Peroxynitrite is formed by the very fast reaction of nitric oxide and superoxide radicals, a reaction that kinetically competes with other routes that chemically consume or physically sequester the reagents. It can behave either as an endogenous cytotoxin toward host tissues or a cytotoxic effector molecule against invading pathogens, depending on the cellular source and pathophysiological setting. Peroxynitrite is in itself very reactive against a few specific targets that range from efficient detoxification systems, such as peroxiredoxins, to reactions eventually leading to enhanced radical formation (e.g., nitrogen dioxide and carbonate radicals), such as the reaction with carbon dioxide. Thus, the chemical biology of peroxynitrite is dictated by the chemical kinetics of its formation and decay and by the diffusion across membranes of the species involved, including peroxynitrite itself. On the other hand, most durable traces of peroxynitrite passing (such as 3-nitrotyrosine) are derived from radicals formed from peroxynitrite by routes that represent extremely low-yield processes but that have potentially critical biological consequences. Here we have reviewed the chemical kinetics of peroxynitrite as a biochemical transient species in order to estimate its rates of formation and decay and then its steady-state concentration in different intra- or extracellular compartments, trying to provide a quantitative basis for its reactivity; additionally, we have considered diffusion across membranes to locate its possible effects. Finally, we have assessed the most successful attempts to intercept peroxynitrite by pharmacological intervention in their potential to increment the existing biological defenses that routinely deal with this cytotoxin.

Peroxynitrite is a short-lived and reactive biological oxidant formed from the diffusion-controlled reaction of the free radicals superoxide (O) and nitric oxide (NO). In this review, we first analyze the biochemical evidence for the formation of peroxynitrite in vivo and the reactions that lead to it. Then, we describe the principal reactions that peroxynitrite undergoes with biological targets and provide kinetic and mechanistic details. In these reactions, peroxynitrite has roles as (1) peroxide, (2) Lewis base, and (3) free radical generator. Physiological levels of CO can change the outcome of peroxynitrite reactions. The second part of the review assesses the formation of protein 3-nitrotyrosine (NOTyr) by peroxynitrite-dependent and -independent mechanisms, as one of the hallmarks of the actions of NO-derived oxidants in biological systems. Moreover, tyrosine nitration impacts protein structure and function, tyrosine kinase signal transduction cascades and protein turnover. Overall, the review is aimed to provide an integrated biochemical view on the formation and reactions of peroxynitrite under biologically relevant conditions and the impact of this stealthy oxidant and one of its major footprints, protein NOTyr, in the disruption of cellular homeostasis.

Background:Hydrogen sulfide (H 2 S) modulates physiological processes in mammals. Results: The reactivity of H 2 S toward disulfides (RSSR) and albumin sulfenic acid (RSOH) to form persulfides (RSSH) was assessed. Conclusion: H 2 S is less reactive than thiols. Persulfides have enhanced nucleophilicity. Significance: This kinetic study helps rationalize the contribution of the reactions with oxidized thiol derivatives to H 2 S biology.

Protein thiol reactivity generally involves the nucleophilic attack of the thiolate on an electrophile. A low pK(a) means higher availability of the thiolate at neutral pH but often a lower nucleophilicity. Protein structural factors contribute to increasing the reactivity of the thiol in very specific reactions, but these factors do not provide an indiscriminate augmentation in general reactivity. Notably, reduction of hydroperoxides by the catalytic cysteine of peroxiredoxins can achieve extraordinary reaction rates relative to free cysteine. The discussion of this catalytic efficiency has centered in the stabilization of the thiolate as a way to increase nucleophilicity. Such stabilization originates from electrostatic and polar interactions of the catalytic cysteine with the protein environment. We propose that the set of interactions is better described as a means of stabilizing the anionic transition state of the reaction. The enhanced acidity of the critical cysteine is concurrent but not the cause of catalytic efficiency. Protein stabilization of the transition state is achieved by (a) a relatively static charge distribution around the cysteine that includes a conserved arginine and the N-terminus of an α-helix providing a cationic environment that stabilizes the reacting thiolate, the transition state, and also the anionic leaving group; (b) a dynamic set of polar interactions that stabilize the thiolate in the resting enzyme and contribute to restraining its reactivity in the absence of substrate; but upon peroxide binding these active/binding site groups switch interactions from thiolate to peroxide oxygens, simultaneously increasing the nucleophilicity of the attacking sulfur and facilitating the correct positioning of the substrate. The switching of polar interaction provides further acceleration and, importantly, confers specificity to the thiol reactivity. The extraordinary thiol reactivity and specificity toward H(2)O(2) combined with their ubiquity and abundance place peroxiredoxins, along with glutathione peroxidases, as obligate hydroperoxide cellular sensors.