Superoxide reactions with nitrogen oxides

Deleterious protein cross-linking can also be induced by reactive nitrogen species (RNS) such as peroxynitrite ONOO formed by the reaction of superoxide with nitric oxide (NO). The cross-links are formed between tyrosine residues following nitration by peroxynitrite (Sitte, 2003). Carnosine appears to play roles not only in NO generation but also in protection against excess NO production by inducible nitric oxide synthetase (NOS), thereby preventing ONOO-mediated protein modification (Fontana et ah, 2002). Evidence for a carnosine-NO adduct has also been published (Nicoletti et al., 2007). 

It was recently reported that the tryptophan residues of proteins could be nitrated by the action of peroxynitrite (67). This reactive nitrogen species (RNS) is generated from the reaction of nitric oxide with superoxide at a rate that is ten times greater than the destruction of superoxide by dismutases. The authors propose that the nitration of tryptophan, although less common than tyrosine nitration, could serve to modulate the function of some proteins. However, at this time the in vivo evidence for tryptophan nitration by RNS has yet to be reported. 

The photolysis of nitrate and nitrite in sea water produces nitrogen dioxide (NO2) and nitric oxide (NO), respectively (eqns [I] and [II]). Previous work indicated that the photolysis of nitrite could act as a small net source of NO to the marine atmosphere under some conditions. However, this conclusion seems to be at odds with estimates of the steady-state concentrations of superoxide and the now known rate constant for the reaction of superoxide with nitric oxide (6.7 x 10 M s ) to form peroxyni-trite in aqueous phases (eqn [V]). 

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