Detection and identification of the oxidizing species generated from the physiologically important Fenton-like reaction of iron(II)-citrate with hydrogen peroxide.

ABSTRACT

The Fenton-like reaction of iron(II)-citrate with hydrogen peroxide is physiologically important because it is associated with the oxidative stress and pathological processes induced by the redox-active iron pool in vivo. However, the oxidizing species generated from this reaction at neutral pH has not been convincingly identified because two extremely unstable and hard-to-differentiate species, the hydroxyl radical (•OH) and iron(IV) (ferryl) species, can be produced. Identifying this species is essential for understanding the reaction mechanism. Although there were few data that reported the detection of •OH from this reaction by using the EPR and fluorescence techniques, most of these data were obtained without the necessary assessment with a •OH scavenger. Furthermore, these two techniques may not be able to differentiate the •OH and iron(IV) species. Thus, these reported data cannot lead to a convincing conclusion that the •OH, not the iron(IV) species, was generated. Therefore, in the study reported herein, we carried out systematic investigations first by using the EPR and fluorescence techniques combined with a •OH scavenger to detect the oxidizing species generated from this Fenton-like reaction. Then we utilized NMR spectroscopy and for the first time obtained convincing evidence to demonstrate that this oxidizing species is the •OH rather than iron(IV) species. We also determined the second-order rate constant of the reaction, 3.6 × 103 M-1s-1 (pH7.0, 25 °C), by using the stopped-flow spectrophotometry. On the basis of these findings, a scheme is proposed for the mechanism of this physiologically important Fenton-like reaction.