Formation of Compound I in the Reaction of Native Myoglobins with Hydrogen Peroxide

Abstract

Reaction of ferric native myoglobin (Mb) with hydrogen peroxide (H(2)O(2)) 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 porphyrin pi-cation radical (Por(+)-Fe(IV)(O)), during their reactions with H(2)O(2). With both kinds of Mbs, formation of compound I was more clearly observed in D(2)O than in H(2)O. The compound thus formed was capable of performing monooxygenation of thioanisole to methyl phenyl sulfoxide and a 2-electron oxidation of H(2)O(2) giving O(2) and H(2)O 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 H(2)O(2) were determined. These results established unambiguously that native metMb can form both compound I and ferryl Mb upon reaction with H(2)O(2) 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 H(2)O(2) by compound I.