Effects of cross-linking and zero-link polymerization on oxygen transport and redox chemistry of bovine hemoglobin

Abstract

Cross-linked hemoglobins (Hbs) were found to have enhanced oxidative reactions which compromise the ability of cell-free Hb to carry oxygen. Zero-link bovine Hb (ZL-HbBv), also known as OxyVita™, a large polymer held together by pseudopeptide bonds on the surface of adjacent tetramers, provides a model in which these reactions can be evaluated. The oxygen affinity of ZL-HbBv was greatly increased, whereas the oxygen binding cooperativity ( n 50) as well as the regulatory responses to pH and chloride ions was diminished. Rapid mixing kinetic studies revealed faster carbon monoxide (CO) and nitric oxide (NO) binding to ZL-HbBv, consistent with a more accessible heme pocket conformation. The rate of autoxidation of ferrous ZL-HbBv was 3 folds faster than the unmodified HbBv (control) but only slightly suppressed by the presence of superoxide dismutase and catalase enzymes. The peroxide (H 2O 2) reaction rates of ferric ZL-HbBv and its degradation were comparable to that of the control. The rate of heme loss from ZL-HbBv to a mutant apomyoglobin (H64Y/V68F) was also very close to that of the control. Taken together, allosteric and redox reactions of this protein are altered due to heme accessibility to solvent, however, the compact tetramer to tetramer interaction of the ZL-HbBv polymer appears to restrict heme loss even in the presence of excess H 2O 2.