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Chem. Res. Toxicol. 1998, 11, 718-719
The Nature of Reactive Species in Systems That Produce Peroxynitrite Giuseppe L. Squadrito* and William A. Pryor* Biodynamics Institute, 711 Choppin Hall, Louisiana State University, Baton Rouge, Louisiana 70803-1800 Received March 19, 1998
Introduction: It is widely accepted that peroxynitrite decomposes to form a reactive intermediate, but the nature of this intermediate is a matter of controversy. The activated intermediate has been described as (a) the hydroxyl radical (S1, S2), (b) having a structure similar to the trans-isomer of peroxynitrous acid (HOONO) (S3), and (c) an intermediate of undefined structure, formed in a steady state from ground-state HOONO and usually represented as HOONO* (1-3). Does Peroxynitrite Decompose to form HO• and • NO2? Experimental evidence against substantial yields of the free radicals HO• and •NO2 from the homolysis of HO-ONO is compelling and can be grouped in three categories: (a) Failure of scavengers to prevent hydroxyl radical-type damage of biotargets at concentrations that would have been effective for the hydroxyl radical (1, 3, S4). (b) The lifetime of the intermediate formed during the decomposition of HO-ONO is substantially longer than that of a classical cage of radicals (2, 3). (c) Increasing viscosity has little or no effect on the rate of disappearance of peroxynitrite (4). Therefore, even if homolysis occurs (Scheme 1A), the caged radicals must recombine to form nitrate in the cage (kN . kdiff), and the free radicals HO• and •NO2 must be formed in small yields or not formed at all (4). Homolysis of HO-ONO also was ruled out (S3) using thermokinetic arguments; however, the validity of this conclusion has been questioned (3, 4, S5). In the original arguments (S3), the calculated ∆G° for homolysis of HOONO was claimed to be too large to allow for homolysis based on forward and reverse rate constants; however, an intermediate precludes the use of this simple argument (2, 3). (The requirement for an intermediate is shown, for example, in Scheme 1A.) Moreover, the revised value for ∆G° for homolysis of HO-ONO allows homolysis to occur (S5). Peroxynitrite may give rise to unusually long-lived solvent cages of radicals (2). First, long-lived solvent cages might be expected because water molecules (unlike the organic solvents used in classical cage studies) can form a network of hydrogen bonds; this could raise the barrier for diffusion of the caged radicals apart, leading to increased formation of cage products and fewer free radicals (2). Second, the highly polar nature of the radical pair in the cage [HO• •NO2 T HO- +NO2] predicts a longer-lived cage relative to one containing less polar, weakly interacting organic radicals (2). Houk et al. calculate the interaction between HO• and •NO2 in the gas phase as 1.9-2.5 kcal/mol (S6), and in solution this * Correspondence and requests for supplementary (S) references should be addressed to either of these authors. Phone: (505) 388-2063. Fax: (504) 388-4936. E-mail:
[email protected] or
[email protected].
Scheme 1. Analysis of Reaction Intermediates Formed during the Decomposition of Peroxynitrite and during Its Reaction with CO2
electron-exchange interaction could be even larger. Thus, the caged radical pair [HO• and •NO2] could be long-lived in water (S2). The experimental evidence for the intermediate HOONO* is consistent with a molecular (nonradical) species, but it can also be accommodated by a homolytic mechanism if kN . kdiff (see Scheme 1A). Thus, if long-lived cages do exist, the intermediate we have described as HOONO* may be this special cage. In summary, the experimental evidence is consistent with either HOONO* or with homolysis to give a long-lived cage of radicals that mainly recombines to form nitrate and with only little or no leakage to free radicals. ONOO-/CO2 Reaction in Biological Systems. Considerable effort continues to be devoted to study the nature of the reactive intermediate formed from HOONO; however, most peroxynitrite formed in vivo reacts with CO2 (5); only hemeproteins compete with CO2 for peroxynitrite (S7-S12). Table 1 shows the reactivities of various biotargets (expressed as pseudo-first-order rate constants) and the concentrations of the biotargets in blood [disregarding compartmentation, since peroxynitrite diffuses freely across cellular membranes (S13)]. Except for the reaction of peroxynitrite with hemoglobin, most peroxynitrite reacts with carbon dioxide. Myeloperoxidase, glutathione peroxidase, serum albumin, and small antioxidant molecules (such as ascorbate and glutathione) react too slowly (3) and/or are present in too low concentrations to compete for peroxynitrite (see Table 1) (S8). Thus, as first suggested by Radi et al. in 1993 (S10), biological oxidations by peroxynitrite are mediated by secondary oxidants formed from the peroxynitrite/ bicarbonate reaction. It is now firmly established that the reaction occurs between ONOO- and carbon dioxide (and not HOONO or HCO3- or other forms) (5, S9). Carbon dioxide reacts with ONOO- catalytically; that is, CO2 is not consumed in its reaction with ONOO-, but it is rapidly recycled (6). The initial adduct ONO-O-CO2likely isomerizes to nitrocarbonate, O2N-OCO2-, and/or decomposes to the radicals CO3•- and •NO2 (5, S7-S11),
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Forum: Reactive Species of Peroxynitrite
Chem. Res. Toxicol., Vol. 11, No. 7, 1998 719
Table 1. Reactivities of CO2, the Hemeproteins Myeloperoxidase and Hemoglobin, and the Selenoprotein Glutathione Peroxidasea biomolecule hemoglobinb CO2c myeloperoxidased glutathione peroxidasee
kapp [biomolecule] k × [biomolecule] (M-1 s-1) (M) (s-1) 2.5 × 104 4.6 × 104 4.8 × 106 2 × 104
2.3 × 10-3 1 × 10-3 2 × 10-7
