Peroxynitrite hydroxyl radical reactivity

At physiological pH, ONOO- protonates to peroxynitrous acid (ONOOH) which disappears within a few seconds, the end product being largely nitrate. The chemistry of peroxynitrite/peroxynitrous acid is extremely complex, although addition of ONOO to cells and tissues leads to oxidation and nitration of proteins, DNA and lipids with a reactivity that is comparable to that of hydroxyl radicals. 

This mode of superoxide-dependent free radical-mediated damaging activity remains an important one although the nature of the generated reactive species (free hydroxyl radicals or perferryl, or ferryl ions) is still obscure. However, after the discovery of the fact that many cells produce nitric oxide in relatively large amounts (see below), it became clear that there is another and possibly a more portent mechanism of superoxide-induced free radical damage, namely, the formation of highly reactive peroxynitrite. 

It should be noted that Reaction (4) is not a one-stage process.) Both free radical N02 and highly reactive peroxynitrite are the initiators of lipid peroxidation although the elementary stages of initiation by these compounds are not fully understood. (Crow et al. [45] suggested that trans-ONOO is protonated into trans peroxynitrous acid, which is isomerized into the unstable cis form. The latter is easily decomposed to form hydroxyl radical.) Another possible mechanism of prooxidant activity of nitric oxide is the modification of unsaturated fatty acids and lipids through the formation of active nitrated lipid derivatives. 

NO may react with superoxide to yield the highly reactive peroxynitrite, ONOO-. Superoxide may also be converted into H202 and the reactive hydroxyl radical, OH. In this way excessive activation of glutamate receptors leads to oxidative damage. The calcium influx has a major effect on mitochondria and causes them to depolarize and swell. This leads to a pore being formed in the outer mitochondrial membrane, which allows the escape of cytochrome c and procaspases from the mitochondria into the cytosol. Cytochrome c activates the caspase cascade, which leads to apoptotic cell death (Ch. 35). 

Peroxynitrite is a potent bactericidal agent for E. coli, with an LD50 of 250 tM at pH 7.4 and 37°C (Zhu et al., 1992). Neither 1 mM HjOj nor 10 mU/ml of xanthine oxidase plus xanthine were toxic in the same system. The toxicity was not due to the hydroxyl radical-like reactivity because lipid soluble radical scavengers either had no effect or slightly increased toxicity of peroxynitrite (Zhu... 

Reactive oxygen species such as the hydroxyl radical, superoxide anion, and peroxynitrite are involved in many cellular processes including the inflammatory response. The best known antiinflammatory compound is ebselen (59, R = Ph). It is shown to be a neuroprotective agent and an inhibitor of free radical-induced apoptosis [20, 248, 249], It has undergone phase III clinical trials and is soon to become the first synthetic organoselenium therapeutic released on the market. 

Another "OH generating reaction is peroxynitrite decomposition to hydroxyl radical and nitrogen dioxide (see sect. 1.12). Also, actions of ionizing radiation and ultrasounds yielding water molecule decomposition are additional sources of "OH. The hydroxyl radical is probably the main reactive species in the so-called metal-catalyzed oxidation systems (or mixed-function oxidation systems) used in experiments where ascorbate and metal ions are employed (though formation of other products such as high-valency iron species ferryl Fe+4 or perferryl Fe+5 forms cannot be excluded) (N2, S50). 

Peroxynitrite is a nonspecific oxidant that reacts with all classes of biomolecules depleting low-molecular-weight antioxidants, initiating lipid peroxidation, damaging nucleic acids and proteins. Its reactions are much slower than those of the hydroxyl radical but are faster than those of hydrogen peroxide. Comparison of peroxynitrite reactivity with various amino acid residues of human serum albumin have shown that cysteine, methionine, and tryptophan are the most reactive... 

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