α-Tocopherol in Rat Brain Subcellular Fractions Is Oxidized Rapidly during Incubations with Low Concentrations of Peroxynitrite ,

The reaction of superoxide (a reactive oxygen species) and nitric oxide (one of the nitrogen oxides with numerous biological functions) results in the production of peroxynitrite. The characteristics of oxidation of alpha-tocopherol (vitamin E) in synaptosomes (nerve ending particles) and mitochondria by peroxynitrite were studied. The subcellular fractions were isolated from brain hemispheres of 4-month-old male Fischer 344 rats by standard centrifugation procedures involving Ficoll gradients. Peroxynitrite treatment oxidized alpha-tocopherol in <5 s. This reaction was selective because another membrane component, cholesterol, was not oxidized at the same time, as observed in our previous studies. Mitochondrial alpha-tocopherol was more susceptible to peroxynitrite-induced oxidation than synaptosomal tocopherol. Measurable and significant (P < 0.05) oxidation of tocopherol occurred when mitochondria or synaptosomes were incubated with peroxynitrite in concentrations as low as 5 or 10 micromol/L, respectively. The oxidation could be readily monitored by estimating the production of tocopherolquinone. Oxidation of tocopherol induced by ferrous iron and ascorbate was much slower and the yield of tocopherolquinone lower than by peroxynitrite. The fast and selective oxidation of alpha-tocopherol by peroxynitrite suggests that vitamin E may play an important role in preventing membrane oxidation induced by peroxynitrite. Literature reports indicate the existence of threshold concentrations of tocopherol below which functional alterations occur. Tocopherol oxidation by peroxynitrite could reduce tocopherol concentrations in tissues and subcellular structures to these threshold levels by different concentrations of peroxynitrite. Hence the sensitivity of tissues to peroxynitrite could vary over a wide range.