Superoxide, diffusion-controlled

Reaction of nitric oxide with superoxide is undoubtedly the most important reaction of nitric oxide, resulting in the formation of peroxynitrite, one of the main reactive species in free radical-mediated damaging processes. This reaction is a diffusion-controlled one, with the rate constant (which has been measured by many workers, see, for example, Ref. [41]), of about 2 x 109 1 mol-1 s-1. Goldstein and Czapski [41] also measured the rate constant for Reaction (11) ... 

Simultaneous generation of nitric oxide and superoxide by NO synthases results in the formation of peroxynitrite. As the reaction between these free radicals proceeds with a diffusion-controlled rate (Chapter 21), it is surprising that it is possible to detect experimentally both superoxide and NO during NO synthase catalysis. However, Pou et al. [147] pointed out that the reason is the fact that superoxide and nitric oxide are generated consecutively at the same heme iron site. Therefore, after superoxide production NO synthase must cycle twice before NO production. Correspondingly, there is enough time for superoxide to diffuse from the enzyme and react with other biomolecules. 

Many transition metal complexes have been considered as synzymes for superoxide anion dismutation and activity as SOD mimics. The stability and toxicity of any metal complex intended for pharmaceutical application is of paramount concern, and the complex must also be determined to be truly catalytic for superoxide ion dismutation. Because the catalytic activity of SOD1, for instance, is essentially diffusion-controlled with rates of 2 x 1 () M 1 s 1, fast analytic techniques must be used to directly measure the decay of superoxide anion in testing complexes as SOD mimics. One needs to distinguish between the uncatalyzed stoichiometric decay of the superoxide anion (second-order kinetic behavior) and true catalytic SOD dismutation (first-order behavior with [O ] [synzyme] and many turnovers of SOD mimic catalytic behavior). Indirect detection methods such as those in which a steady-state concentration of superoxide anion is generated from a xanthine/xanthine oxidase system will not measure catalytic synzyme behavior but instead will evaluate the potential SOD mimic as a stoichiometric superoxide scavenger. Two methodologies, stopped-flow kinetic analysis and pulse radiolysis, are fast methods that will measure SOD mimic catalytic behavior. These methods are briefly described in reference 11 and in Section 3.7.2 of Chapter 3. 

It has now been more than a decade since Beckman and his collaborators first disclosed their observations that the combination of two relatively unreactive, yet biologically relevant free radicals, superoxide anion and nitric oxide, would produce a new highly reactive physiologically important reagent. The interaction of these two presumably innocuous species appears to be diffusion controlled and produces a thermally stable peroxy anion, peroxynitrite (equation 1). ... 

As noted earlier, peroxynitrite is formed with a diffusion-controlled rate from superoxide and nitric oxide (Reaction 10). As both these radicals are ubiquitous species, which present practically in all cells and tissues, peroxynitrite can be the most important species responsible for free radical-mediated damage in biological systems. Moreover, it is now known that NO synthases are capable of producing superoxide and nitric oxide simultaneously (see Chapter 22), greatly increasing the possible rate of peroxynitrite production. In addition, another enzyme xanthine dehydrogenase is also able to produce peroxynitrite in the presence of nitrite... 

Furthermore, this combination caused a multifold increase in proteolytic susceptibility of oxidatively damaged BSA [7]. These findings are really confusing because superoxide and hydroxyl radicals react with each other with a diffusion-controlled rate to form inactive hydroxyl anion and dioxygen ... 

The reduction of oxygen by copper(I) is faster than that of the iron(II) complexes 5x 104M 1 s-1 for CuI(phen)2 [52] and 4xl04M 1 s 1 for Cu1 (histidine [76]. It is this relatively fast autoxidation that limits the usefulness of copper complexes as mimics of superoxide dismutase under conditions of high superoxide concentrations [77]. Copper(II) catalyses the dismutation of superoxide at near diffusion-controlled rates kcat = 8 x 109 M 1 s-1 [78,79],... 

The yellow ONOO- ion can be made in pure form by passing NO through a solution of tetramethylammonium superoxide in ammonia56 ONOO from other syntheses can be contaminated by reactants and nitrate. ONOOH is a weak acid with a pKa near 6.8 and isomerizes to N03 and H+ in seconds.57 At pH 12 the anion is stable for several hours at 0°C. It occurs solely as the cis isomer, due to a partial double bond between N and the first peroxide O.58 Peroxynitrite is toxic in vivo it is formed through the diffusion-controlled reaction of 02 with NO-.57 ONOOH oxidizes via direct and indirect pathways.59 Under physiological conditions the main reaction partners for ONOO" and ONOOH appear to be carbon dioxide60 and thiols,61 respectively. The ONOO—C02—adduct nitrates tyrosines. 

Finally, one of the most highly studied acid-base radical pairs is the superoxide/perhydroxyl (H0 /0 ) radical. In the presence of oxygen, the electron is scavenged in a diffusion-controlled reaction ... 

The prototropic equilibrium constant (pRa) for the equilibrium between 0 and HO radicals is 4.7. Therefore at physiological pH, the superoxide radicals exist predominantly in the form of O radicals. HO radicals are more reactive than Oj radicals, and react with substrates by hydrogen abstraction or by addition to the double bonds. 0 radicals do not exhibit these reactions but participate in a number of redox reactions with metal ions and substrates like quinone, ascorbate, etc. The rate constants for these reactions are considerably lower than diffusion-controlled limits (10 to 10 s ). Oj ... 

S. A. Allison, G. Ganti and J. A. McCammon, Simulation of the diffusion controlled reaction between superoxide and superoxide dismutase. 1. Simple models. Biopolymers, 24 (1985) 1323-1336. 

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