Quantitative mapping of the N-linked sialyloligosaccharides of recombinant erythropoietin: combination of direct high-performance anion-exchange chromatography and 2-aminopyridine derivatization

The Pre- and Post-Column Derivatization on Monosaccharide Composition Analysis, a Review

Polysaccharides, as common metabolic products in organisms, play a crucial role in the growth and development of living organisms. For humans, polysaccharides represent a class of compounds with diverse applications, particularly in the medical field. Therefore, the exploration of the monosaccharide composition and structural characteristics of polysaccharides holds significant importance in understanding their biological functions. This review provides a comprehensive overview of extraction methods and hydrolysis strategies for polysaccharides. It systematically analyzes strategies and technologies for determining polysaccharide composition and discusses common derivatization reagents employed in further polysaccharide studies. Derivatization is considered a fundamental strategy for determining monosaccharides, as it not only enhances the detectability of analytes but also increases detection sensitivity, especially in liquid chromatography (LC), capillary electrophoresis (CE), and gas chromatography (GC) techniques. The review meticulously examines pre-column and post-column derivatization techniques for monosaccharide analysis, categorizing them based on diverse detection methodologies. It delves into the principles and distinctive features of various derivatization reagents, offering a comparative analysis of their strengths and limitations. Ultimately, the aim is to provide guidance for selecting the most suitable derivatization approach, taking into account the structural nuances, biological functions, and reaction dynamics of polysaccharides.

Evidence for aluminum-binding erythropoietin by size-exclusion chromatography coupled to electrothermal absorption atomic spectrometry

Erythropoietin (EPO) is a glycoprotein that stimulates erythropoiesis and is clinically used for treating anemia during chronic renal failure and for anemia in preterm infants. EPO formulations usually have elevated rates of contamination due to aluminum (Al), which is toxic to both types of patients. Size-exclusion chromatography (SEC) coupled with graphite furnace atomic absorption spectrometry (GF AAS) was employed to separate proteins and to quantify the amount of aluminum present in the elution volume corresponding to EPO and, therefore, to evaluate possible binding. Because EPO formulations contain human serum albumin (HSA), a chromatographic method was optimized for the separation of these proteins. Subsequent to the chromatographic separation, 1-mL fractions of the column effluent were collected, and the Al content in these aliquots was measured by GF AAS. EPO and HSA samples were incubated with Al for 4h at 4°C and 37°C as well as for 16 h at 4°C and 37°C. Afterwards, they were injected into the chromatographic system. These samples were also submitted to ultrafiltration (10 and 50 kDa membranes), and Al was measured in the ultrafiltrates. The results showed that Al was present in the eluent volume corresponding to the EPO peak but not in the HSA peak in the chromatograms. Temperature strengthened the interaction because the Al present in the EPO fraction was 3 times higher at 37°C compared to 4°C. Thirty-eight percent of the Al present in a 2.4 μg/mL EPO standard solution, and approximately 50% of the Al in formulation samples containing approximately 11 μg/mL EPO and either citrate or phosphate, were non-ultrafiltrable, which suggests that EPO is an effective Al acceptor in vitro.

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.

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.

Testing for recombinant erythropoietin

Erythropoietin (Epo) is a glycoprotein hormone that promotes the production of red blood cells. Recombinant human Epo (rhEpo) is illicitly used to improve performance in endurance sports. Doping in sports is discouraged by the screening of athletes for rhEpo. Both direct tests (indicating the presence of exogeneous Epo isoforms) and indirect tests (indicating hematological changes induced by exogenous Epo administration) can be used for Epo detection. At present, the test adopted by the World Anti Doping Agency is based on a combination of isoelectric focusing and double immunoblotting, and distinguishes between endogenous and rhEpo. However, the adopted monoclonal anti-Epo antibodies are not monospecific. Therefore, the test can occasionally lead to the false-positive detection of rhEpo (epoetin-beta) in post-exercise, protein-rich urine, or in case of contamination of the sample with microorganisms. An improved preanalytical care may counteract a lot of these problems. Adaptation of the criteria may be helpful to further refine direct Epo testing. Indirect tests have the disadvantage that they require blood instead of urine samples, but they can be applied to detect a broader range of performance improving techniques which are illicitly used in sports.

The prolonged half-lives of new erythropoietin derivatives via peptide addition

Erythropoietin, or Epo, is a hematopoietic cytokine that promotes erythropoiesis, and recombinant human Epo has been used in the treatment of anemia in various chronic diseases. Here, we have constructed novel Epo derivatives with prolonged half-lives by adding peptides to the carboxy terminus of Epo without using linkers. The fused peptides were selected from the carboxy terminal region of human chorionic gonadotropin (hCG) or human thrombopoietin (hTpo), which promote the proper folding, secretion, and stabilization of bioactive glycoproteins. Addition of these peptides did not interfere with secretion or receptor binding, and significantly increased the in vivo half-life of human Epo, as measured by intravenous administration in rats. The plasma half-life of the Epo constructs was longest when the carboxy terminal 28 aa of the beta subunit of hCG was added (Epo-CGC), a half-life that was slightly longer than NESP (Aranesp), which is the most effective Epo product in current clinical use. The transformation of four Ser glycosylation sites to Ala on the CGC sequence also lengthened the plasma half-life of Epo, indicating that the in vivo stabilizing effect of the hCG peptide was due to both structures within the peptide itself and its O-glycosylations. The application of the carboxy terminal half of hTpo also resulted in remarkably reduced elimination of the Epo chimera (Epo-TpC), possibly due to protection by the TpC sequence. The in vivo hematopoietic activity of Epo derivatives in mice was consistent with their pharmacokinetic profiles. Therefore, these derivatives with prolonged half-lives may provide opportunities for developing new Epo therapeutics with less frequent administration.

New urinary EPO drug testing method using two-dimensional gel electrophoresis

We present a two-dimensional electrophoresis (2DE) method for the detection of the drug, recombinant erythropoietin (rHuEPO) in urine and its separation from endogenous erythropoietin (HuEPO). This method involves a one-step acetonitrile precipitation of the proteins in urine, addition of an internal standard, two-dimensional gel electrophoresis (2D PAGE), a single Western blot and chemiluminescent immunodetection.

RESULTS

The 2DE method separates HuEPO and rHuEPO isoforms by both iso-electric point and molecular mass. We have identified several urinary proteins with which the monoclonal EPO antibody used in the current test has non-specific binding. The iso-electric points of these cross-reactive proteins overlap with HuEPO and rHuEPO however, they separate distinctly by the 2DE method. Alpha-2-HS-glycoprotein (HSGP) was identified by peptide mass fingerprinting as one of the urinary cross-reacting proteins, and commercially available purified HSGP was chosen to be added into urine samples as an internal standard prior to separation. Software (EpIQ) was specifically developed that applies four separate criteria to the detection of the migration of rHuEPO and HuEPO relative to the internal standard.

CONCLUSION

The combination of sample preparation, two-dimensional separation, internal standard, standardized blotting procedures and image analysis software enables the 2DE test for rHuEPO in urine to be performed reproducibly and accurately.

Relationships between the N-glycan structures and biological activities of recombinant human erythropoietins produced using different culture conditions and purification procedures

Eight preparations of recombinant human erythropoietin (EPO) with differing isoform compositions were produced by using different culture conditions and purification procedures. The N-glycan structures of these EPOs were analysed using a recently developed profiling procedure and identified using matrix-assisted laser desorption ionization mass spectrometry. The specific activities of each of the EPOs were estimated by in vivo and in vitro mouse bioassays. The eight EPOs were found to differ in their isoform compositions (as judged by isoelectric focusing), their N-glycan profiles, and in their in vivo and in vitro bioactivities. N-glycan analyses identified at least 23 different structures among these EPOs, including bi-, tri- and tetra-antennary N-glycans, with or without fucosylation or N-acetyllactosamine extensions, and sialylated to varying degrees. Mass spectrometry also indicated the presence of N-glycans with incomplete outer chains, terminating in N-acetylglucosamine residues, and of molecular masses consistent with phosphorylated or sulphated oligomannoside structures. The tetrasialylated tetra-antennary N-glycan contents of the eight rEPOs were found to be significantly and positively correlated with their specific activities as estimated by mouse in vivo bioassay, and significantly and negatively correlated with their specific activities as estimated by mouse in vitro bioassay. It was concluded that the tetrasialylated tetra-antennary N-glycan content of EPO is a major determinant for its in vivo biological activity in the mouse.

Detection of isoelectric profiles of erythropoietin in urine: differentiation of natural and administered recombinant hormones

Erythropoietin (EPO) is normally present in urine at a low concentration (about 1IU/L, i.e., about 10ng/L) for a total protein concentration of at least 50mg/L. A method to study the isoelectric profile of this hormone from 20-ml urine aliquots without previous purification was developed. This method involves isoelectric focusing of the retentate from ultrafiltered urine. Both the ultrafiltration and the isoelectric focusing required precautionary measures to prevent EPO degradation by the proteases that are present in urine. Because classical immunoblotting gave rise to an unspecific detection of various urinary proteins in the focused retentate, it was essential to use the "double-blotting" process developed to solve this problem. Sufficient sensitivity was achieved using amplified chemiluminiscent detection after the blotting membrane was treated with dithiotreitol. The patterns that were revealed from various urinary samples proved to be highly heterogeneous as they were composed of more than 10 isoforms in a pI range of 3.7-4.7. Clear transformation of the patterns was observed in the case of treatment by the recombinant hormone, suggesting that this method can be regarded an efficient tool for indicating recombinant EPO misuse in sports. It may also open new investigations in the field of physiologic or pathologic exploration.

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.

Mild tagging procedures for the structural analysis of glycans

The reductive oxyamination of model glycan structures has been investigated as a mild, alternative tagging procedure to reductive amination using O-(4-nitrobenzyl)-hydroxylamine. Oxime formation was quantitative, but the reduction step did not always go to completion. Novel O- and N-substituted 7-hydroxycoumaryl- and 3-methoxybenzylhydroxylamines were synthesized and shown to couple quantitatively with model saccharides by oxime formation and reductive hydroxyamination, respectively, under very mild, aqueous conditions. The fluorescent derivatives produced show good chromatographic and mass spectrometric properties. Both procedures are suitable for the labeling of carbohydrates and oligosaccharide fragments from glycosaminoglycan structures, such as heparin and heparan sulfate.

Selective glycopeptide mapping of erythropoietin by on-line high-performance liquid chromatography--electrospray ionization mass spectrometry

Selective glycopeptide mapping of recombinant human erythropoietin (rhEPO) used as a model glycoprotein was successfully carried out by on-line high-performance liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) using a Vydac C18 column eluted in acetonitrile-1 mM ammonium acetate, pH 6.8. rhEPO expressed in a Chinese hamster ovary clone was exhaustively digested into four glycopeptides and nine peptides with endoproteinase Glu-C. Both glycopeptides and peptides were eluted with trifluoroacetic acid as the eluent, whereas only glycopeptides were eluted selectively with ammonium acetate in the following order: N38, N24, 0126, and N83. Furthermore, many glycoforms included in each glycopeptide were found to be separated by differences in the numbers of sialic acid and N-acetyllactosaminyl repeats. Twenty, 16 and 22 different N-linked oligosaccharides were determined at Asn24, 38, and 83, respectively, and two different O-linked oligosaccharides were observed at Ser126. Our method is simple, rapid, and useful for determining the carbohydrate structures at each glycosylation site and for elucidating the site-specific carbohydrate heterogeneity.

Effects of ammonia on CHO cell growth, erythropoietin production, and glycosylation

The effect of ammonium chloride was determined on a culture of CHO cells transfected with the human erythropoietin (EPO) gene. Cell growth was inhibited above a culture concentration of 5 mM NH(4)Cl with an IC-50 determined to be 33 mM. The specific production of EPO increased with the addition of NH(4)Cl above 5 mM. At 10 mM NH(4)Cl, the final cell density after 4 days in culture was significantly lower but the final yield of EPO was significantly higher. This appeared to be due to continued protein production after cell growth had ceased. The metabolic effects of added NH(4)Cl included higher specific consumption rates of glucose and glutamine and an increased rate of production of alanine, glycine, and glutamate. The EPO analyzed from control cultures had a molecular weight range of 33-39 kDa and an isoelectric point range of 4.06-4.67. Seven distinct isoforms of the molecule were identified by two-dimensional electrophoresis. This molecular heterogeneity was ascribed to variable glycosylation. Complete enzymatic de-glycosylation resulted in a single molecular form with a molecular mass of 18 kDa. Addition of NH(4)Cl to the cultures caused a significant increase in the heterogeneity of the glycoforms as shown by an increased molecular weight and pI range. Enzymatic de-sialylation of the EPO from the ammonia-treated and control cultures resulted in identical electrophoretic patterns. This indicated that the effect of ammonia was in the reduction of terminal sialylation of the glycan structures which accounted for the increased pI. Selective removal of the N-glycan structures by PNGase F resulted in two bands identified as the O-glycan linked structure (19 kDa) and the completely de-glycosylated structure (18 kDa). The proportion of the O-linked glycan structure was reduced, and its pI increased in cultures to which ammonia was added. Thus, the glycosylation pattern altered by the presence of ammonia included a reduction in terminal sialylation of all the glycans and a reduction in the content of the O-linked glycan. The addition of a sialidase inhibitor to the cultures had no effect on the ammonia-induced increase in EPO heterogeneity. Also, the effect of ammonia on glycosylation could not be mimicked using the weak base chloroquine in our system.

Unusual N-glycosylation of a recombinant human erythropoietin expressed in a human lymphoblastoid cell line does not alter its biological properties

Erythropoietin (Epo) is a 166 amino acids protein containing three N-glycosylation sites (Asn-24, Asn-38, and Asn-83) and 1 O- glycosylation site (Ser-126) and involved in the regulation of the level of red blood cells. Today, only one recombinant human Epo (rHuEpo), produced in CHO cell line, is extensively used in therapy to cure severe anemia. The structure of the glycan chains of this rHuEpo slightly differ of those of the urinary human Epo (uHuEpo), considered as the natural Epo molecule. In an attempt to produce a rHuEpo as close as possible to the uHuEpo, Epo gene was expressed in a human lymphoblastoid cell line, named RPMI 1788. In order to fully characterize the Epo-RPMI, structural characterizations of the protein skeleton as well as glycan chains were undergone. As expected, the amino acid sequence of the Epo-RPMI conformed to that of uHuEpo. Surprisingly, the structure of some N-glycan chains, as mainly determined by ESI-MS, revealed some unusual characteristics. Thus, 80% of N-glycans possess a bisecting GlcNAc residue, 25% bear a second fucose residue which is present, in a large part, in a sialyl Le(x)motif, and 13% contain more than three LacNAc repeats (up to five per molecule). Despite these unusual structural characteristics, the data concerning the in vitro and in vivo biological activities were not impaired when compared to Epo-CHO and uHuEpo.

Xanthosoma sagittifolium tubers contain a lectin with two different types of carbohydrate-binding sites

An unusual lectin possessing two distinctly different types of carbohydrate-combining sites was purified from tubers of Xanthosoma sagittifolium L. by consecutive passage through two affinity columns, i.e. asialofetuin-Sepharose and invertase-Sepharose. SDS-polyacrylamide gel electrophoresis, N-terminal amino acid sequencing, and gel filtration chromatography of the purified lectin showed that the X. sagittifolium lectin is a heterotetrameric protein composed of four 12-kDa subunits (alpha(2)beta(2)) linked by noncovalent bonds. The results obtained by quantitative precipitation and hapten inhibition assays revealed that the lectin has two different types of carbohydrate-combining sites: one type for oligomannoses, which preferentially binds to a cluster of nonreducing terminal alpha1,3-linked mannosyl residues, and the other type for complex N-linked carbohydrates, which best accommodates a non-sialylated, triantennary oligosaccharide with N-acetyllactosamine (i.e. Galbeta1,4GlcNAc-) or lacto-N-biose (i.e. Galbeta1,3GlcNAc-) groups at its three nonreducing termini.

An IgG monoclonal antibody against Dictyostelium discoideum glycoproteins specifically recognizes Fucalpha1,6GlcNAcbeta in the core of N-linked glycans. Localized expression of core-fucosylated glycoconjugates in human tissues

Core fucosylation of N-linked oligosaccharides (GlcNAcbeta1, 4(Fucalpha1,6)GlcNAcbeta1-Asn) is a common modification in animal glycans, but little is known about the distribution of core-fucosylated glycoproteins in mammalian tissues. Two monoclonal antibodies, CAB2 and CAB4, previously raised against carbohydrate epitopes of Dictyostelium discoideum glycoproteins (Crandall, I. E. and Newell, P. C. (1989) Development 107, 87-94), specifically recognize fucose residues in alpha1,6-linkage to the asparagine-bound GlcNAc of N-linked oligosaccharides. These IgG3 antibodies do not cross-react with glycoproteins containing alpha-fucoses in other linkages commonly seen in N- or O-linked sugar chains. CAB4 recognizes core alpha1,6 fucose regardless of terminal sugars, branching pattern, sialic acid linkage, or polylactosamine substitution. This contrasts to lentil and pea lectins that recognize a similar epitope in only a subset of these structures. Additional GlcNAc residues found in the core of N-glycans from dominant Chinese hamster ovary cell mutants LEC14 and LEC18 progressively decrease binding. These antibodies show that many proteins in human tissues are core-fucosylated, but their expression is localized to skin keratinocytes, vascular and visceral smooth muscle cells, epithelia, and some extracellular matrix-like material surrounding subpopulations of lymphocytes. The availability of these antibodies now allows for an extended investigation of core fucose epitope expression in development and malignancy and in genetically manipulated mice.

Effect of active oxygen radicals on protein and carbohydrate moieties of recombinant human erythropoietin

Our previous study showed that active oxygen radicals generated from a Fenton system and a xanthine plus xanthine oxidase system caused serious loss of in vivo bioactivity of recombinant human erythropoietin (EPO), a highly glycosylated protein. In the present study, we characterized the oxidative modifications to the protein and carbohydrate moiety of EPO, which lead to a reduction of its bioactivity. In vitro bioactivity was reduced when EPO was treated with oxygen radicals generated from a Fenton system in the presence of 0.016 mM H2O2, and the reduction was directly proportional to the loss of in vivo bioactivity. SDS-PAGE analysis showed that dimer formation and degradation was observed under more severe conditions (Fenton reaction with 0.16 mM H2O2). The tryptophan destruction was detected at 0.016 mM H2O2 and well correlated with the loss of in vitro bioactivity, whereas loss of other amino acids were occurred under more severe conditions. Treatment with the Fenton system did not result in any specific damage on the carbohydrate moiety of EPO, except a reduction of sialic acid content under severe condition. These results suggest that active oxygen radicals mainly react with the protein moiety rather than the carbohydrate moiety of EPO. Destruction of tryptophan residues is the most sensitive marker of oxidative damage to EPO, suggesting the importance of tryptophan in the active EPO structure. Deglycosylation of EPO caused an increased of susceptibility to oxygen radicals compared to intact EPO. The role of oligosaccharides in EPO may be to protect the protein structure from active oxygen radicals.

Erythropoietin: physico- and biochemical analysis

A hormone, erythropoietin, mainly produced in adult kidneys and fetal livers, acts on bone marrow erythroid progenitor cells to regulate the production of erythrocyte in mammals. As a result, the oxygen carrying capacity of blood increases and the increased oxygen supply raises the cardiac function and physiological working capacity. Erythropoietin is possibly misused by athletes in sports for the purpose of improving performance. Presently there is no discernible and specific method to identify erythropoietin administration for doping control. To address this practical problem, this paper presents a summary of the applications of analytical biotechnology, especially the structural characterization of erythropoietin.

In vitro bioassay for human erythropoietin based on proliferative stimulation of an erythroid cell line and analysis of carbohydrate-dependent microheterogeneity

The human erythroleukemia cell line TF-1 was employed for the determination of proliferative stimulation induced by recombinant human erythropoietin (rhEpo). Potencies of various intact and sugar-trimmed rhEpo preparations were estimated using the International Standard for Human r-DNA-derived Epo (87/684) as a reference for activity. The cellular response was measured in a multi-channel photometer using a colorimetric microassay, based on the metabolism of the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide to formazan, by viable cells. The linear part of the log dose-response relationship encompassed 2.5-90 pM and activity of rhEpo preparations was measured at doses between 3 and 60 pM. The assay was designed as a parallel line test, using three or four concentrations for potency determinations, which fulfills pharmacopoeial requirements for assay validity. Inter-assay relative standard deviation varied between 4.1% and 12.6% and most assays revealed potencies with limits of error within 87-113%. In order to acquire an additional means for an efficient probing of physiologically relevant features of rhEpo, a luminiscence-dependent Western detection system, based on a combined isoelectric focusing/sodium dodecyl sulphate-polyacrylamide gel electrophoresis separation, was established. As opposed to conventional electrophoresis the two dimensional approach enabled the disclosure of minor truncations in the rhEpo-attached glycan moieties using picomolar quantities of the hormone. Moreover, the separated isoforms of rhEpo were quantified by computer-assisted densitometry and compared with the 87/684 standard. Accordingly, results obtained by the cellular response were balanced against the general pattern observed and the relative amounts of separated rhEpo isomers as determined by the quantitative Western analysis. The method described should be suitable for potency assessments of pharmaceutical formulations of rhEpo.

Three-dimensional mapping of N-linked oligosaccharides using anion-exchange, hydrophobic and hydrophilic interaction modes of high-performance liquid chromatography

To determine the structure of N-linked oligosaccharides, a three-dimensional (3-D) sugar mapping technique for pyridylaminated neutral and sialyl oligosaccharides is proposed. The pyridylaminated oligosaccharide mixture is first separated by HPLC on a diethylaminoethyl anion-exchange column and the elution data are placed on the Z-axis. Neutral and mono-, di-, tri- and tetrasialyloligosaccharides are then individually separated on both a hydrophobic octadecylsilylsilica column and a hydrophilic amide-silica column under the same conditions as described previously for neutral oligosaccharides. The validity of the 3-D mapping technique was tested using sialyl pyridylaminated oligosaccharides from human serum glycoproteins.

Preparative purification of tyrosinamide N-linked oligosaccharides

N-linked oligosaccharides from glycoproteins can be either analyzed on a sub-nanomole scale or preparatively purified on a multi-micromole scale. Each goal necessitates a unique analytical strategy often involving oligosaccharide derivatization to enhance separation and detection. Tyrosinamide-oligosaccharides were developed to facilitate the preparative purification of N-linked oligosaccharides. These have found many uses in oligosaccharide remodeling, in the preparation of neoglycoconjugates, in developing receptor probes, and even as analytical standards in chromatography. This review discusses progress in the preparation of tyrosinamide-oligosaccharides from different glycoproteins and their utility in glycobiology research.

Characterization of changes in the glycosylation pattern of recombinant proteins from BHK-21 cells due to different culture conditions

The N-glycosylation patterns of a genetically engineered human interleukin-2 variant glycoprotein (IL-Mu6), produced by BHK-21 cells from long-term suspension and microcarrier cultures in the presence and absence of fetal calf serum were compared. IL-Mu6 was used as a model protein in studying the effect of different controlled cell culture conditions on the expression of N-glycans in recombinant glycoproteins. IL-Mu6 contains a single amino acid substitution (Glu100<==>Asn) generating a potential N-glycosylation recognition site (Asn100-Xxx-Thr/Ser) in addition to the natural O-glycosylation at position Thr3. Parallel cell cultivations were carried out in two continuously perfused 2.5-liter stirred bioreactors under defined culture conditions. Major differences were found in the glycoprotein products obtained during these different cultivation conditions. Serum-free cultures resulted in a higher level of terminal sialylation and proximal alpha 1-6 fucosylation. The ratio of O- to N-glycans as well as the amount of nonglycosylated product and the antennarity of N-linked carbohydrates in the model protein exhibited major differences depending on the presence or absence of serum, the condition of growth and the cultivation procedure.

Structural analysis of the sialylated N- and O-linked carbohydrate chains of recombinant human erythropoietin expressed in Chinese hamster ovary cells. Sialylation patterns and branch location of dimeric N-acetyllactosamine units

The N-linked carbohydrate chains of recombinant human erythropoietin expressed in CHO cells were quantitatively released with peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F, separated from the remaining O-glycoprotein by gel-permeation chromatography, and subsequently fractionated via FPLC on Mono Q, HPLC on Lichrosorb-NH2 and high-pH anion-exchange chromatography on CarboPac PA1. The purified sialylated oligosaccharides were analyzed by one-dimensional and two-dimensional 500-MHz 1H-NMR spectroscopy. When necessary, oligosaccharides were treated with endo-beta-galactosidase (and N-acetyl-beta-glucosaminidase) followed by 1H-NMR analysis of the incubation products, to obtain additional structural information. Di-, tri-, tri'- and tetraantennary N-acetyllactosamine-type oligosaccharides occur which can be completely (major) or partially (minor) sialylated. Three different types of alpha 2-3-linked sialic acids are present, namely, N-acetylneuraminic acid (95%), N-glycolylneuraminic acid (2%) and N-acetyl-9-O-acetylneuraminic acid (3%). In the case of partial sialylation, a non-random distribution of the sialic acids over the branches is observed. One or two extra N-acetyllactosamine units, being exclusively located in the branches attached to the alpha 1-6-linked Man residue, can be present in completely or partially sialylated di-, tri'-, and tetraantennary oligosaccharides. Tetraantennary oligosaccharides with N-acetyllactosamine repeats could be digested quantitatively with endo-beta-galactosidase from Bacteroides fragilis, whereas under the same conditions tri' antennary oligosaccharides hardly reacted (< 15%). Using endo-beta-galactosidase from Escherichia freundii, these tri'antennary oligosaccharides could be digested more extensively (> 75%). The O-linked carbohydrate chains were released from the O-glycoprotein by alkaline borohydride treatment, and purified via FPLC on Mono Q and HPLC on Lichrosorb-NH2. Two O-glycans were found, namely, Neu5Ac alpha 2-3Gal beta 1-3GalNAc-ol and Neu5Ac alpha 2-3Gal beta 1-3(Neu5Ac alpha 2-6)GalNAc-ol.

Analysis of protein modifications: recent advances in detection, characterization and mapping

The past year has seen several contributions, both in methods for determining and characterizing chemical modifications of proteins and in related technologies used to map peptides. These contributions mainly involve improvements in capillary zone electrophoresis and in various applications of mass spectrometry.

Glycosidase digestion, electrophoresis and chromatographic analysis of recombinant human granulocyte colony-stimulating factor glycoforms produced in Chinese hamster ovary cells

Recombinant human granulocyte colony stimulating factor (G-CSF) produced in Chinese hamster ovary cells is glycosylated. The carbohydrate compositional analysis indicated that G-CSF molecule contains sialic acid, galactose and galactosamine. By isolation and characterization of the purified glycopeptides obtained from cleavages by Staphylococcal aureus V-8 protease and cyanogen bromide, the O-linked glycosylation site was confirmed to be a Thr residue at position 133. Neuraminidase and O-glycanase digestion followed by sodium dodecyl sulfate polyacrylamide and isoelectric focusing gel electrophoreses distinguished two possible carbohydrate structures attached at Thr-133: structure A, NeuNAc-Gal-beta(1,3)-GalNAc-O-Thr; and structure B, NeuNAc-Gal-beta(1,3)-[NeuNAc]-GalNAc-O-Thr. Different glycoforms, undigested or after glycosidase digestion, can also be separated by ion-exchange or reversed-phase high-performance liquid chromatography. The approach described in this report provides a simple and valuable procedure to characterize glycoprotein structures containing simple carbohydrate moieties.