Intra- and intermolecular oxidation of oxymyoglobin and oxyhemoglobin induced by hydroxyl and carbonate radicals

The mechanism of the reactions of myoglobin and hemoglobin with ^.OH and CO₃^.- in the presence of oxygen was studied using pulse and γ-radiolysis. Unlike ^.NO₂, which adds to the porphyrin iron, ^.OH and CO₃^.- form globin radicals. These secondary radicals oxidize the Fe^II center through both intra- and intermolecular processes. The intermolecular pathway was further demonstrated when BSA radicals derived from ^.OH or CO₃^.- oxidized oxyhemoglobin and oxymyoglobin to their respective ferric states. The oxidation yields obtained by pulse radiolysis were lower compared to γ-radiolysis, where the contribution of radical-radical reactions is negligible. Full oxidation yields by ^.OH-derived globin radicals could be achieved only at relatively high concentrations of the heme protein mainly via an intermolecular pathway. It is suggested that CO ₃^.- reaction with the protein yields Tyr and/or Trp-derived phenoxyl radicals, which solely oxidize the porphyrin iron under γ-radiolysis conditions. The ^.OH particularly adds to aromatic residues, which can undergo elimination of H₂O forming the phenoxyl radical, and/or react rapidly with O₂ yielding peroxyl radicals. The peroxyl radical can oxidize a neighboring porphyrin iron and/or give rise to superoxide, which neither oxidize nor reduce the porphyrin iron. The potential physiological implications of this chemistry are that hemoglobin and myoglobin, being present at relatively high concentrations, can detoxify highly oxidizing radicals yielding the respective ferric states, which are not toxic.