PEPTIDE-MAPPING AND EVALUATION OF GLYCOPEPTIDE MICROHETEROGENEITY DERIVED FROM ENDOPROTEINASE DIGESTION OF ERYTHROPOIETIN BY AFFINITY HIGH-PERFORMANCE CAPILLARY ELECTROPHORESIS

High-performance capillary electrophoresis (HPCE) has been employed to characterize the peptide map of recombinant human erythropoietin (rHuEPO) expressed from Chinese hamster ovary (CHO) cells. The methodology employs an ion pairing agent, 100 mM heptanesulfonic acid in 40 mM sodium phosphate buffer, pH 2.5, to increase peptide resolution, to decrease analyte wall interactions, and to evaluate glycopeptide microheterogeneity. The total tryptic map is segregated into two regions, nonglycosylated and glycosylated peptides. Reproducibility of the peptide map is excellent; the map results in baseline separation of 16 tryptic peptides and one doublet peak composed of two peptides (resolution 0.22). The map furthermore allows for the evaluation of the microheterogeneity associated with the three rHuEPO glycopeptides. At least 12 glycopeptide forms were separated in the initial peptide map. Peptides were identified by Edman sequencing, and the glycopeptides were further subjected to Dionex anion-exchange chromatography. To simplify the level of complexity associated with the glycopeptides, much of the characterization employed asialoglycopeptides and employed several endoproteolytic digestions. The relative percent distribution for each purified asialoglycopeptide was calculated to define the level of complexity and to tentatively assign a known structure to the HPCE peak. The level of structural complexity of the asialoglycopeptides appears to increase from the simplest O-linked form to the more complex N83, N38, and N24 glycosylation positions, respectively. HPCE evaluation of glycopeptide microheterogeneity appears to be simpler, faster, and just as sensitive as other more frequently employed methods for glycopeptide characterizations.