Ferric-superoxide complexes

Fig. 9. Possible electron transfer mechanism for NOS utilizing a pterin radical. The oxy-complex in 2 is shown as the ferric (Fe +)-superoxide complex. The role of the pterin then is to donate an electron to the iron, thus giving the peroxy dianion in 3. The dianion is a potent base that abstracts a proton from the substrate, giving 4. The system is now set up for a peroxidase-like heterolytic cleavage of the 0-0 bond to give the active hydroxylating intermediate in 5 and, finally, the first product in 6.

A recent study of the interaction of superoxide anion with Fe(II) porphyrins in dimethyl sulphoxide or acetonitrile has suggested the formation of an complex (por-phyrin)Fe02, which is formulated on the basis of infra-red, U.V.-visible, n.m.r., and E.P.R. spectroscopic measurements as (porphyrin)-Fe (high spin)-02 . The E.P.R. spectrum differs from that of other high spin ferric porphyrin complexes but is... 

E° = —196 mV) to the ferric P450CAM heme iron to produce the ferrous state of the protein, 3. Dioxygen binds to the ferrous heme iron to form the ferrous oxy complex, 4a/4b, whose valence structure can be presented either as the ferrous-02, 4a, or as the ferric superoxide, 4b, complex. Addition of carbon monoxide to 3 yields a ferrous carbon monoxide adduct, 5, with its characteristic absorbance peak at 450 nm [37],... 

Although the coordination and orientation of the metal complex are still not understood, extensive studies have been conducted concerning the remarkable chemistry of this species. The overall mechanism of action is described in Figure 8.19. In the presence of oxygen, the Fe(II) O2 species is formed and is likely rapidly converted to a ferric superoxide species. The one-electron reduction of this species, using either an organic reductant or another equivalent of Fe(II)-bleomycin, leads formally to an Fe(III)-peroxide, which then undergoes... 

Step e. The ferric superoxide anion undergoes further reduction by accepting a second electron from the flavoprotein (or possibly cytochrome bS) to form the equivalent of a two-electron-reduced complex, ... 

FRAP Superoxide Ferric chromogenic complex reduction at 593 nm Scavenging of Oj" generated by xantbine oxidase No substrate. Not specific measure of reducing capacity Not specific, no equihbrium when 0 is generated continuously Benzie and Strain (1996) Costantmo et al. (1992)... 

Addition of molecular oxygen to reduced the complex. Resonance forms exist for ferrous-dioxygen and ferric-superoxide with the latter favored. 

In contrast, superoxide reacts with ferrous iron in SOR, and in this case, the fer-rous-NO <-> ferric-NO interaction must model formation of a ferric-peroxyl complex, but with one less electron [55] ... 

In the process, the iron is reduced to the ferrous form. Ferric cytochrome c is reduced by nitric oxide through a nitrosyl intermediate to produce ferrous cytochrome c and nitrite (Orii and Shimada, 1978). The nitrosyl cytochrome c absorbs at 560 nm, which is slightly higher than the 550-nm peak observed for reduced cytochrome c. Nitric oxide may be an interference in the assay of superoxide from cultured cells by the cytochrome c method. When nitric oxide reacts with cytochrome c, there is an initial decrease in absorbance at 550 nm as the nitrosyl complex is formed followed by a rise in absorbance as the complex decomposes to nitrite and reduced cytochrome c. This is a potential artifact in studies measuring the release of superoxide from cultured endothelial cells or other cells that make nitric oxide. 

Iron-containing superoxide dismutases are present in many species of bacteria (Hassan and Fridovitch, 1978). These nonheme iron proteins have a characteristic set of EPR lines split about g = 4.2 in the ferric state, arising from the middle Kramers doublet of a rhombic high-spin site. Ferrous iron superoxide dismutase forms an S = I complex with NO that resembles the lipoxygenase-NO adduct by EPR criteria (I. Fridovich, T. Kirby, and J. C. Salerno, (1978) unpublished observations). 

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