Redox Reactions of Cross-linked Haemoglobins with Oxygen and Nitrite
Redox reactions of haemoglobin (Hb) with oxygen can initiate a cascade of oxidative reactions that appear to underlie the adverse side reactions observed when cell-free Hbs are introduced into the circulation to enhance oxygen delivery to respiring tissues. Redox reactions of cell-free Hbs with nitrite may also be of significance in vivo, as these reactions can lead to formation of nitrosylated Hb (NO-Hb) along with oxidised Hb (MetHb). To clarify the factors governing these redox reactions we measured the kinetics of nitrite-induced and oxygen-induced heme oxidation and obtained oxygen binding and oxidation curves for unmodified human Hb and four cross-linked Hbs. The four cross-linked Hbs studied were generated by cross-linking Hb with glutaraldehyde, dextran, O-raffinose or bis(3,5-dibromosalicyl)fumarate. Oxygen binding by the cross-linked Hbs occurred with reduced oxygen affinity, reduced cooperativity and reduced responses to organic phosphate effectors. The redox potentials of the cross-linked Hbs were shifted to higher potentials relative to unmodified Hb in the absence of allosteric effectors, indicating a reduced thermodynamic driving force for oxidation. In spite of this, these Hbs showed increased rates in oxidative reactions. Elevated rates of heme oxidation were observed for their oxy derivatives under aerobic conditions, and upon exposure to nitrite under both aerobic and anaerobic conditions. These results show that heme accessibility rather than heme redox potential is the major determinant of the kinetics of redox reactions of Hb with both oxygen and nitrite.
KeywordsOxygen Carrier Oxygen Affinity Oxygen Binding Blood Substitute Hill Plot
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