Identification of the residues involved in homodimer formation of recombinant human erythropoietin

Recombinant human erythropoietin (rHuEPO) is biologically functional when in a monomeric state; upon extensive heating, rHuEPO forms a dimer. The nature of this dimeric linkage was investigated after isolation of the dimer by gel filtration. The dimer fraction was subjected to tryptic digestion, and the peptides were separated by reversed-phase HPLC. SDS-PAGE, N-terminal sequencing, capillary electrophoresis and mass spectrometry (both liquid-chromatographic electrospray and matrix-assisted laser desorption ionization) were employed to compare the tryptic peptides from heat-treated rHuEPO and untreated rHuEPO. Results demonstrated that elevated heat broke the intramolecular disulfide bond between Cys-7 and Cys-161 and an intermolecular disulfide bond then formed from these residues, producing a covalently linked rHuEPO homodimer. Dimer formation was also mathematically modeled and shown to fit a simple equilibrium.