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Abstract
Polyethylene glycol conjugation to proteins and peptides (PEGylation) has been shown to promote increased retention time in the circulation as well as to blunt immune or allergic reactions. PEGylated bovine hemoglobin (PEG-Hb) is being explored in human clinical trials as an oxygen delivering agent for the sensitization of solid tumors to radiation therapy. In this study the functional properties of PEG-Hb were compared to those of bovine hemoglobin (Hb), the mutant human hemoglobin Rothchild and bovine hemoglobin crosslinked between the beta chains. The rate of heme transfer from Hb to serum albumin at pH 9.0 was greatly increased by PEGylation, suggesting destabilization of the heme-globin linkage and of the bonds between aB dimers. Measurement of oxygen binding equilibrium showed that the oxygen affinity of Hb became unusually dependent on temperature and Hb concentration after PEGylation. Evidence is presented to suggest that PEGylation of lysine β-81 at the entrance to the central cavity of the Hb tetramer might be responsible for these observations. The alterations of the functional properties of Hb induced by PEGylation are consistent with the beneficial effects of PEG-Hb in exchange transfusion and radiation sensitization models of human conditions.