OXIDATION OF VITAMIN-E, VITAMIN-C, AND THIOLS IN RAT-BRAIN SYNAPTOSOMES BY PEROXYNITRITE

Peroxynitrite is formed by the reaction of superoxide with nitric oxid e, an important neuro-transmitter. Incubation of rat brain synaptosome s with peroxynitrite resulted in the consumption of antioxidant substa nces such as alpha-tocopherol, ascorbate, and thiols. Membrane cholest erol was not oxidized under the same conditions. alpha-Tocopherol and ascorbate in synaptosomes were oxidized very rapidly by peroxynitrite. In contrast, previous reports in the literature have shown that perox ynitrite treatment did not oxidize tocopherol in human plasma. Peroxyn itrite in sufficient concentrations oxidized all of the tocopherol and ascorbate in synaptosomes. Thus, the oxidant is able to diffuse to th e different membranes in synaptosomes and oxidize tocopherol in all of them. alpha-Tocopherol is converted quantitatively to tocopherolquino ne during the oxidation. Significant amounts of thiols (at least 30% o f the total thiols) do not seem to be accessible to oxidation by perox ynitrite. However, the concentration of thiols is much higher than tho se oi tocopherol and ascorbate. Addition of the hydroxyl radical quenc hers benzoate or mannitol or the enzymes superoxide dismutase or catal ase (alone or together) did not affect the oxidation of tocopherol and ascorbate by peroxynitrite, whereas cysteine and glutathione blocked the oxidation. Therefore, reactive oxygen species may not be directly involved as intermediates in oxidations induced by peroxynitrite. The latter is a potent oxidizing agent that can oxidize substances such as tocopherols, ascorbate, and thiols in the immediate vicinity of its f ormation. The antioxidant nutrients ascorbate and tocopherol could pla y important roles in protecting brain from oxidative damage induced by peroxynitrite.