Formation of compound I in the reaction of native myoglobins with hydrogen peroxide

Reaction of ferric native myoglobin (Mb) with hydrogen peroxide (H₂O₂) was studied by the aid of stopped-flow rapid-scan spectrophotometry. In contrast to the results in previous studies where compound I was reported to be undetectable, both sperm whale and horse heart metmyoglobins (metMbs) formed a significant quantity of compound I, an oxoferryl porphyrln π-cation radical (Por⁺-Fe (IV)(O)), during their reactions with H₂O₂. With both kinds of Mbs, formation of compound I was more clearly observed in D₂O than in H₂O. The compound thus formed was capable of performing monooxygenation of thioanisole to methyl phenyl sulfoxide and a 2-electron oxidation of H₂O₂ giving O₂ and H₂O as products. It was also converted into ferryl myoglobin (Por-Fe (IV)(O)-globin⁺) spontaneously. Rate constants for these reactions and that for a direct conversion of metMb to ferryl Mb through the homolysis of H₂O₂ were determined. These results established unambiguously that native metMb can form both compound I and ferryl Mb upon reaction with H₂O₂ and that these high valent iron compounds serve as essential intermediates in Mb-assisted peroxidative reactions. The observed deuterium effect on the apparent stability of compound I was attributable to that effect on the hydrogen abstraction step in the 2-electron oxidation of H₂O₂ by compound I.