Nitric Oxide and Nitroxides Can Act as Efficient Scavengers of Protein-Derived Free Radicals

Nitric oxide (•NO) may act as either a pro-oxidant or an antioxidant in biological systems. Although •NO and nitroxide radicals react slowly with most molecules, they react at near diffusion-controlled rates with other radicals and may therefore be efficient protective agents. This study assessed the ability of •NO and nitroxides to intercept specific protein-derived radicals and compared the efficacy of these species. Three protein radical systems were investigated as follows: BSA-derived radicals generated via radical transfer from H₂O₂-activated horseradish peroxidase, radicals formed on myoglobin via reaction with H₂O₂, and carbon-centered radicals formed from amino acid hydroperoxides on exposure to Fe²⁺-EDTA. In each case, radicals were generated in the absence or presence of •NO or nitroxides of different size and charge. Concentration-dependent loss of the protein radicals was detected by electron paramagnetic resonance with both •NO and nitroxides and time-dependent consumption of •NO using an •NO electrode. The protein oxidation product dityrosine was significantly reduced by •NO and nitroxides, and 3,4-dihydroxyphenylalanine levels were reduced by nitroxides but not •NO. Overall, these studies demonstrate that •NO and nitroxides are efficient near-stoichiometric scavengers of protein radicals and, hence, are potential protective agents against protein oxidation reactions and resulting damage. These reactions show little dependence on nitroxide structure or charge.