Spin trap studies on the decomposition of peroxynitrite.

The observation of the hydroxyl radical spin adduct of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) is evaluated as a probe for the production of hydroxyl radicals from the decomposition of peroxynitrite. Although a weak signal corresponding to the DMPO-hydroxyl radical spin adduct (.DMPO-OH) is observed when peroxynitrite is allowed to decompose in the presence of DMPO, it is concluded that this does not constitute proof of the presence of free hydroxyl radicals. The observed rate constant for the decay of peroxynitrite increases from 0.35 to 0.51 s-1 (46% increase) when the concentration of DMPO is increased from 0 to 75 mM. This strongly suggests there is a reaction between DMPO and HOONO, or between DMPO and an activated intermediate of HOONO, to produce the hydroxyl radical spin adduct. The addition of glutathione or cysteine produces a large increase in the intensity of the .DMPO-OH spin adduct signal; experiments employing superoxide dismutase suggest that the increases in the amounts of .DMPO-OH adduct are produced from the decomposition of the spin adduct of the superoxide radical (.DMPO-OOH). The superoxide adduct arises as a result of the autoxidation of thiols, a process known to produce superoxide. The results presented here are incompatible with the formation of free hydroxyl radicals but can be explained in terms of an intermediate of HOONO that is less reactive and more selective than the free hydroxyl radical.