Peroxynitrite-Induced DNA Strand Scission Mediated by a Manganese Porphyrin John T. Groves* and Sudhakar S. Marla Department of Chemistry Princeton University Princeton NJ 08544 J. Am. Chem. Soc., 1995, 117, 9578 Peroxynitrite (ONOO^-) is a potent oxidant generated by the reaction between nitric oxide (NO) and superoxide ion (O2-). Thus, the macrophage immune response3 or pathological conditions such as endotoxic shock and ischemia/reperfusion, which raise the concentrations of both NO and O2-, could generate significant levels of ONOO-; in vivo . Peroxynitrite and its conjugate acid are capable of nitrating tyrosine residues in proteins and oxidizing DNA, lipids, sulfhydryls and methionine. Hence, the toxicity of ONOO- has been considered in light of this reactivity. Oxidative damage in cells has been linked to the metal ion-catalyzed generation of free radicals and peroxides. Thus, we have examined the metal-mediated activation of peroxynitrite toward biological targets. Described here is evidence that a manganese porphyrin-peroxynitrite system generates reactive metal-oxo species which potentiate oxidative cleavage of plasmid DNA and catalyze the nitration of phenols.

We find that peroxynitrite reacts rapidly and efficiently with Mn(III)TMPyP under physiological conditions to generate an oxo-manganese intermediate (Figure 1). A 1:1 stoichiometry is observed for the reaction even though the half-life of ONOO- is about 1 s under these conditions. The high-valent species decayed back to the starting Mn(III) porphyrin with clear isosbestic behavior. The intermediate observed was identified to be an oxoMn(IV)TMPyP species (2). The spectroscopically similar oxoMn(V) intermediate (1) has a very short lifetime under these conditions.

The reactive oxo-manganese species efficiently mediated the oxidative cleavage of DNA. Interception of 1 by NO2- diverted the oxidation reaction (via 2 and NO2-) toward the nitration of phenols. ONOO- is formed by a reaction between nitric oxide (NO) and superoxide (O2-) and can therefore be formed in cells experiencing oxidative stress. The results indicate that adventitious metal ions could play a role in determining ONOO- toxicity.