretic separation subsequent to the selective injection of the phosphopeptides was performed prior to analysis by ESI-MS. This allowed us to resolve the multiply phosphorylated peptides present in the samples, predominantly based on the number of phosphorylation sites on the peptides.

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.

CZE of human alpha-1-acid glycoprotein for qualitative and quantitative comparison of samples from different pathological conditions

Alpha-1-acid glycoprotein (AGP) presents different forms, which may arise from differences in the amino acid sequence and/or in the glycosidic part of the protein. Changes in forms of AGP have been described in literature as a possible tumor marker. While most previous works have approached the study of glycopeptides and/or glycans obtained after fragmentation of the protein, in this work, a CZE method is developed to separate up to eleven peaks of intact forms of AGP. A computer program developed in our laboratory is used to select the migration parameters that make possible an accurate assignment of AGP peaks. Electropherograms of AGP samples purified from sera of cancer patients and healthy donors are qualitatively and quantitatively compared. Percentages of correct assignment of AGP peaks close to 100% are achieved by using either the migration time of each peak relative to that of the EOF marker or the effective electrophoretic mobility of the peaks. The computer program permits to select, among different hypotheses for peak allotment, that one providing the highest accuracy of assignment. In this way, some peaks with different charge-to-mass ratio and a different distribution of area percentage of AGP forms are observed when comparing samples from sick and healthy individuals. Thus, a method that permits to compare AGP forms existing in sera of individuals with different pathophysiological situations has been developed. A potential for using AGP forms analyzed by CZE as a disease marker and for using this technique for screening purposes is envisaged.

Capillary electrophoresis for the analysis of glycoprotein pharmaceuticals

Carbohydrate chains in glycoprotein pharmaceuticals play important roles for the expression of their biological activities, but the structure and compositions of carbohydrate chains are dependent on the conditions for their production. Therefore, evaluation of the carbohydrate chains is quite important for productive process development, characterization of product for approval application, and routine quality control. The oligosaccharides themselves have complex structure including blanching and various glycosidic linkages, and oligosaccharides in one glycoprotein pharmaceutical generally have high heterogeneity, and characterization of oligosaccharide moiety in glycoprotein has been a challenging target. In these situations, CE has been realized as a powerful tool for oligosaccharide analysis due to its high resolution and automatic operating system. This review focuses on the application of CE to the glycoform analysis of glycoproteins and profiling of the N-linked glycans released from glycoprotein pharmaceuticals. Current applications for structure analysis using CE-MS(n) technique and glycan profiling method for therapeutic antibody are also described.

Comparative studies on the analysis of urinary trypsin inhibitor (ulinastatin) preparations

Urinary trypsin inhibitor (ulinastatin) is a characteristic protein pharmaceutical which contains both glycosaminoglycans and N-linked glycans in its molecule and has been used for treatment of acute pancreatitis. The comparability of ulinastatin preparations of different lots or from different companies was studied by using conventional analytical approaches such as SDS-PAGE, cellulose acetate membrane electrophoresis, and HP size-exclusion chromatography (SEC) and also by using newly developed techniques such as CE and MALDI-TOF MS. The methods using SEC and SDS-PAGE according to The Japanese Pharmacopoeia showed similar molecular masses for two different preparations, and the estimated molecular masses were significantly different from those observed with MALDI-TOF MS. We also showed that the electrophoretic methods using cellulose acetate membrane electrophoresis and CE can be used for comparability assessments of ulinastatin preparations. In addition, we analyzed the unsaturated disaccharides derived from glycosaminoglycan (chondroitin 4-sulfate chain) and N-linked oligosaccharides attached to ulinastatin by CE after releasing them by enzymatic digestion followed by fluorescent labeling with 2-aminoacridone and 2-aminobenzoic acid, respectively. The results indicated that carbohydrate chains are important as markers for comparability assessments of ulinastatin pharmaceutical preparations.

Isolation and Identification of Sialylated Glycopeptides from Bovine α1-Acid Glycoprotein by Off-Line Capillary Electrophoresis MALDI-TOF Mass Spectrometry

Sialylated glycopeptides contained in liquid chromatographic fractions of bovine alpha1-glycoprotein tryptic digests were isolated from asialo peptides using capillary electrophoresis (CE). CE effluents were deposited directly onto a metallic target and analyzed using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. This method allowed the characterization of four N-glycosylation sites in the glycoprotein, and each site was observed as a set of sialylated peptide glycoforms. Tandem mass spectrometry was used to confirm peptide sequences and glycan content in glycoforms. The CE method developed for this study resulted in a very clear separation of the sialylated from the asialo content of glycoprotein digests and proved very useful in the determination of the nature and location of sialylated glycans along the protein chain. This article is the first report describing the use of on-target CE fraction collection using a MALDI removable sample concentrator.

Rapid analysis of oligosaccharides derived from glycoproteins by microchip electrophoresis

A novel method for fast profiling of complex oligosaccharides released from glycoproteins based on microchip electrophoresis (mu-CE) is presented here. The characterization of separation conditions, i.e., the composition, concentration and pH of running buffer as well as the applied voltage, has been performed using maltose (G2), cellobiose ( G2'), maltriose (G3) and panose (G3') as oligosaccharide isomer models. In mu-CE, much better separation of oligosaccharide isomers and oligosaccharide ladder was obtained in phosphate buffer than in borate buffer over a wide pH range. Under optimal conditions, high-performance separation of the N-linked complex oligosaccharides released from ribonuclease B, fetuin, alpha1-acid glycoprotein (AGP) and IgG was achieved using polymethylmethacrylate (PMMA) microchips with an effective separation channel of 30 mm. These results represent the first reported analysis of the N-linked oligosaccharides derived from glycoproteins by mu-CE, indicating that the present mu-CE-based method is a promising alternative for characterization of the N-linked oligosaccharides in glycoproteins.

Analysis of glycoprotein-derived oligosaccharides in glycoproteins detected on two-dimensional gel by capillary electrophoresis using on-line concentration method

Capillary electrophoresis (CE) is an effective tool to analyze carbohydrate mixture derived from glycoproteins with high resolution. However, CE has a disadvantage that a few nanoliters of a sample solution are injected to a narrow capillary. Therefore, we have to prepare a sample solution of high concentration for CE analysis. In the present study, we applied head column field-amplified sample stacking method to the analysis of N-linked oligosaccharides derived from glycoprotein separated by two-dimensional gel electrophoresis. Model studies demonstrated that we achieved 60-360 times concentration effect on the analysis of carbohydrate chains labeled with 3-aminobenzoic acid (3-AA). The method was applied to the analysis of N-linked oligosaccharides from glycoproteins separated and detected on PAGE gel. Heterogeneity of alpha1-acid glycoprotein (AGP), i.e. glycoforms, was examined by 2D-PAGE and N-linked oligosaccharides were released by in-gel digestion with PNGase F. The released oligosaccharides were derivatized with 3-AA and analyzed by CE. The results showed that glycoforms having lower pI values contained a larger amount of tetra- and tri-antennary oligosaccharides. In contrast, glycoforms having higher pI values contained bi-antennary oligosaccharides abundantly. The result clearly indicated that the spot of a glycoprotein glycoform detected by Coomassie brilliant blue staining on 2D-PAGE gel is sufficient for quantitative profiling of oligosaccharides.

Hybrid Dynamic Coating with n-Dodecyl β-d-Maltoside and Methyl Cellulose for High-Performance Carbohydrate Analysis on Poly(methyl methacrylate) Chips

Hybrid dynamic coating using n-dodecyl beta-d-maltoside (DDM) and methyl cellulose (MC) has been developed for suppression of analyte adsorption and electroosmotic flow (EOF) in a poly(methyl methacrylate) (PMMA) channel. The adsorption of APTS-labeled sugars in a PMMA channel was obviously suppressed with DDM dynamic coating; however, EOF was reduced only by a factor of approximately 25%, resulting in irreproducible separations. In contrast, both analyte adsorption and EOF in a PMMA channel were efficiently minimized with MC coating; however, concentrated MC above 0.3% was required to achieve high-performance separations, which greatly increased viscosity of the solution and caused difficulties during buffer loading and rinsing. In addition, n-dodecyltrimethylammonium chloride did not show observable effects on reducing analyte adsorption, although it has the same hydrophobic alkyl chain as DDM. These results strongly indicated that the polysaccharide moiety of surface modifiers has a specific affinity to surface charges and is crucial to achieving efficient and stable dynamic coating on the PMMA surface. Hybrid dynamic coating with 0.25% DDM and 0.03% MC was found to minimize both analyte adsorption and EOF in a PMMA channel to a negligible level, while still keeping a low viscosity of the solution. High-speed and high-throughput profiling of the N-linked glycans derived from alpha1-acid glycoprotein, fetuin, and ribonuclease B was demonstrated in both single-channel and 10-channel PMMA chips using DDM-MC hybrid coating. We propose that DDM-MC hybrid coating might be a general method for suppressing analyte adsorption and EOF in polymer MCE devices. The current MCE-based method might be a promising alternative for high-throughput screening of carbohydrate alterations in glycoproteins.

Capillary affinity electrophoresis using lectins for the analysis of milk oligosaccharide structure and its application to bovine colostrum oligosaccharides

Animal colostrum and milk contain complex mixtures of oligosaccharides, which have species-specific profiles. Milk oligosaccharides have various types of structure related to the core structures of glycolipids and N- and O-glycans of glycoproteins and provide a good library to examine the binding of oligosaccharides to various lectins. Recently, we reported a capillary affinity electrophoresis (CAE) method for analyzing the interactions between lectins and complex mixtures of N-linked oligosaccharides prepared from serum glycoproteins. The present paper reports the interactions between 24 milk oligosaccharides and six lectins (PA-I, RCA(120), SBA, WGA, UEA-I, and AAL) analyzed using CAE. Based on the resulting data, we constructed a library that enables us to determine nonreducing terminal monosaccharides, such as Gal, GalNAc, GlcNAc, and Fuc, and to differentiate Gal- or Fuc-linked isomers, such as lacto-N-tetraose, lacto-N-neotetraose, and lacto-N-fucopentaose II and III. In addition, using the library, we show that a combination of the lectins can characterize the neutral oligosaccharides derived from bovine colostrum.

Synthesis and molecular recognition of carbohydrate-centered multivalent glycoclusters by a plant lectin RCA120

Water soluble and lectin-recognizable carbohydrate-centered glycoclusters were prepared efficiently by the Huisgen 1,3-cycloaddition reaction of methyl-2,3,4,6-tetra-O-propargyl beta-D-galactopyranoside with 2-azidoethyl glycosides of lactose and N-acetyllactosamine. Their binding by a plant lectin RCA120 was examined by capillary affinity electrophoresis using fluorescence-labeled asialoglycans from human alpha1-acid glycoprotein. The glycoclusters showed 400-fold stronger inhibitory effect than free lactose, manifesting strong multivalency effect.

Sample preparation for peptides and proteins in biological matrices prior to liquid chromatography and capillary zone electrophoresis

The determination of peptides and proteins in a biological matrix normally includes a sample-preparation step to obtain a sample that can be injected into a separation system in such a way that peptides and proteins of interest can be determined qualitatively and/or quantitatively. This can be a rather challenging, labourious and/or time-consuming process. The extract obtained after sample preparation is further separated using a compatible separation system. Liquid chromatography (LC) is the generally applied technique for this purpose, but capillary zone electrophoresis (CZE) is an alternative, providing fast, versatile and efficient separations. In this review, the recent developments in the combination of sample-preparation procedures with LC and CZE, for the determination of peptides and proteins, will be discussed. Emphasis will be on purification from and determination in complex biological matrices (plasma, cell lysates, etc.) of these compounds and little attention will be paid to the proteomics area. Additional focus will be put on sample-preparation conditions, which can be 'hard' or 'soft', and on selectivity issues. Selectivity issues will be addressed in combination with the used separation technique and a comparison between LC and CZE will be made.

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.

?1-Acid glycoprotein fucosylation as a marker of carcinoma progression and prognosis

BACKGROUND

Serum alpha1-acid glycoprotein (AGP), an acute-phase protein secreted by the liver, carries alpha(1,3)-fucosylated structures on its 5 highly branched, N-linked sugar chains.

METHODS

Serum AGP levels in patients with various types of malignancies (n=214 patients) were measured using an enzyme-linked immunosorbent assay with anti-AGP antibody. To investigate glycoforms that differed in their degree of branching and extent of fucosylation, serum AGP samples were analyzed by crossed affinoimmunoelectrophoresis (CAIE) with concanavalin A, and Aleuria aurantia lectin (AAL), and anti-AGP antibody.

RESULTS

A significant difference (P <0.001) in serum AGP levels was observed in preoperative patients compared with levels in the healthy control group, but the levels in individual patients did not reflect their clinical status. Conversely, it was found not only that the patterns of AGP glycoforms differed widely in the patient group compared with the healthy control group, but they also changed depending on each patient's clinical status. Furthermore, AGP glycoforms seemed to be appropriate markers of disease progression and prognosis according to follow-up studies of 45 patients during prolonged preoperative and postoperative periods.

CONCLUSIONS

Patients with advanced malignancies who had AGP glycoforms that contained highly fucosylated triantennary and tetraantennary sugar chains for long periods after surgery were likely to have a poor prognosis. However, patients who had AGP glycoforms without such changes were expected to have a good prognosis.

Sample clean-up method for analysis of complex-type N-glycans released from glycopeptides

N-Glycans in glycoprotein can be liberated either from glycoproteins or from their glycopeptides with glycoamidases. The latter approach is preferable, because it requires a smaller amount of the enzyme, and yields N-glycans in excellent yields. Moreover it alleviates the necessity of removing from the reaction mixture the detergents needed to denature the glycoproteins. On the other hand, this approach necessitates removal of interfering peptidic materials, because some of the peptide peaks often overlap with the peaks of carbohydrate chains in high-performance anion-exchange chromatography (HPAEC). These peptidic materials also hinder labeling of N-glycans by reductive amination. We have tried to remove the interfering peptidic materials by several different methods--octadecyl (C18) silica cartridge, cation-exchange resin column, and graphitized carbon cartridge. Unfortunately, none of these could completely remove the interfering peptidic materials. Therefore, we resorted to modify the amino groups of the peptidic materials with sodium 2,4,6-trinitro-benzene-1-sulfonate (TNBS) to render them more hydrophobic, so that they can be retained more strongly on the C18 or graphitized carbon cartridges. In the model study presented here, we were able to obtain N-glycans for HPAEC analyses without any interfering materials by a combination of TNBS reaction and graphitized carbon treatment.

Ultrafast analysis of oligosaccharides on microchip with light-emitting diode confocal fluorescence detection

We have developed a new method for the high-speed separation and high-sensitivity detection of complex oligosaccharides based on microchip electrophoresis (nu-CE) with light-emitting diode (LED) confocal fluorescence detection. Oligosaccharides labeled with 8-aminopyrene-1,3,6-trisulfonate (APTS) were found to strongly adsorb to the surface of polymethylmethacrylate (PMMA) microchips. Accordingly, three classes of major dynamic coating additives were systematically investigated, and cellulose derivatives were found to specifically suppress such adsorption and allow high-performance separation on PMMA chips. Additive concentration, buffer pH and applied field strength were found to be key factors in the high-performance separation& of APTS-labeled oligosaccharides on PMMA chips. Under optimal conditions, 15 oligosaccharides in dextrin hydrolysate can be separated within 45 s with an electrophoretic separation efficiency of over 400 000 theoretical plates per meter. The relative standard deviation (RSD) values of migration times of fourteen oligosaccharides were less than 0.50% between six different channels, and the detection limit for APTS-labeled glucose was about 1.98 x 10(-8) mol/L or 8.61 amol with a signal-to-noise ratio (S/N) of 3. The high speed, high efficiency and high sensitivity of this micro-CE-based method indicate that it can be widely applied to analysis of complex oligosaccharides.

Capillary affinity electrophoresis for the screening of post-translational modification of proteins with carbohydrates

Glycosylation is one of the most important post-translational events for proteins, affecting their functions in health and disease, and plays significant roles in various information traffics for intracellular and intercellular biological events (Hancock, W. S. J. Proteome Res. 2002, 1, 297). We have attempted to obtain the information on the numbers and amounts of carbohydrate chains. Interaction between carbohydrate chains and proteins that recognize them is a target to understand the biological roles of glycosylation. To date, there have been a few strategies for simultaneous analysis of the interactions between complex mixtures of carbohydrates and proteins. Here, we report an approach to categorize carbohydrate chains using a few glycoprotein samples as models for the studies on the analysis of post-translational modification of proteins with carbohydrates. A combination of some specific lectins was used as carbohydrate-binding proteins. The method is based on high-resolution separation of fluorescent-labeled carbohydrates by capillary electrophoresis with laser-induced fluorescent detection in the presence of carbohydrate-binding proteins at different concentrations. The present technique affords (1) simultaneous determination of carbohydrate chains, (2) binding specificity of the constituent carbohydrate chains to specific proteins, and (3) kinetic data such as the association constant of each carbohydrate. We found that the lectins employed in the present study could discriminate subtle difference in linkages and resolved the carbohydrate mixtures. The results will be useful, for example, to understand the biological events expressed with carbohydrate changes on the cell surface.

Collection of alpha1-acid glycoprotein molecular species by capillary electrophoresis and the analysis of their molecular masses and carbohydrate chains. Basic studies on the analysis of glycoprotein glycoforms

A highly heterogeneous glycoprotein, alpha1-acid glycoprotein, was resolved into their glycoforms by capillary electrophoresis using a surface-modified capillary in 20 mM acetate buffer (pH 4.2) containing 0.5% (w/v) hydroxypropylmethylcellulose. We collected the fractions containing each glycoform as nearly pure state by capillary electrophoresis, and examined the molecular masses of these glycoforms by matrix assisted laser desorption time-of-flight mass spectrometry. We also analyzed carbohydrate chains after releasing them with N-glycosidase F followed by fluorescent labeling with 8-aminopyrene-1,3,6-trisulfonate. We found that the separation of glycoforms was mostly due to the presence of multiantennary carbohydrate chains. We propose that the present technique is useful for the analysis of post translational modification of proteins with carbohydrate chains.

Analysis of glycoproteins and the oligosaccharides thereof by high-performance capillary electrophoresis-significance in regulatory studies on biopharmaceutical products

This review describes the recent development in the analysis of glycoproteins using capillary electrophoresis with various separation techniques, and focuses especially on the analysis of recombinant glycoprotein pharmaceuticals. We include the analysis of glycoprotein multiforms (ie glycoform) as well as glycan analysis. The relationship between glycoprotein multiforms and oligosaccharide distributions in a glycoprotein sample is also discussed. Further, recent development in capillary electrophoresis-mass spectrometry is described.