Oxidation of hemoglobin and the enhancement produced by nitroblue tetrazolium.

The autoxidation of hemoglobin as a function of oxygen pressure and the effect of added nitroblue tetrazolium have been studied. It has been shown that the enhanced autoxidation at intermediate oxygen pressures can only be partially explained by the outer-sphere reaction of oxygen with deoxygenated chains. An additional enhancement associated with the properties of partially oxygenated hemoglobins has been attributed to the mobility on the distal side of the heme which facilitates the nucleophilic displacement of bound oxygen by the distal histidine. Nitroblue tetrazolium, in addition to reacting with the superoxide formed during autoxidation, is shown to oxidize directly both deoxygenated and oxygenated hemoglobin in two steps, proceeding through the one-electron reduced intermediate, a tetrazolinyl radical. Even though the oxygenated chains are oxidized by nitroblue tetrazolium, much less overall reduction of nitroblue tetrazolium is observed at high oxygen pressures. This phenomenon is attributed to the reoxidation by oxygen of both the tetrazolinyl radical and the formazan. The reaction with nitroblue tetrazolium is also found to contribute to enhanced oxidation at intermediate oxygen pressures. This behavior is explained by the two-electron reaction with nitroblue tetrazolium and the oxygen dependence of the various processes involved in the reaction of nitroblue tetrazolium.