Usefulness of sugar mapping by liquid chromatography/mass spectrometry in comparability assessments of glycoprotein products

Characterization of N-glycan heterogeneities of erythropoietin products by liquid chromatography/mass spectrometry and multivariate analysis

RATIONALE

Glycan heterogeneity on recombinant human erythropoietin (rEPO) product is considered to be one of the critical quality attributes, and similarity tests of glycan heterogeneities are required in the manufacturing process changes and developments of biosimilars. A method for differentiating highly complex and diverse glycosylations is needed to evaluate comparability and biosimilarity among rEPO batches and products manufactured by different processes.

METHODS

The glycan heterogeneities of nine rEPO products (four innovator products and five biosimilar products) were distinguished by multivariate analysis (MVA) using the peak area ratios of each glycan to the total peak area of glycans in mass spectra obtained by liquid chromatography/mass spectrometry (LC/MS) of N-glycans from rEPOs.

RESULTS

Principal component analysis (PCA) using glycan profiles obtained by LC/MS proved to be a useful method for differentiating glycan heterogeneities among nine rEPOs. Using PC values as indices, we were able to visualize and digitalize the glycan heterogeneities of each rEPO. The characteristic glycans of each rEPO were also successfully identified by orthogonal partial least-squares discrimination analysis (OPLS-DA), an MVA method, using the glycan profile data.

CONCLUSIONS

PCA values were useful for evaluating the relative differences among the glycan heterogeneities of rEPOs. The characteristic glycans that contributed to the differentiation were also successfully identified by OPLS-DA. PCA and OPLS-DA based on mass spectrometric data are applicable for distinguishing glycan heterogeneities, which are virtually indistinguishable on rEPO products.

Differences in sialic acid O-acetylation between human urinary and recombinant erythropoietins: a possible mass spectrometric marker for doping control

Development of a mass spectrometric method for the unambiguous detection of doping with recombinant human erythropoietins (rhEPO) has been attempted for many years. Unfortunately, progress in this field was hampered by the unavailability of highly purified human endogenous EPOs (urinary[uhEPO], serum/plasma EPO)--a prerequisite for generating detailed mass spectrometric glycosylation data necessary for revealing significant differences between uhEPO and rhEPOs. The paper presents the worldwide first analytical data on purified human urinary EPO generated with a high resolution high accuracy mass spectrometer (LTQ-Orbitrap). The focus is on the tryptic O-glycopeptide (E117-R131) and its degree of sialic acid O-acetylation. Data are compared with results obtained from 40 rhEPO pharmaceuticals. It could be demonstrated that the O-glycopeptide of uhEPO (ca 100 IU) contains only trace amounts of mono-acetylated mono-and di-sialylated O-glycans but no other O-acetylated structures and in this respect significantly differs from all rhEPOs. Moreover, Dynepo--a rhEPO previously thought to be not O-acetylated--also contains small amounts of O-acetylations within the O-glycan structure. The results might be useful for anti-doping purposes as well as the development of EPO pharmaceuticals with closer structural similarity to the endogenous hormone.

The overlooked difference between human endogenous and recombinant erythropoietins and its implication for sports drug testing and pharmaceutical drug design

Sequential deglycosylation by exoglycosidase treatment (Reagent Array Analysis Method, RAAM) and subsequent sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed a profound structural difference between human endogenous and recombinant erythropoietins. While both proteins behaved similarly upon digestion with Arthrobacter ureafaciens α-sialidase and Steptococcus pneumoniae β-D-galactosidase, the action of N-acetyl-β-D-glucosaminidase from Steptococcus pneumoniae was partly blocked by endogenous but not recombinant erythropoietins. Consequently, further treatment with Jack bean α-D-mannosidase and Helix pomatia β-D-mannosidase led to only very limited additional deglycosylation of endogenous EPO, while rhEPO glycans continued to be degraded. The behaviour was visualized by SDS-PAGE combined with Western blotting. While the apparent molecular masses of most endogenous glycoforms did not further decrease after treatment with the first three enzymes, masses of most rhEPO glycoforms continued to drop after digestion with the two mannosidases. Both, human urinary and serum EPO showed this blocking effect, and all of the tested 28 recombinant epoetins were accessible to further degradation by exo-mannosidases. The majority of EPO pharmaceuticals is produced in Chinese hamster ovary (CHO) cell lines, few in other ones (i.e. baby hamster kidney (BHK) or human fibrosarcoma (HT-1080) cells). Since human endogenous EPO is primarily produced by the kidneys, tissue specific glycosylation might explain the altered deglycosylation behaviour. This difference was overlooked since EPO was first isolated from human urine in 1977. The results might prove useful for anti-doping testing and future EPO drug development.

Recent developments in doping testing for erythropoietin

The constant development of new erythropoiesis-stimulating agents (ESAs), since the first introduction of recombinant erythropoietin (rhEpo) for clinical use, has also necessitated constant development of methods for detecting the abuse of these substances. Doping with ESAs is prohibited according to the World Anti-Doping Code and its prohibited list of substances and methods. Since the first publication of a direct and urine-based detection method in 2000, which uses changes in the Epo isoform profile as detected by isoelectric focusing in polyacrylamide slab gels (IEF-PAGE), the method has been constantly adapted to the appearance of new ESAs (e.g., Dynepo, Mircera). Blood had to be introduced as an additional matrix, because Mircera (a PEGylated Epo) is best confirmed in serum or plasma after immunoaffinity purification. A Mircera ELISA was developed for fast screening of sera. With the appearance of Dynepo and copy epoetins, the additional application of sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE or equivalent) became necessary. The haematological module of the Athlete Biological Passport is the latest development in multivariable indirect testing for ESA doping. The article summarizes the main strategies currently used in Epo anti-doping testing with special focus on new developments made between 2009 and 2010.

Solvent effects on the retention of oligosaccharides in porous graphitic carbon liquid chromatography

Porous graphitic carbon (PGC) is known as well suited adsorbent for liquid chromatography of carbohydrates. In this work we report on systematic investigations of solvent effects on the retention mechanism of fluorescence labeled malto-oligosaccharides on PGC. The adsorption mechanism was found to depend on the type of organic modifier used in the mobile phase. Positive adsorption enthalpies and entropies, which have already been reported in the literature, were solely produced using acetonitrile. Both alternative solvents (tetrahydrofuran, 2-propanol) yielded in contrast negative enthalpies. As plausible retention mechanism for oligosaccharides on PGC applying acetonitrile as mobile phase component we propose the formation of a dense and highly ordered solvation layer of the PGC surface with the linear acetonitrile molecules. Adsorption of analyte molecules requires a displacement of numerous acetonitrile molecules, which explains the positive enthalpy and entropy values measured. The interplay of enthalpic and entropic contributions to the overall adsorption phenomena results in strongly temperature dependent chromatographic selectivity values.

Erythropoietin and analogs

Erythropoietin (EPO), a glycoprotein hormone, stimulates the growth of red blood cells and as a consequence it increases tissue oxygenation. This performance enhancing effect is responsible for the ban of erythropioetin in sports since 1990. Especially its recombinant synthesis led to the abuse of this hormone, predominatly in endurance sports. The analytical differentiation of endogenously produced erythropoietin from its recombinant counterpart by using isoelectric focusing and double blotting is a milestone in the detection of doping with recombinant erythropoietin. However, various analogous of the initial recombinant products, not always easily detectable by the standard IEF-method, necessitate the development of analytical alternatives for the detection of EPO doping. The following chapter summarizes its mode of action, the various forms of recombinant erythropoietin, the main analytical procedures and strategies for the detection of EPO doping as well as a typical case report.

Identification of glycoproteins carrying a target glycan-motif by liquid chromatography/multiple-stage mass spectrometry: identification of Lewis x-conjugated glycoproteins in mouse kidney

Certain glycan motifs in glycoproteins are involved in several biological events and diseases. To understand the roles of these motifs, a method is needed to identify the glycoproteins that carry them. We previously demonstrated that liquid chromatography-multiple-stage mass spectrometry (LC-MSn) allowed for differentiation of oligosaccharides attached to Lewis-motifs, such as Lewisx(Lex, Galbeta1-4(Fucalpha1-3)GlcNAc) from other glycans. We successfully discriminated Lex-conjugated oligosaccharides from other N-linked oligosaccharides derived from mouse kidney proteins by using Lewis-motif-distinctive ions, a deoxyhexose (dHex)+hexose (Hex)+N-acetylhexsosamine (HexNAc) fragment (m/z 512), and a Hex+HexNAc fragment (m/z 366). In the present study, we demonstrated that this method could be used to identify the Lex-conjugated glycoproteins. All proteins in the mouse kidney were digested into peptides, and the fucosylated glycopeptides were enriched by lectin-affinity chromatography. The resulting fucosylated glycopeptides were subjected to two different runs of LC-MSn using a Fourier- transform ion cyclotron resonance mass spectrometer (FTICR-MS) and an ion trap-type mass spectrometer. After the first run, we picked out product ion spectra of the expected Lex-conjugated glycopeptides based on the presence of Lewis-motif-distinctive ions and assigned a peptide+HexNAc or peptide+(dHex)HexNAc fragment in each spectrum. Then the fucosylated glycopeptides were subjected to a second run in which the peptide-related fragments were set as precursor ions. We successfully identified gamma-glutamyl transpeptidase 1 (gamma-GTP1), low-density lipoprotein receptor-related protein 2 (LRP2), and a cubilin precursor as Lex-conjugated glycoproteins by sequencing of 2-5 glycopeptides. In addition, it was deduced that cadherin 16, dipeptidase I, H-2 class I histocompatibility antigen, K-K alpha precursor (H2-Kk), and alanyl (membrane) aminopeptidase could be Lex-conjugated glycoproteins from the good agreement between the experimental and theoretical masses and fragment patterns. The results indicated that our method could be applicable for the identification and screening of glycoproteins carrying target glycan-motifs, such as Lewis epitopes.

The significance of glycosylation analysis in development of biopharmaceuticals

Many glycoproteins and glycosaminoglycans are approved for clinical use. Carbohydrate moieties in biopharmaceuticals affect not only their physicochemical properties and thermal stability, but also their reactivity with their receptors and circulating half-life. Modification of glycans is one target of drug design for enhancement of efficacy. Meanwhile, there have been reports of serious adverse events caused by some carbohydrates. It is crucial to maintain the constancy of carbohydrate moieties for the efficient and safe use of glycosylated biopharmaceuticals. On the other hand, for scientific, safety-related, and economic reasons, changes in the manufacturing process are frequently made either during the development or after the approval of new biopharmaceuticals. Furthermore, the development of biosimilar glycoprotein products has been attempted by different manufacturers. Changes in pharmaceutical manufacturing processes possibly cause alteration of glycosylation and raise concerns about alteration of their quality, safety, and efficacy. In this review we provide some current topics of glycosylated biopharmaceuticals from the viewpoints of efficacy, safety, and the manufacturing process and discuss the significance of glycosylation analysis for development of biopharmaceuticals.

Oligosaccharide analysis by graphitized carbon liquid chromatography-mass spectrometry

Structural analysis of complex mixtures of oligosaccharides using tandem mass spectrometry is regularly complicated by the presence of a multitude of structural isomers. Detailed structural analysis is, therefore, often achieved by combining oligosaccharide separation by HPLC with online electrospray ionization and mass spectrometric detection. A very popular and promising method for analysis of oligosaccharides, which is covered by this review, is graphitized carbon HPLC-ESI-MS. The oligosaccharides may be applied in native or reduced form, after labeling with a fluorescent tag, or in the permethylated form. Elution can be accomplished by aqueous organic solvent mixtures containing low concentrations of acids or volatile buffers; this enables online ESI-MS analysis in positive-ion or negative-ion mode. Importantly, graphitized carbon HPLC is often able to resolve many glycan isomers, which may then be analyzed individually by tandem mass spectrometry for structure elucidation. While graphitized carbon HPLC-MS for glycan analysis is still only applied by a limited number of groups, more users are expected to apply this method when databases which support structural assignment become available.

Influence of electrosorption, solvent, temperature, and ion polarity on the performance of LC-ESI-MS using graphitic carbon for acidic oligosaccharides

Porous graphitic carbon (PGC) emerges as an ideal stationary phase for LC-ESI-MS of complex oligosaccharides. Therefore, we studied the factors influencing detection and elution of charged oligosaccharides from PGC columns coupled to an ESI source. Electrosorption by the carbon surface leads to total retention of very acidic glycans on instruments where voltage is applied to the spray needle. This problem can be eliminated by thorough electrical grounding. A point of general importance is the influence of ionic strength on the elution and peak shape of glycans containing several carboxylic acid groups in the form of sialic acids or uronic acids. Solvent pH had a marginal effect on the ionization efficiency in both ion polarities, but the content of organic solvent strongly influenced signal intensity of acidic glycans in the negative mode. As a consequence, detection in the positive ion mode appears preferable when neutral and charged glycans shall be quantitated in the same sample. While retention of neutral glycans is not affected by pH, sialylated species are retained somewhat stronger at acidic pH resulting in a larger spread of the entire elution range of N-glycans. Remarkably, retention of glycans on PGC increased at higher temperatures.

Low LC-MS/MS detection of glycopeptides released from pmol levels of recombinant erythropoietin using nanoflow HPLC-chip electrospray ionization

The test used by anti-doping laboratories to detect the misuse of recombinant erythropoietin (rhEPO) is based on its different migration pattern on isoelectric focusing (IEF) gel compared with the endogenous human erythropoietin (hEPO) that can possibly be explained by structural differences. While there is definitely a need to identify those differences by LC-MS/MS, the extensive characterization that was achieved for the rhEPO was never performed on human endogenous EPO because its standard is not available in sufficient amount. The goal of this study was to develop an analytical method to detect pmol amounts of N-linked and O-linked glycopeptides of the recombinant hormone as a model. Using a nanoflow HPLC-Chip electrospray ionization/ion trap mass spectrometer, the diagnostic ion at m/z 366 of oligosaccharides was monitored in the product ion spectra to identify the four theoretical glycosylation sites, Asn24, Asn38, Asn83 and Ser126, respectively, on glycopeptides 22-37, 38-55, 73-96 and 118-136. With 3 pmol of starting material applied on Chip, only the desialylated N-glycopeptides 22-37 and 38-55/38-43 could be observed, and of all the glycan isoforms, those with the smaller structures were predominantly detected. While the preservation of the sialic acid moieties decreased the detection of all the N-glycopeptides, it allowed a more extensive characterization of the O-linked glycopeptide 118-136. The technique described herein provides a mean to detect glycopeptides from commercially available pharmaceutical preparations of rhEPO with the sensitivity required to analyze pmol amounts of hEPO, which could ultimately lead to the identification of structural differences between the recombinant and the human forms of the hormone.

Pronase-immobilized enzyme reactor: an approach for automation in glycoprotein analysis by LC/LC-ESI/MSn

An automated analytical approach is proposed for simultaneous characterization of glycan and peptide moieties in pronase-generated glycopeptides. The proposed method is based on the use of a new pronase-immobilized enzyme reactor for the on-line rapid digestion of the target glycoprotein. By coupling the bioreactor to a Hypercarb chromatographic trap column, on-line selective glycopeptide enrichment prior to normal-phase liquid chromatography-mass spectrometry was obtained. A detailed study was carried out for integration and automation of each phase of the proposed analytical procedure. On-line digestion allowed extensive cleavage of the model protein (ribonuclease B), yielding to glycopeptides with peptide moieties up to eight amino acids, carrying the Man5-Man9 N-glycans each, selectively resolved on an Amide-80 column. The use of a linear ion trap instrument resulted in efficient ion capture and led to MS3 acquisition times and spectra quality similar to those for MS2, allowing the unambiguous identification of glycan (MS2) and peptide (MS3) sequences. The proposed procedure reduces the glycoprotein analysis time from approximately 3 days, as in most of the traditional off-line methods, to approximately 1 h.

Simple separation of isomeric sialylated N-glycopeptides by a zwitterionic type of hydrophilic interaction chromatography

Asparagine-linked oligosaccharides (N-glycans) usually show structural heterogeneity, especially in proteins with sialylated N-glycans and, therefore, their structural analysis is still very difficult. A zwitterionic type of hydrophilic interaction chromatography column with sulfobetaine functional groups (called a ZIC-HILIC column) was applied to the separation of tryptic peptides of alpha-1-acid glycoprotein. It was demonstrated that the ZIC-HILIC separation column has a selectivity for sialylated N-glycopeptides and a high capability for separation based on the structural recognition of sialylated N-glycan isomers as well as for the previously reported neutral N-glycans and N-glycopeptides. The retention characteristics of neutral and sialylated N-glycans derivatized with 2-aminopyridine (PA N-glycans) demonstrate that the retentions of the N-glycans are based primarily on hydrophilic interaction with the water-rich liquid layer generated on the surface of the ZIC-HILIC column. In addition, the electrostatic repulsion interaction shielded with counter ions effectively tunes the separation and recognition of sialylated N-glycan isomers.

Mass Spectrometry-Based Glycoproteomic Approach Involving Lysine Derivatization for Structural Characterization of Recombinant Human Erythropoietin

Lysine-containing peptides comprising glycosylation sites derived from recombinant human erythropoietin (rHuEPO) by trypsin or Lys-C and PNGase F dual digestion were derivatized with 2-methoxy-4,5-dihydro-1H-imidazole and its deuterated analogues. In the same reaction, under reducing conditions (beta-mercaptoethanol), cysteines were converted into methyl-cysteines and lysines into Lys-4,5-dihydro-1H-imidazole. Both modifications on cysteines and lysines simplified the CID-MS/MS spectra, while preserving the structural information by yielding y-series ions and improved the mass spectral signal intensity up to 25 times. Moreover, by this approach, the N-glycan occupation sites were unambiguously determined. O-Glycosylation sites as well as O-glycan structures were determined by a LC-MS/MS experiment carried out on dually digested rHuEPO. N-Glycan mixture purified on a graphitized carbon column using a newly developed method that extracted only sialylated carbohydrates was analyzed first using MALDI-TOF in negative linear ion mode with low mass accuracy but without interferences and metastabile ions and then a reflectron with high mass accuracy. After defining the precursor ions, we performed the nanoESI QTOF MS/MS analysis on N-glycans, mainly targeting the distinction between carbohydrates with sialylated antennae and those lacking sialic acid moieties.

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.

Glycoform characterization of erythropoietin combining glycan and intact protein analysis by capillary electrophoresis – electrospray – time-of-flight mass spectrometry

Glycosylation of recombinant human erythropoietin (rHuEPO) is a post-translational process that alters biological activity, solubility and lifetime of the glycoprotein in blood, and strongly depends on the type of cell and the cell culture conditions. A fast and simple method providing extensive carbohydrate information about the glycans present in rHuEPO and other glycoproteins is needed in order to improve current methods in drug development or product quality control. Here, an improved method for intact rHuEPO glycoform characterization by CZE-ESI-TOF MS has been developed using a novel capillary coating and compared to a previous study. Both methods allow a fast separation in combination with accurate mass characterization of the single protein isoforms. The novel dynamic coating provides a separation at an EOF close to zero, enabling better separation. This results in an improved mass spectrometric resolution and the detection of minor isoforms. In order to assign an unequivocal carbohydrate composition to every intact glycoform, a CZE-ESI-MS separation method for enzymatically released underivatized N-glycans has been developed. The TOF MS allows the correct identification of the glycans due to its high mass accuracy and resolution. Therefore, glycan modifications such as acetylation, oxidation, sulfation and even the exchange of OH by NH(2) are successfully characterized. Information of the protein-backbone molecular mass has been combined with results from peptide analysis (revealing information about O-glycosylation) and from the glycan analysis, including the detection of as yet undescribed glycans containing four antennae and five sialic acids. This allows an unequivocal assignment of an overall glycosylation composition to the molecular masses obtained for the intact rHuEPO glycoforms.

Separation of isomeric 2-aminopyridine derivatized N-glycans and N-glycopeptides of human serum immunoglobulin G by using a zwitterionic type of hydrophilic-interaction chromatography

Isomeric oligosaccharides and isomeric glycopeptides are sometimes difficult to separate on normal-phase (NP) and reversed-phase (RP) columns. A zwitterionic type of hydrophilic-interaction chromatography column with sulfobetaine groups (called ZIC-HILIC column) was first applied to the separation of 2-aminopyridine derivatized (PA) N-glycans and tryptic peptides of human serum immunoglobulin G (IgG). It is shown that the ZIC-HILIC column has high capability for structural recognition of isomeric N-glycans as well as high selectivity for glycopeptides. The former feature (i.e., structural recognition) was proven by sufficient separation of neutral PA N-glycan isomers, which are usually difficult to separate on NP and RP columns. In addition, it is noteworthy that IgG glycopeptides consisting of isomeric N-glycans and the same peptide sequences can be sufficiently separated on a ZIC-HILIC column. The latter feature (i.e., selectivity) was also demonstrated by easily separating two peptide groups with/without N-glycans. Thus, we note that the ZIC-HILIC column is highly promising for a simple analysis of N-glycans and N-glycopeptide samples.

Glycomic/glycoproteomic analysis by liquid chromatography/mass spectrometry: Analysis of glycan structural alteration in cells

The alteration of glycosyltransferase expression and the subsequent changes in oligosaccharide structures are reported in several diseases. The analysis of glycan structural alteration in glycoproteins is becoming increasingly important in the discovery of therapies and diagnostic markers. In this study, we propose a strategy for glycomic/glycoproteomic analysis based on oligosaccharide profiling by LC/MS followed by proteomic approaches, including 2-DE and 2-D lectin blot. As a model of aberrant cells, we used Chinese hamster ovary cells transfected with N-acetylglucosaminyltransferase III (GnT-III), which catalyzes the addition of a bisecting N-acetylglucosamine (GlcNAc) to beta-mannose of the mannosyl core of N-linked oligosaccharides. LC/MS equipped with a graphitized carbon column (GCC) enabled us to elucidate the structural alteration induced by the GnT-III expression. Using 2-D lectin blot followed by LC/MS/MS, the protein carrying an extra N-acetylhexosamine in cells transfected with GnT-III was successfully identified as integrin alpha3. Thus, oligosaccharide profiling by GCC-LC/MS followed by proteomic methods can be a powerful tool for glycomic/glycoproteomic analysis.

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.

Analysis of site-specific glycosylation in recombinant human follistatin expressed in Chinese hamster ovary cells

Follistatin (FS), a glycoprotein, plays an important role in cell growth and differentiation through the neutralization of the biological activities of activins. In this study, we analyzed the glycosylation of recombinant human FS (rhFS) produced in Chinese hamster ovary cells. The results of SDS-PAGE and MALDI-TOF MS revealed the presence of both non-glycosylated and glycosylated forms. FS contains two potential N-glycosylation sites, Asn95 and Asn259. Using mass spectrometric peptide/glycopeptide mapping and precursor-ion scanning, we found that both N-glycosylation sites were partially glycosylated. Monosaccharide composition analyses suggested the linkages of fucosylated bi- and triantennary complex-type oligosaccharides on rhFS. This finding was supported by mass spectrometric oligosaccharide profiling, in which the m/z values and elution times of some of the oligosaccharides from rhFS were in good agreement with those of standard oligosaccharides. Site-specific glycosylation was deduced on the basis of the mass spectra of the glycopeptides. It was suggested that biantennary oligosaccharides are major oligosaccharides located at both Asn95 and Asn259, whereas the triantennary structures are present mainly at Asn95.

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.

Separation of polar compounds using carbon columns

The objective of this review with 122 references is to provide structure and retention mechanisms of porous graphitic carbon by chromatographic analysis and computational chemical analysis of retention mechanisms. Synthesis methods of porous graphitic carbon are described. Applications for use as matrix for dynamic coating on porous graphitic carbon and direct separation of polar compounds on porous graphitic carbon demonstrated that the physical and chemical stability of graphitic carbons performed in both chromatography and extraction, especially for polar compounds, those are difficult on both silica-based and organic polymer-based packing materials. The disadvantage is difficult desorption of non-polar compounds adsorbed on the surface. The development of 3.5-microm particles improves the separation power of graphitic carbon columns with the high theoretical plate number.

Structural analysis of a glycoprotein by liquid chromatography-mass spectrometry and liquid chromatography with tandem mass spectrometry. Application to recombinant human thrombomodulin

Using recombinant human thrombomodulin (rhTM) expressed in Chinese hamster ovary (CHO) cells, we studied the structural analysis of a glycoprotein by liquid chromatography-mass spectrometry (LC-MS) and liquid chromatography with tandem mass spectrometry (LC-MS-MS). First, we analyzed the structure of both the O- and N-linked glycans in rhTM by oligosaccharide mapping using LC-MS equipped with a graphitized carbon column (GCC-LC-MS). Major O- and N-linked glycans were determined to be core 1 structure and fucosyl biantennary containing NeuAc(0-2) respectively. Next, the post-translational modifications and their heterogeneities, including the site-specific glycosylation, were analyzed by mass spectrometric peptide/glycopeptide mapping of trypsin-digested rhTM and precursor-ion scanning. Precursor-ion scanning was successful in the detection of five glycopeptides. Four N-glycosylation sites and their site-specific carbohydrate heterogeneity were determined by their mass spectra. O-Glycosylation could be estimated on the basis of its mass spectrum. We were able to identify partial beta-hydroxylation on Asn324 and Asn439, and O-linked glucose on Ser287 from the peptide/glycopeptide map and their mass spectra. We demonstrated that a sequential analysis of LC-MS and LC-MS-MS are very useful for the structural analysis of O- and N-linked glycans, polypeptides, and post-translational modifications and their heterogeneities, including site-specific glycosylation in a glycoprotein. Our method can be applied to a glycoprotein in biological samples.

Usefulness of glycopeptide mapping by liquid chromatography/mass spectrometry in comparability assessment of glycoprotein products

We previously reported on glycopeptide mapping of erythropoietin (EPO) by liquid chromatography/mass spectrometry (LC/MS). Using this method, glycopeptides in proteolytic digestion can be eluted before peptides, and are further separated on the basis of the carbohydrate structure. The detailed glycosylation at each glycosylation site can be elucidated based on mass chromatography and mass spectroscopy. In this study, we evaluated glycopeptide mapping with regard to its use in comparability assessment of glycoprotein products possessing multiple glycosylation sites. Models of closely related glycoprotein products used in this study are EPOs produced from three different sources. We previously reported that there are differences in the carbohydrate heterogeneity of these EPOs with regard to sialylation, acetylation, and sulphation patterns, using sugar mapping by LC/MS. In this paper, we demonstrated that glycopeptide mapping can distinguish site-specific glycosylation among these three EPOs and reveal the differences in acetylation, sialylation, and sulphation at each glycosylation site in one analysis. Our method can thus be useful in comparability assessment of therapeutic glycoproteins in terms of glycosylation.