Detoxification of superoxide without production of H_2O_2: Antioxidant activity of superoxide reductase complexed with ferrocyanide

摘要

The superoxide radical O_2~- is a toxic by-product of oxygen metabolism. Two O_2~- detoxifying enzymes have been described so far, superoxide dismutase and superoxide reductase (SOR), both forming H_2O_2 as a reaction product. Recently, the SOR active site, a ferrous iron in a [Fe~(2+) (N-His)_4 (S-Cys)] pentacoordination, was shown to have the ability to form a complex with the organome-tallic compound ferrocyanide. Here, we have investigated in detail the reactivity of the SOR-ferrocyanide complex with O_2~- by pulse and γ-ray radiolysis, infrared, and UV-visible spectroscopies. The complex reacts very efficiently with O_2~-. 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 H_2O_2 species is no longer the reaction product. Accordingly, in vivo experiments 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 O_2~- detoxification activity, catalyzed by the SOR-ferrocyanide complex, which does not conduct to the production of the toxic H_2O_2 species.