NO-redox paradox: direct oxidation of alpha-tocopherol and alpha-tocopherol-mediated oxidation of ascorbate.

Nitric-oxide (NO) can act as both a pro- or an antioxidant, yielding either cytotoxic or protective effects, respectively. The previously unrecognized redox interactions of NO with antioxidants, and not solely its well-known reactions with oxygen radicals, peroxyl radicals and transition metal centers, may be essential for its dual mechanisms in cells. Since the alpha-tocopherol/ascorbate redox cycle is central to antioxidant protection, we studied the direct effects of NO on alpha-tocopherol, ascorbate and combinations thereof in aqueous, micellar environments using ESR spectral and HPLC quantitative techniques. We found that NO does not directly oxidize ascorbate under anaerobic conditions. alpha-Tocopherol, however, in the presence of NO and under anaerobic conditions, was oxidized to the alpha-tocopheroxyl radical. Under conditions where NO oxidized alpha-tocopherol, the subsequent production of the alpha-tocopheroxyl radical depleted ascorbate, yielding the semidehydroascorbyl radical and regenerating alpha-tocopherol. Thus, NO interacts with the redox cycle involving alpha-tocopherol and ascorbate in a pro-oxidant manner.