Detoxification of superoxide without production of H2O2: Antioxidant activity of superoxide reductase complexed with ferrocyanide

摘要

The superoxide radical O2^·̅ is a toxic by-product of oxygen metabolism. Two O2^·̅ detoxifying enzymes have been described so far, superoxide dismutase and superoxide reductase (SOR), both forming H2O2 as a reaction product. Recently, the SOR active site, a ferrous iron in a [Fe²⁺ (N-His)4 (S-Cys)] pentacoordination, was shown to have the ability to form a complex with the organometallic compound ferrocyanide. Here, we have investigated in detail the reactivity of the SOR–ferrocyanide complex with O2^·̅ by pulse and γ-ray radiolysis, infrared, and UV-visible spectroscopies. The complex reacts very efficiently with O2^·̅. However, the presence of the ferrocyanide adduct markedly modifies the reaction mechanism of SOR, with the formation of transient intermediates different from those observed for SOR alone. A one-electron redox chemistry appears to be carried out by the ferrocyanide moiety of the complex, whereas the SOR iron site remains in the reduced state. Surprisingly, the toxic H2O2 species is no longer the reaction product. Accordingly, in vivoexperiments showed that formation of the SOR–ferrocyanide complex increased the antioxidant capabilities of SOR expressed in an Escherichia coli sodA sodB recA mutant strain. Altogether, these data describe an unprecedented O2^·̅ detoxification activity, catalyzed by the SOR–ferrocyanide complex, which does not conduct to the production of the toxic H2O2 species.