Cytochrome nitrosyl complexes

The kinetics of reactions of NO with ferri- and ferro-heme proteins and models under ambient conditions have been studied by time-resolved spectroscopic techniques. Representative results are summarized in Table I (22-28). Equilibrium constants determined for the formation of nitrosyl complexes of met-myoglobin (metMb), ferri-cytochrome-c (Cyt111) and catalase (Cat) are in reasonable agreement when measured both by flash photolysis techniques (K= konlkQff) and by spectroscopic titration in aqueous media (22). Table I summarizes the several orders of magnitude range of kon and kQs values obtained for ferri- and ferro-heme proteins. Many k0f[ values were too small to determine by flash photolysis methods and were determined by other means. The small values of kQ result in very large equilibrium constants K for the... 

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. 

The spectrum of reduced cytochrome d in E. coli membranes is affected by added nitrate. The new spectrum formed may be that of a nitrosyl complex. Similar but faster changes were brought about by nitrite, trioxodinitrate or NO.723 The nitrate may be reduced to nitrite by a nitrate reductase in a slow step, followed by further reduction by reduced d. These nitrate-induced spectral changes in cytochrome d had been attributed previously to Ag+ added as silver nitrate. 

The higher stability of ferrous heme nitrosyl complexes compared to the corresponding ferric species is well documented [see Refs (44-46, 69) for reviews]. Since cytochrome P450, catalase, and cytochrome c oxidase are commonly associated with reducing agents, a mechanism exists for longer term inhibition of the activity of these proteins. 

Both Hb and cytochrome c produce stable nitrosyl complexes that in some cases serve as an NO depot in the organism [168-170]. The described nitrosyl complexes are photosensitive and can be destroyed upon irradiation with visible light. 

Behan RK, Hoffart LM et al (2007) Reaction of cytochrome P450BM3 and peroxynitrite yields nitrosyl complex. J Am Chem Soc 129 5855-5859... 

The dissimilatory nitrite reductase (cytochrome cdlt see below) catalyzes the nitrosation of several nucleophiles by nitrite.1531 This may occur through a heme-nitrosyl in which an NO+ group is present. Coordinated nitrite in simple metal complexes such as [Run(NH3)5(N02)]+... 

Studies on nitrosylation reactions of metalloporphyrins are emerging and have been reviewed. 21 Reactions involving FeNO 7 and FeNO 6 species (considered as ferroheme and ferriheme, respectively) are crucial to enzymatic functions (activation of guanylyl cyclase, cytochrome oxidase, catalase inhibition). Reversible photodissociations of NO from nitrosyl metalloporphyrins have been studied by ns-pulsed laser techniques, providing values for kt and kA (Equation (12)). Dissociation processes are very slow, particularly for ferroheme complexes however, kA values in the 10-5—101 s 1 range have been measured for several five-coordinated FeNO 7 tetraarylpor-phyrins.103 The spread in the kA values is not well understood yet. 

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