Reaction between peroxynitrite and hydrogen peroxide: formation of oxygen and slowing of peroxynitrite decomposition.

Peroxynitrite, the reaction product of nitric oxide and superoxide, is a potent and versatile oxidant that can attack a wide range of targets. In this work, we studied the oxidation of hydrogen peroxide by peroxynitrite, which led to oxygen evolution. Oxygen yields increased at alkaline pH with an apparent pKa of 7.05 +/- 0.04. The maximum yields were 16% and 32% of added peroxynitrite at pH 5.9 and 7.4, respectively, assuming that two molecules of peroxynitrite are needed to produce one of oxygen. Hydroxyl radical scavengers (dimethyl sulfoxide, mannitol, ethanol, formate, and acetate) inhibited oxygen evolution to a similar extent to that predicted from their rate constants with hydroxyl radical. The apparent rate constant of peroxynitrite decomposition was zero-order in hydrogen peroxide at acidic pH. At neutral and alkaline pH, the rate of peroxynitrite disappearance decreased in the presence of millimolar concentrations of hydrogen peroxide by up to 50%. The apparent activation enthalpy and entropy for peroxynitrite decomposition increased by 1.7 kcal mol-1 and 4.7 cal mol-1 K-1, respectively, in the presence of hydrogen peroxide. We propose that an activated intermediate of peroxynitrous acid is responsible for hydrogen peroxide oxidation at acidic pH, while at more alkaline pH the formation of a stabilizing complex between hydrogen peroxide and transperoxynitrite anion is involved.