Production of Recombinant Human Erythropoietin in Mammalian Cells: Host–Cell Dependency of the Biological Activity of the Cloned Glycoprotein

Biotechnology Applications of Cell-Free Expression Systems

Cell-free systems are a rapidly expanding platform technology with an important role in the engineering of biological systems. The key advantages that drive their broad adoption are increased efficiency, versatility, and low cost compared to in vivo systems. Traditionally, in vivo platforms have been used to synthesize novel and industrially relevant proteins and serve as a testbed for prototyping numerous biotechnologies such as genetic circuits and biosensors. Although in vivo platforms currently have many applications within biotechnology, they are hindered by time-constraining growth cycles, homeostatic considerations, and limited adaptability in production. Conversely, cell-free platforms are not hindered by constraints for supporting life and are therefore highly adaptable to a broad range of production and testing schemes. The advantages of cell-free platforms are being leveraged more commonly by the biotechnology community, and cell-free applications are expected to grow exponentially in the next decade. In this study, new and emerging applications of cell-free platforms, with a specific focus on cell-free protein synthesis (CFPS), will be examined. The current and near-future role of CFPS within metabolic engineering, prototyping, and biomanufacturing will be investigated as well as how the integration of machine learning is beneficial to these applications.

A synthetic approach to 'click' neoglycoprotein analogues of EPO employing one-pot native chemical ligation and CuAAC chemistry

The clinical significance of batch-wise variability on the pharmacokinetics and potency of commercial erythropoietin (EPO), prepared recombinantly as a heterogeneous mixture of glycoforms, necessitates the development of synthetic strategies to afford homogenous EPO formulations. Herein we present a previously unexplored and divergent route towards 'click' neoglycoprotein analogues of EPO, employing one-pot native chemical ligation (NCL) of alkynylated peptides and copper-catalysed azide-alkyne cycloaddition (CuAAC) with azido monosaccharides. By design, our synthetic platform permits glycosylation at virtually any stage, providing flexibility for the synthesis of various glycoforms for biological analysis. Insights obtained from attempted folding of our 'click' neoglycoprotein EPO analogue, bearing four different neutral sugar moieties, highlight the important role played by the charged oligosaccharides present in native EPO glycoproteins.

Biomanufacturing: history and perspective

Biomanufacturing is a type of manufacturing that utilizes biological systems (e.g., living microorganisms, resting cells, animal cells, plant cells, tissues, enzymes, or in vitro synthetic (enzymatic) systems) to produce commercially important biomolecules for use in the agricultural, food, material, energy, and pharmaceutical industries. History of biomanufacturing could be classified into the three revolutions in terms of respective product types (mainly), production platforms, and research technologies. Biomanufacturing 1.0 focuses on the production of primary metabolites (e.g., butanol, acetone, ethanol, citric acid) by using mono-culture fermentation; biomanufacturing 2.0 focuses on the production of secondary metabolites (e.g., penicillin, streptomycin) by using a dedicated mutant and aerobic submerged liquid fermentation; and biomanufacturing 3.0 focuses on the production of large-size biomolecules—proteins and enzymes (e.g., erythropoietin, insulin, growth hormone, amylase, DNA polymerase) by using recombinant DNA technology and advanced cell culture. Biomanufacturing 4.0 could focus on new products, for example, human tissues or cells made by regenerative medicine, artificial starch made by in vitro synthetic biosystems, isobutanol fermented by metabolic engineering, and synthetic biology-driven microorganisms, as well as exiting products produced by far better approaches. Biomanufacturing 4.0 would help address some of the most important challenges of humankind, such as food security, energy security and sustainability, water crisis, climate change, health issues, and conflict related to the energy, food, and water nexus.

Glycosylation and antiproliferative activity of hyperglycosylated IFN-α2 potentiate HEK293 cells as biofactories

Both CHO and HEK cells are interesting hosts for the production of biotherapeutics due to their ability to introduce post-translational modifications such as glycosylation. Even though oligosaccharide structures attached to proteins are conserved among eukaryotes, many differences have been found between therapeutic glycoproteins expressed in hamster and human derived cells. In this work, a hyperglycosylated IFN-α2b mutein (IFN4N) was produced in CHO and HEK cell lines and an extensive characterization of their properties was performed. IFN4N_CHO exhibited a higher average molecular mass and more acidic isoforms compared to IFN4N_HEK. In agreement with these results, a 2-times higher sialic acid content was found for IFN4N_CHO in comparison with the HEK-derived protein. This result was in agreement with monosaccharide quantification and glycan's analysis using WAX chromatography and HILIC coupled to mass spectrometry; all methods supported the existence of highly sialylated and also branched structures for IFN4N_CHO glycans, in contrast with smaller and truncated structures among IFN4N_HEK glycans. Unexpectedly, those remarkable differences in the glycosylation pattern had not a considerable impact on the clearance rate of both molecules in rats. In fact, although IFN4N_HEK reached maximum plasma concentration 3-times faster than IFN4N_CHO, their elimination profile did not differ significantly. Also, despite the in vitro antiviral specific biological activity of both proteins was the same, IFN4N_HEK was more efficient as an antiproliferative agent in different tumor-derived cell lines. Accordingly, IFN4N_HEK showed a higher in vivo antitumor activity in animal models. Our results show the importance of an appropriate host selection to set up a bioprocess and potentiate the use of HEK293 cells for the production of a new hyperglycosylated protein-based pharmaceutical.

Post-translational modification of a chimeric EPO-Fc hormone is more important than its molecular size in defining its in vivo hematopoietic activity

BACKGROUND

Recombinant erythropoietin (EPO) has been marketed as biopharmaceutical for anemia and chronic renal failure. Long-acting EPO variants that aimed at achieving less frequent dosing have been generated, either by the addition of glycosylation sites or increasing its molecular weight.

METHODS

The hEPO cDNA linked to the human IgG Fc fragment was cloned as a single codifying gene on the pAdtrack-CMV vector, yielding the recombinant adenoviral genome. For in vitro and in vivo expression assays cervical cancer cell line (SiHa) and nulliparous goats were used, respectively. The hematopoietic activity of EPO-Fc, expressed as the differential increment of hematocrit was evaluated in B6D2F1 mice. NP-HPLC of the 2AB-labeled N-glycan was carried out to profile analysis.

RESULTS

The direct transduction of mammary secretory cells with adenoviral vector is a robust methodology to obtain high levels of EPO of up to 3.5mg/mL in goat's milk. SiHa-derived EPO-Fc showed significant improvement in hematopoietic activity compared to the commercial hEPO counterpart or with the homologous milk-derived EPO-Fc. The role of the molecular weight seemed to be important in enhancing the hematopoietic activity of SiHa-derived EPO-Fc. However, the lack of sialylated multi-antennary glycosylation profile in milk-derived EPO-Fc resulted in lower biological activity.

CONCLUSIONS

The low content of tri- or tetra-antennary sialylated N-glycans linked to the chimeric EPO-Fc hormone, expressed in the goat mammary gland epithelial cells, defined its in vivo hematopoietic activity.

GENERAL SIGNIFICANCE

The sialylated N-glycan content plays a more significant role in the in vivo biological activity of hEPO than its increased molecular weight.

Plant-produced human recombinant erythropoietic growth factors support erythroid differentiation in vitro

Clinically available red blood cells (RBCs) for transfusions are at high demand, but in vitro generation of RBCs from hematopoietic stem cells requires significant quantities of growth factors. Here, we describe the production of four human growth factors: erythropoietin (EPO), stem cell factor (SCF), interleukin 3 (IL-3), and insulin-like growth factor-1 (IGF-1), either as non-fused proteins or as fusions with a carrier molecule (lichenase), in plants, using a Tobacco mosaic virus vector-based transient expression system. All growth factors were purified and their identity was confirmed by western blotting and peptide mapping. The potency of these plant-produced cytokines was assessed using TF1 cell (responsive to EPO, IL-3 and SCF) or MCF-7 cell (responsive to IGF-1) proliferation assays. The biological activity estimated here for the cytokines produced in plants was slightly lower or within the range cited in commercial sources and published literature. By comparing EC50 values of plant-produced cytokines with standards, we have demonstrated that all four plant-produced growth factors stimulated the expansion of umbilical cord blood-derived CD34+ cells and their differentiation toward erythropoietic precursors with the same potency as commercially available growth factors. To the best of our knowledge, this is the first report on the generation of all key bioactive cytokines required for the erythroid development in a cost-effective manner using a plant-based expression system.

Human ubiquitin C promoter based expression of erythropoietin in CHO K1 cell lines: a simple transfectants screening approach

Erythropoietin (EPO), a glycoprotein hormone that regulates the production of erythrocytes in the human body, is of clinical importance in the treatment of anemia. Low expression levels of this recombinant hormone and time-consuming screening methods have made its commercial production expensive. Cloning of human EPO gene in a shuttle vector pUB6/V5-HisB driven by human ubiquitin C promoter and its transfection in CHO K1 cell lines by electroporation resulted in a moderate level of EPO expression. The limiting-dilution screening method required several months to obtain high expression stable transfectants but needed only short duration for selection in contrast to the present screening strategy. The supernatants of stably transfected cells were found to be biologically active by in vitro erythroid cluster forming activity.

Erythropoietin after a century of research: younger than ever

In the light of the enthusiasm regarding the use of recombinant human erythropoietin (Epo) and its analogues for treatment of the anaemias of chronic renal failure and malignancies it is worth remembering that today's success has been based on a century of laborious research. The concept of the humoral regulation of haematopoiesis was first formulated in 1906. The term 'erythropoietin' for the erythropoiesis-stimulating hormone was introduced in 1948. Native human Epo was isolated in 1977 and its gene cloned in 1985. During the last 15 yr, major progress has been made in identifying the molecules controlling Epo gene expression, primarily the hypoxia-inducible transcription factors (HIF) that are regulated by specific O2 and oxoglutarate requiring Fe2+-containing dioxygenases. With respect to the action of Epo, its dimeric receptor (Epo-R) has been characterised and shown to signal through protein kinases, anti-apoptotic proteins and transcription factors. The demonstration of Epo-R in non-haematopoietic tissues indicates that Epo is a pleiotropic viability and growth factor. The neuroprotective and cardioprotective potentials of Epo are reviewed with a focus on clinical research. In addition, studies utilising the Epo derivatives with prolonged half-life, peptidic and non-peptidic Epo mimetics, orally active drugs stimulating endogenous Epo production and Epo gene transfer are reviewed.

Cholesterol is required for the polarized secretion of erythropoietin in Madin-Darby canine kidney cells

It has already been reported that stably expressed exogenous human wild-type EPO (wtEPO) is preferentially secreted to the apical side and one of the three N-linked carbohydrate chains critically acts as an apical sorting determinant in Madin-Darby canine kidney (MDCK) cells. It has been suggested that lipid rafts are involved in the apical sorting of membrane and secretory proteins. To investigate the involvement of lipid rafts in the apical sorting of wtEPO, we examined the effect of cholesterol depletion with methyl-beta-cyclodextrin on the secretion polarity of EPO and analyzed Triton X-100 insoluble cell extracts by sucrose density gradients centrifugation in MDCK cells. We found that wtEPO was shifted in non-polarized direction by cholesterol depletion. Most of the wtEPO was not detectable in the raft fractions by sucrose density gradients centrifugation analysis. These results indicate that apical secretion of EPO involves a cholesterol-dependent mechanism probably not involving lipid rafts.

Epididymis is a novel site of erythropoietin production in mouse reproductive organs

The epididymis consists of the interstitial tissue and the ductus epididymidis, an extremely tortuous duct, in which spermatozoa exported from the testis gain motility and fertilizing capacity. We found that the cultured mouse epididymis produces erythropoietin (Epo). The content of Epo mRNA in the epididymis from the adult mouse (8-week-old) amounts to 40% of that in the kidney. The epididymal Epo mRNA dramatically increased upon growth; its level increased 120-fold from the age of 3 weeks to 7 weeks when they complete sexual maturation, while the increase in the total RNA was 3-fold. Hypoxia induced a 5-fold increase in the epididymal Epo mRNA transiently, which is much lower than the induction in the kidney (28-fold). In situ hybridization technique elucidated that the site of Epo production was located in the interstitial space between ductus epididymidis. The epididymal Epo may have an unidentified function in the male reproductive organ.

Erythropoietin receptor is expressed in rat hippocampal and cerebral cortical neurons, and erythropoietin prevents in vitro glutamate-induced neuronal death

Recently, erythropoietin has been shown to be produced by astrocytes and its production is hypoxia-inducible. In the present study, we demonstrated, using a reverse transcription-polymerase chain reaction assay and immunostaining of the cells, that the erythropoietin receptor was expressed in cultured hippocampal and cerebral cortical neurons of day 19 rat embryo. Erythropoietin protected the cultured neurons from glutamate neurotoxicity. Neurons cultured for seven to 10 days were exposed to glutamate for 15 min and after culture for a further 24 h in the absence of glutamate the neuron survival was assayed. Significant protection was observed with erythropoietin from 3 pM (c. 100 pg/ml) in a dose-dependent manner. The protection was completely reversed by co-application of a soluble erythropoietin receptor, an extracellular domain capable of binding with erythropoietin. For exhibition of the neuroprotective effect, exposure of neurons to erythropoietin approximately 8 h prior to exposure to glutamate was required. Experiments with the inhibitors indicated that RNA and protein syntheses were necessary for the protection. However, exposure to erythropoietin for a short period (5 min or less) was sufficient to elicit the protective effect. The protective effect of erythropoietin was blocked by the simultaneous addition of EGTA. These findings and the previous finding that erythropoietin induces a rapid and transient increase in intracellular Ca2+ concentration in neuronal cells suggest that erythropoietin plays a neuroprotective role in brain injury caused by hypoxia or ischemia and that erythropoietin-induced Ca2+ influx from outside of the cells is a critical initial event yielding an enhanced resistance of the neurons to glutamate toxicity.

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.

Characterization of haemolyser-resistant cells increased in the blood of erythropoietin-treated mice

Recently, we reported a new in vivo assay method for erythropoietin (Epo) by means of counts of the number of haemolyser-resistant cells (HRCs) increased in Epo-injected mice. Here, we attempted to characterize the HRCs. Flow-cytometric studies revealed that HRCs obtained from Epo-injected mice expressed the transferrin receptor on their surface membranes. Furthermore, a fluorophotometric study suggested that the number of transferrin receptor-positive cells increased in a dose-dependent manner in response to treatment with Epo. On the other hand, flowcytometric and fluorophotometric studies of glycophorin A on HRCs using a rabbit antiglycophorin A antibody also showed a high expression of glycophorin A on them as compared with on HRCs from vehicle-treated animals (control). The results indicated that HRCs could be defined by their expression of both transferrin receptors and glycophorin A. We concluded that HRCs might be immature reticulocytes.

In vivo biological activities of recombinant human erythropoietin analogues produced by CHO cells, BHK cells and C127 cells

The in vivo biological activity of four pharmaceutical preparations of recombinant human erythropoietin was compared. Two of the erythropoietins were produced by Chinese hamster ovary cells, CHO-K1, and the others were produced by mouse mammary cells, C127, and baby hamster kidney cells, BHK-21. The activities of the analogues were estimated by a simple cell counting method with conventional automated microcell counters. The amounts of these analogues gave straight logarithmic dose-response curves when plotted against the count of particles resistant to hemolysing reagent, which particles were mostly immature reticulocytes. The lines from the four analogues were parallel to each other. The relative activities of these analogues were 1.02, 1.19 and 1.21 when one of the analogues was arbitrarily used as the standard. These differences in the extent of the activity were not significant. Thus, the four recombinant human erythropoietin analogues, produced by four different mammalian cell lines, expressed the same biological potencies in vivo corresponding to their units, and the units used up to now by the manufacturers are equivalent. These results also draw the conclusion that the new simple in vivo bioassay can replace the existing accepted assay methods.

Simple in vivo bioassay without radioisotopes for recombinant human erythropoietins

A simple in vivo bioassay suitable for routine testing of quality control of recombinant human erythropoietin (rHuEPO) analogues was developed. The assay took four days, normal mice were used and radioactive compounds were not needed. EPO activity was measured by the increased number of some part of reticulocytes which increased specifically and dose-dependently by the injection of rHuEPO. They were considered to be mostly immature reticulocytes and were counted as the residual particles from blood cells after treatment with a hemolysing reagent. These particles could be counted by conventional automated microcell counters. The assay procedure was simple and easy. The sensitivity, reliability and reproducibility of this method were acceptable for routine in vivo bioassay of rHuEPOs. This method was economical, and can be used instead of the existing bioassays for rHuEPOs.

Production and secretion of high levels of recombinant human acetylcholinesterase in cultured cell lines: microheterogeneity of the catalytic subunit

To allow for structural analysis of the human acetylcholinesterase (hAChE) subunit, a series of eukaryotic vectors was designed for efficient expression. Several eukaryotic multicistronic expression vectors were tested in various mammalian cell lines. All expression vectors contained the selectable neo gene under control of a weak promoter, while the hAChE cDNA was under control of the cytomegalovirus (CMV) immediate-early or Rous sarcoma virus long terminal repeat (RSV LTR) or simian virus 40 (SV40) early promoters. Optimal production and secretion of recombinant hAChE (rehAChE) was achieved in the embryonal kidney 293 cell line transfected either with the RSV-hAChE or with CMV-hAChE expression vectors. Clones expressing and secreting as much as 5-25 pg of enzyme per cell per 24 h were obtained without resorting to coamplification techniques or continuous maintenance of cells under selective pressure. The purified (specific activity of 6000 units per mg protein) homodimer and tetramer enzyme molecules displayed typical AChE biochemical properties: a Km value of 120 microM for acetylthiocholine; a kcat value of 3.9 x 10(5)/min, and selective by AChE-specific inhibitors. Catalytic subunit dimers (130 kDa) exhibit differential N-glycosylation patterns, and upon reduction resolve into 67- and 70-kDa monomeric subunits. These two forms appear as a single discrete 62-kDa band following deglycosylation by N-glycanase. The N-terminal amino acid sequence analysis of the purified mature enzyme suggests the existence of two alternative cleavage sites for the removal of the signal peptide, in which the 'mature' position 1 is either Ala31 or Gly33. Both of these positions conform with the consensus signal peptide recognition sequences and demonstrate bidirected processing of signal peptides on a native molecule.

Production and ligand-binding characteristics of the soluble form of murine erythropoietin receptor

A recombinant soluble form (sEPO-R) of erythropoietin (EPO) receptor (EPO-R) was produced by Chinese hamster ovary cells and isolated in high yield with the EPO-fixed gel. Ligand binding assays were done using three methods; precipitation of sEPO-R radiolabeled EPO complex and competition of sEPO-R for the binding of radiolabeled EPO with the cellular EPO-R. The results showed a Kd of 17 nM which was much lower than those for cellular EPO-R. One N-glycosylation site exists in sEPO-R but the glycosylation did not affect the binding affinity to EPO. A complex with a molecular size that corresponded to a 1:1 complex of EPO and sEPO-R was detected.

In vitro reconstitution of an erythropoietin gene transcription system using its 5'-flanking sequence and a nuclear extract from anemic kidney

We have developed an in vitro transcription system for the erythropoietin (Epo) gene. This system uses a plasmid carrying 0.2 kb of 5'-flanking sequence from the human Epo gene, rNTPs and a nuclear extract from mouse kidney. The transcribed RNA was assayed by primer extension with an end-labeled primer complementary to the sequence of the plasmid, dNTPs and reverse transcriptase. The primer extension product corresponding to the transcript was detected on a sequencing gel. The in vitro promoter activity of the Epo 5'-flanking sequence was observed with a nuclear extract from anemic kidney but not with that from normal kidney.

Expression of the erythropoietin receptor on a human myeloma cell line

We demonstrated the expression of the erythropoietin (EPO) receptor by a human myeloma cell line (MM-S1) which was established in our laboratory. EPO dose-dependently stimulated the proliferation of MM-S1 cells. Binding of radioiodinated EPO (125I-Epo) to MM-S1 cells was competitively inhibited by unlabeled EPO, but not by other recombinant cytokines. Specific binding of 125I-Epo to MM-S1 cells was saturable, and the Scatchard analysis revealed 330 EPO binding sites per cell with a Kd of 0.56 nmol/L. Bound EPO was internalized by MM-S1 cells during incubation at 37 degrees C. This is the first report describing the expression of the EPO receptor by human cells other than those of the erythroid lineage.

Enhanced growth of clonogenic cells from acute myeloblastic leukaemia by erythropoietin

The effect of erythropoietin (Epo) on colony formation by blast progenitors in acute myeloblastic leukaemia other than erythroleukaemia was studied using a blast colony assay. Epo alone did not induce colony formation, but when it was used together with phytohaemagglutinin-stimulated leucocyte-conditioned medium (PHA-LCM) the number of leukaemic colonies significantly increased in nine out of 12 cases studies. Preincubation of Epo with anti-Epo antibody completely abolished this enhancement, indicating that the increase in colony numbers was caused by Epo itself. Cell surface phenotype analysis of colonies produced by Epo plus PHA-LCM showed no increase in percentages of erythroid and megakaryocyte lineages. The addition of Epo also increased the self-renewal capacity of leukaemic blast cells. Fresh leukaemic cells did not express Epo receptors, but they were induced after incubation with PHA-LCM. The present study thus showed that the proliferative response to Epo is not restricted only to the erythroid lineage, but also extends to AML blast cells other than those in erythroleukaemia in the presence of colony stimulating factors.

Chemical modification of erythropoietin: an increase in in vitro activity by guanidination

Human recombinant erythropoietin (rHuEPO) was chemically modified with several group-specific reagents in order to study the role of each kind of amino-acid residue in its biological activity. Guanidination of the amino groups of the lysine residues yielded derivatives that showed higher activities in vitro than native rHuEPO, whereas amidination had no effect on the activity. By contrast, modification of the positive charges of the lysine residues to neutral or negative charges, such as in carbamylation, trinitrophenylation, acetylation or succinylation, caused a significant loss of rHuEPO activity. Chemical modification of other amino-acid residues, such as arginine and tyrosine residues or carboxyl groups, also led to loss of activity.

Development of new cell lines for animal cell biotechnology

Mammalian cell culture has been an important technique in laboratory-scale experimentation for many decades. Developments in large-scale culture have been due to the need to grow large numbers of cells to support the growth of viruses for vaccine production, and more recently, for growing hybridoma cells as a source of monoclonal antibody. Increasingly, however, pharmaceutical products such as hormones, enzymes, growth factors, and clotting factors are being produced from cell lines which have been manipulated by recombinant DNA techniques. It is clear, therefore, that the high cost of growing mammalian cells on a large scale does not necessarily prohibit their use for biotechnology, and indeed there is considerable evidence to suggest that animal cell biotechnology will continue to be a major growth area in the future.

Post-translational modifications of proteins: some problems left to solve

Three major questions regarding the post-translational modification of amino acid side chains in proteins are briefly considered: (1) What are the biological functions of the reactions, (2) what is the specificity of the processing reactions in selecting only a few or sometimes even only one residue for modification, and (3) how do we solve the uniqueness of the processing steps in the production of recombinant proteins? The answers to these questions are not obvious at this time.

Erythropoietin receptor of a human leukemic cell line with erythroid characteristics

A new cell line of human erythroleukemia cells differentiates spontaneously so that 30% of the cells are always hemoglobinized. Erythropoietin did not affect the percentage of such cells but stimulated the cell growth, indicating that the cells have functioning receptors. Binding of human recombinant radioiodinated erythropoietin to the receptors was specific. The bound ligand was internalized into cells at 37 degrees C but not at 15 degrees C. Scatchard analysis showed two classes of binding sites. Covalent binding of erythropoietin to its receptors yielded two products detected on sodium dodecyl sulfate-polyacrylamide gels electrophoresed under reducing conditions. Under non-reducing conditions, these species disappeared and larger products appeared.

Comparative structural study of N-linked oligosaccharides of urinary and recombinant erythropoietins

The structures of the N-linked oligosaccharides of the urinary erythropoietin (u-EPO) purified from urine of aplastic anemic patients were analyzed and compared with those for recombinant erythropoietin (r-EPO) prepared with baby hamster kidney (BHK) cells. Asparagine-linked neutral oligosaccharides were released from each EPO protein by N-oligosaccharide glycopeptidase (almond) digestion. The reducing ends of the oligosaccharide chains thus obtained were aminated with a fluorescent reagent, 2-aminopyridine, and the mixture of pyridylamino derivatives of the oligosaccharides was separated by high-performance liquid chromatography (HPLC) on an ODS silica column. More than 8 and 13 kinds of oligosaccharide fractions for u-EPO and r-EPO (BHK), respectively, were completely separated by the one-step HPLC procedure. The structure of each oligosaccharide thus isolated was analyzed by a combination of sequential exoglycosidase digestion and another kind of HPLC with an amide-silica column. Furthermore, high-resolution proton nuclear magnetic resonance (1H NMR) spectroscopy and methylation analyses were carried out in the case of r-EPO (BHK).(ABSTRACT TRUNCATED AT 250 WORDS)