Identification and structural characterization of a mannose-6-phosphate containing oligomannosidic N-glycan from human erythropoietin secreted by recombinant BHK-21 cells

An Integrated Strategy Reveals Complex Glycosylation of Erythropoietin Using Mass Spectrometry

The characterization of therapeutic glycoproteins is challenging due to the structural heterogeneity of the therapeutic protein glycosylation. This study presents an in-depth analytical strategy for glycosylation of first-generation erythropoietin (epoetin beta), including a developed mass spectrometric workflow for N-glycan analysis, bottom-up mass spectrometric methods for site-specific N-glycosylation, and a LC-MS approach for O-glycan identification. Permethylated N-glycans, peptides, and enriched glycopeptides of erythropoietin were analyzed by nanoLC-MS/MS, and de-N-glycosylated erythropoietin was measured by LC-MS, enabling the qualitative and quantitative analysis of glycosylation and different glycan modifications (e.g., phosphorylation and O-acetylation). The newly developed Python scripts enabled the identification of 140 N-glycan compositions (237 N-glycan structures) from erythropoietin, especially including 8 phosphorylated N-glycan species. The site-specificity of N-glycans was revealed at the glycopeptide level by pGlyco software using different proteases. In total, 114 N-glycan compositions were identified from glycopeptide analysis. Moreover, LC-MS analysis of de-N-glycosylated erythropoietin species identified two O-glycan compositions based on the mass shifts between non-O-glycosylated and O-glycosylated species. Finally, this integrated strategy was proved to realize the in-depth glycosylation analysis of a therapeutic glycoprotein to understand its pharmacological properties and improving the manufacturing processes.

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.

Chromatographic purification of recombinant human erythropoietin

Recombinant human erythropoietin is a valuable therapeutic protein used in the treatment of several serious diseases. It exists in different isoforms and is produced by genetically modified mammalian cells such as Chinese hamster ovary or human embryonic kidney cells. As for other biopharmaceutical drugs, a key factor for its successful industrial production is to achieve a high degree of purity and to decrease the content of critical impurities to trace amounts. This goal is achieved in the separation sequence which substantial part is formed by chromatographic steps. Therefore, downstream processing forms an essential part of production costs. This review presents the overview of published separation sequences and, analyzes the use of different types of chromatographic media such as affinity, ion-exchange, reversed-phase, hydrophobic interaction, multimodal, and size-exclusion chromatography adsorbents. Their application is discussed with regard to their place in the purification stages generally denoted as capture, intermediate purification and polishing.

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.

N-/O-glycosylation analysis of human FVIIa produced in the milk of transgenic rabbits

Human coagulation factor VIIa is a glycoprotein that promotes haemostasis through activation of the coagulation cascade extrinsic pathway. Most haemophilia A/B patients with inhibitors are treated by injection of plasma-derived or recombinant FVIIa. The use of recombinant products raises questions about the ability of the host cell to produce efficiently post-translationally modified proteins. Glycosylation is especially critical considering that it can modulate protein safety and efficacy. The present paper reports the N-/O-glycosylation pattern of a new recombinant human factor VIIa expressed in the mammary glands of transgenic rabbits. Glycosylation was investigated by chromatography and advanced mass spectrometry techniques for glycan identification and quantitation. Mass spectrometry (MS)/MS analyses were performed to confirm the glycan structures as well as the position and branching of specific monosaccharides or substituents. The two N-glycosylation sites were found to be fully occupied mostly by mono- and bi-sialylated biantennary complex-type structures, the major form being A₂G₂S₁. Some oligomannose/hybrid structures were retrieved in lower abundance, the major ones being GlcNAcα1,O-phosphorylated at the C6-position of a Man residue (Man-6-(GlcNAcα1,O-)phosphate motif) as commonly observed on lysosomal proteins. No immunogenic glycotopes such as Galili (Galα1,3Gal) and HD antigens (N-glycolylneuraminic acid (NeuGc)) were detected. Concerning O-glycosylation, the product exhibited O-fucose and O-glucose-(xylose)_{0, 1, 2} motifs as expected. The N-glycosylation consistency was also investigated by varying production parameters such as the period of lactation, the number of consecutive lactations and rabbit generations. Results show that the transgenesis technology is suitable for the long-term production of rhFVIIa with a reproducible glycosylation pattern.

The sweet tooth of biopharmaceuticals: importance of recombinant protein glycosylation analysis

Biopharmaceuticals currently represent the fastest growing sector of the pharmaceutical industry, mainly driven by a rapid expansion in the manufacture of recombinant protein-based drugs. Glycosylation is the most prominent post-translational modification occurring on these protein drugs. It constitutes one of the critical quality attributes that requires thorough analysis for optimal efficacy and safety. This review examines the functional importance of glycosylation of recombinant protein drugs, illustrated using three examples of protein biopharmaceuticals: IgG antibodies, erythropoietin and glucocerebrosidase. Current analytical methods are reviewed as solutions for qualitative and quantitative measurements of glycosylation to monitor quality target product profiles of recombinant glycoprotein drugs. Finally, we propose a framework for designing the quality target product profile of recombinant glycoproteins and planning workflow for glycosylation analysis with the selection of available analytical methods and tools.

Biosimilar recombinant human erythropoietins ("epoetins") and future erythropoiesis-stimulating treatments

INTRODUCTION

Recombinant human erythropoietin (rhEPO, epoetin) has prospered in the treatment of renal and chemotherapy-associated anemias. Since the patents of the original epoetins expired, biosimilars have been launched. Because these are not fully identical to the original products, non-clinical and clinical studies are necessary to show similarity with respect to quality, safety, and efficacy.

AREAS COVERED

The article summarizes experiences with EU-approved biosimilar epoetins. In particular, the issue of immunogenicity is considered. Neutralizing anti-EPO antibodies can cause pure red cell aplasia (PRCA). Further, a first view is offered on future erythropoiesis-stimulating therapies.

EXPERT OPINION

The term "biosimilar" should only be used for follow-on biopharmaceuticals approved under a defined regulatory pathway. The primary rationale for the therapy with biosimilars is cost saving. Two biosimilar epoetins are available in the EU that are used at the same dose(s) and dosing regimen(s) for indications of the reference product. Their advent has stimulated innovator companies to develop second-generation products with improved pharmacokinetic properties. EPO-mimicking peptides are a new therapeutic option. Other strategies focus on orally active chemical drugs that induce endogenous EPO production ("HIF stabilizers"). Epo gene transfer is also possible, but needs to be further explored with respect to efficacy and safety.

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.

Biosimilar epoetins and other "follow-on" biologics: update on the European experiences

After the patents of biopharmaceuticals have expired, based on specific regulatory approval pathways copied products ("biosimilars" or "follow-on biologics") have been launched in the EU. This article summarizes experiences with hematopoietic medicines, namely the epoetins (two biosimilars traded under five different brand names) and the filgrastims (two biosimilars, six brand names). Physicians and pharmacists should be familiar with the legal and pharmacological specialities of biosimilars: The production process can differ from that of the original, clinical indications can be extrapolated, glycoproteins contain varying isoforms, the formulation may differ from the original, and biopharmaceuticals are potentially immunogenic. Only on proof of quality, efficacy and safety, biosimilars are a viable option because of their lower costs.

SDS-PAGE of recombinant and endogenous erythropoietins: benefits and limitations of the method for application in doping control

Doping of athletes with recombinant and genetically modified erythropoietins (EPO) is currently detected by isoelectric focusing (IEF). The application of these drugs leads to a significant change in the isoform profile of endogenous urinary erythropoietin (uhEPO). Dynepo, MIRCERA, biosimilars with variable IEF-profiles as well as active urines and effort urines have made additional testing strategies necessary. The new generation of small molecule EPO-receptor stimulating agents like Hematide will also challenge the analytical concept of detecting the abuse of erythropoiesis stimulating agents (ESA). By determining their apparent molecular masses with SDS-PAGE a clear differentiation between endogenous and exogenous substances also concerning new EPO modifications is possible. Due to the orthogonal character of IEF- and SDS-PAGE both methods complement each other. The additional benefits of SDS-PAGE especially in relation to active and effort urines as well as the detection of Dynepo were investigated. Due to significant differences between the apparent molecular masses of uhEPO/serum EPO (shEPO) and recombinant, genetically or chemically modified erythropoietins the presence of active or effort urines was easily revealed. The characteristic band shape and apparent molecular mass of Dynepo on SDS-PAGE additionally evidenced the presence of this substance in urine. A protocol for the detection of EPO-doping in serum and plasma by SDS-PAGE was developed. Blood appears to be the ideal matrix for detecting all forms ESA-doping in the future.

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.

Effect of culture temperature on erythropoietin production and glycosylation in a perfusion culture of recombinant CHO cells

To investigate the effect of culture temperature on erythropoietin (EPO) production and glycosylation in recombinant Chinese hamster ovary (CHO) cells, we cultivated CHO cells using a perfusion bioreactor. Cells were cultivated at 37 degrees C until viable cell concentration reached 1 x 10(7) cells/mL, and then culture temperature was shifted to 25 degrees C, 28 degrees C, 30 degrees C, 32 degrees C, 37 degrees C (control), respectively. Lowering culture temperature suppressed cell growth but was beneficial to maintain high cell viability for a longer period. In a control culture at 37 degrees C, cell viability gradually decreased and fell below 80% on day 18 while it remained over 90% throughout the culture at low culture temperature. The cumulative EPO production and specific EPO productivity, q(EPO), increased at low culture temperature and were the highest at 32 degrees C and 30 degrees C, respectively. Interestingly, the cumulative EPO production at culture temperature below 32 degrees C was not as high as the cumulative EPO production at 32 degrees C although the q(EPO) at culture temperature below 32 degrees C was comparable or even higher than the q(EPO) at 32 degrees C. This implies that the beneficial effect of lowering culture temperature below 32 degrees C on q(EPO) is outweighed by its detrimental effect on the integral of viable cells. The glycosylation of EPO was evaluated by isoelectric focusing, normal phase HPLC and anion exchange chromatography analyses. The quality of EPO at 32 degrees C in regard to acidic isoforms, antennary structures and sialylated N-linked glycans was comparable to that at 37 degrees C. However, at culture temperatures below 32 degrees C, the proportions of acidic isoforms, tetra-antennary structures and tetra-sialylated N-linked glycans were further reduced, suggesting that lowering culture temperature below 32 degrees C negatively affect the quality of EPO. Thus, taken together, cell culture at 32 degrees C turned out to be the most satisfactory since it showed the highest cumulative EPO production, and moreover, EPO quality at 32 degrees C was not deteriorated as obtained at 37 degrees C.

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.

N-glycans of recombinant human acid alpha-glucosidase expressed in the milk of transgenic rabbits

Pompe disease is a lysosomal glycogen storage disorder characterized by acid alpha-glucosidase (GAA) deficiency. More than 110 different pathogenic mutations in the gene encoding GAA have been observed. Patients with this disease are being treated by intravenous injection of recombinant forms of the enzyme. Focusing on recombinant approaches to produce the enzyme means that specific attention has to be paid to the generated glycosylation patterns. Here, human GAA was expressed in the mammary gland of transgenic rabbits. The N-linked glycans of recombinant human GAA (rhAGLU), isolated from the rabbit milk, were released by peptide-N(4)-(N-acetyl-beta-glucosaminyl)asparagine amidase F. The N-glycan pool was fractionated and purified into individual components by a combination of anion-exchange, normal-phase, and Sambucus nigra agglutinin-affinity chromatography. The structures of the components were analyzed by 500 MHz one-dimensional and 600 MHz cryo two-dimensional (total correlation spectroscopy [TOCSY] nuclear Overhauser enhancement spectroscopy) (1)H nuclear magnetic resonance spectroscopy, combined with two-dimensional (31)P-filtered (1)H-(1)H TOCSY spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, and high-performance liquid chromatography (HPLC)-profiling of 2-aminobenzamide-labeled glycans combined with exoglycosidase digestions. The recombinant rabbit glycoprotein contained a broad array of different N-glycans, comprising oligomannose-, hybrid-, and complex-type structures. Part of the oligomannose-type glycans showed the presence of phospho-diester-bridged N-acetylglucosamine. For the complex-type glycans (partially) (alpha2-6)-sialylated (nearly only N-acetylneuraminic acid) diantennary structures were found; part of the structures were (alpha1-6)-core-fucosylated or (alpha1-3)-fucosylated in the upper antenna (Lewis x). Using HPLC-mass spectrometry of glycopeptides, information was generated with respect to the site-specific location of the various glycans.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update covering the period 1999-2000

This review describes the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates and continues coverage of the field from the previous review published in 1999 (D. J. Harvey, Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates, 1999, Mass Spectrom Rev, 18:349-451) for the period 1999-2000. As MALDI mass spectrometry is acquiring the status of a mature technique in this field, there has been a greater emphasis on applications rather than to method development as opposed to the previous review. The present review covers applications to plant-derived carbohydrates, N- and O-linked glycans from glycoproteins, glycated proteins, mucins, glycosaminoglycans, bacterial glycolipids, glycosphingolipids, glycoglycerolipids and related compounds, and glycosides. Applications of MALDI mass spectrometry to the study of enzymes acting on carbohydrates (glycosyltransferases and glycosidases) and to the synthesis of carbohydrates, are also covered.

Differentiating factors between erythropoiesis-stimulating agents: a guide to selection for anaemia of chronic kidney disease

Endogenous erythropoietin (EPO) consists of a central polypeptide core covered by post-translationally linked carbohydrates. Three of the four currently available erythropoiesis stimulating agents (ESA)--epoetin-alpha, epoetin-beta and epoetin-omega- are composed of an identical amino acid sequence, but glycosylation varies as a result of type- and host cell-specific differences in the production process. Epoetin-alpha and epoetin-beta resemble each other with respect to molecular characteristics and pharmacokinetic data, although epoetin-beta has a higher molecular weight, a lower number of sialylated glycan residues and possibly slight pharmacokinetic advantages such as a longer terminal elimination half-life. A serious adverse effect of long-term administration of ESA is pure red cell aplasia. This effect has been observed predominantly with subcutaneous use of epoetin-alpha produced outside the US after albumin was removed from the formulation. In comparison with the intravenous route, subcutaneous administration of epoetin has been reported to have a dose-sparing effect in some studies. Epoetin-beta has been the subject of studies aimed at proving efficacy with a reduced administration frequency but results are not unequivocal. Epoetin-omega is produced in a different host cell than all other erythropoietic agents, hence glycosylation and pharmacokinetics are different. Small-scale clinical studies found epoetin-omega to be slightly more potent than epoetin-alpha. Epoetin-delta is a recently approved agent produced by human cells that are genetically engineered to transcribe and translate the EPO gene under the control of a newly introduced regulatory DNA sequence. However, epoetin-delta is not yet on the market and few data are available. The erythropoietin analogue darbepoetin-alpha carries two additional glycosylation sites that permit a higher degree of glycosylation. Consequently, in comparison with the other epoetins, darbepoetin-alpha has a longer serum half-life and a higher relative potency, which further increases with extension of the administration interval. Dosage requirements of darbepoetin-alpha do not appear to differ between the intravenous and subcutaneous routes of administration. The less frequent administration of darbepoetin-alpha in comparison to the other epoetins may reduce drug costs in the long term, but the variability in dosage or dosage frequency required within a single patient is high. Further studies should be aimed at defining predictors of the individual demand for erythropoietic agents, thereby allowing nephrologists to prescribe a cost-effective, individualised regimen.

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.

The enigma of the metabolic fate of circulating erythropoietin (Epo) in view of the pharmacokinetics of the recombinant drugs rhEpo and NESP

Recombinant human erythropoietin (rhEpo) is a mainstay in the treatment of anaemia, primarily in renal failure. Because the half-life of circulating rhEpo is relatively short (4-8 h), the drug is usually administered 2-3 times weekly. Recently, a novel erythropoiesis-stimulating protein (NESP) with a longer half-life (24-26 h) has been approved. NESP possesses two additional N-glycans compared to endogenous Epo or rhEpo. The pharmacokinetics of rhEpo and NESP in humans have been investigated in detail. The composition of the N-glycans is clearly important in determining the biological activity and the velocity of the degradation of Epo and its analogues. However, due to the lack of knowledge of the main site and mechanism of the removal of Epo from circulation, the difference in survival of rhEpo and NESP has remained phenomenological. Investigators have implicated the liver, kidneys, and bone marrow as possible sites of the catabolism of Epo. However, while hepatocytes take up desialylated Epo, the liver does not appear to play a major role in the degradation of intact Epo. Likewise, renal Epo clearance is apparently of secondary importance. Studies showing non-linear pharmacokinetics of Epo suggest that Epo is eliminated by saturable mechanisms. The hormone, as well as the recombinant drugs, can be incorporated by erythrocytic progenitors and other tissues expressing the Epo receptor. The affinity of the Epo receptor for rhEpo is 4.3-fold higher than for NESP. Taken together, it seems most likely that native Epo, rhEpo and NESP are degraded following Epo receptor-mediated uptake, mainly in bone marrow.

Application of liquid chromatography/mass spectrometry and liquid chromatography with tandem mass spectrometry to the analysis of the site-specific carbohydrate heterogeneity in erythropoietin

High-performance liquid chromatography with electrospray ionization mass spectrometry (LC/MS) and liquid chromatography with tandem mass spectrometry (LC/MS/MS) were applied to the analysis of the site-specific carbohydrate heterogeneity in erythropoietin (EPO) used as a model of the sialylated glycoprotein. N-linked oligosaccharides were released from recombinant human EPO expressed in Chinese hamster ovary cells enzymatically and reduced with NaBH(4). Many different sialylated oligosaccharides of EPO were separated and characterized by LC/MS equipped with a graphitized carbon column (GCC). Glycosylation sites and the preliminary glycosylation pattern at each glycosylation site were determined by LC/MS of endoproteinase Glu-C-digested EPO. The detailed site-specific carbohydrate heterogeneity caused by the differences in the molecular weight, branch, linkage, and sequence was elucidated by GCC-LC/MS of the N-linked oligosaccharides released from the isolated glycopeptides. Structural details of the isomers were analyzed by LC/MS/MS, and it was indicated that di- and trisialylated tetraantennary oligosaccharides are attached to Asn24, 38, and 83, whereas their isomers, di- and trisialylated triantennary oligosaccharides containing N-acetyllactosamines, are combined with Asn24. Our method is useful for the determination of glycosylation sites, the site-specific carbohydrate heterogeneity of glycoproteins, and the carbohydrate structure.

H (0) blood group determinant is present on soluble human L-selectin expressed in BHK-cells

In the present study we show that the H (0) blood group determinant Fuc alpha1-2Gal beta1-4GlcNAc beta1-R is present on N-linked glycans of soluble human L-selectin recombinantly expressed in baby hamster kidney (BHK) cells. The glycans were isolated using complementary HPLC techniques and characterized by a combination of exoglycosidase digestion and mass spectrometry. The linkage of the fucose residues was determined by incubation of the glycans with specific fucosidases. The H blood determinant Fuc alpha1-2Gal beta1-4GlcNAc beta1 was detected for bi-, 2,4 branched tri- and tetraantennary structures. To our knowledge, the proposed oligosaccharide structures represent a new glycosylation motif for recombinant glycoproteins expressed on BHK cells.

Characterization of protein glycosylation by mass spectrometry

Electrospray ionization, a natural interface with microbore and capillary high-pressure liquid chromatography, has become the method of choice for the reliable structural characterization of protein glycosylation by mass spectrometry at the picomole level. Its advantages include inherent sensitivity in the femtomole range, compatibility with collisional activation methods that both permit the detection and monitoring of structurally specific ions and enable the induction of glycopeptide fragmentation that facilitates determination of glycoform sequence and branching. Developments in high-performance electrospray mass spectrometry include sample introduction at nanoliter flow rates, tandem magnetic sector/orthogonal time-of-flight instruments, Fourier transform instruments, and new ion optical strategies, including ion traps. Although a sensitive and important complementary technique, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry suffers from matrix-dependent deposition of excess internal energies, which produce extensive metastable fragmentation and (photo)adduct formation. These metastable fragments may be focused into a mass spectrum by employing an ion mirror (reflectron) in time-of-flight instrumentation. In favorable cases, structural information may be obtained.