Reversible reduction of nitroxides to hydroxylamines: Roles for ascorbate and glutathione

Biological applications of stable nitroxyl radicals, NR, include their use as contrast agents for magnetic resonance imaging, spin labels, superoxide dismutase mimics, and antioxidants. The rapid reduction of NR in biological samples into hydroxylamines (HA) significantly limits their application. In turn, reoxidation of HA back to the NR has been used for detection of reactive oxygen species (ROS). In this work comparative studies of the reduction of pyrrolidine, imidazoline, and imidazolidine NR by ascorbate were performed taking advantage of recently synthesized tetraethyl-substituted NR with much higher stability toward reduction both in vitro and in vivo. Surprisingly, these NR kept 10-50% of initial intensity of electron paramagnetic resonance signal for about 1 h in the presence of 100-fold excess of ascorbate. To explain these data, reoxidation of the corresponding HA by ascorbate radical and dehydroascorbic acid back to the NR was proposed. This hypothesis was supported by direct measurement of the NR appearance from the HA on ascorbate radical generation by ascorbate oxidase, or in the presence of the dehydroascorbic acid. The reversible reaction between NR and ascorbate was observed for the various types of NR, and the rate constants for direct and reverse reactions were determined. The equilibrium constants for one-electron reduction of the tetraethyl-substituted NR by ascorbate were found to be in the range from 2.65 × 10^{- 6} to 10^{- 5} which is significantly lower than corresponding values for the tetramethyl-substituted NR (more or about 10^{- 4}). This explains the establishment of an EPR-detectable quasi-equilibrium level of tetraethyl-substituted NR in the presence of an excess of ascorbate. The redox reactions of the NR-HA couple in ascorbate-containing media were found to be significantly affected by glutathione (GSH). This effect was attributed to the reduction of ascorbate radicals by GSH, and the rate constant of this reaction was found to be equal to 10 M^{- 1} s^{- 1}. In summary, the data provide new insight into the redox chemistry of NR and HA, and significantly affect interpretation and strategy of their use as redox- and ROS-sensitive probes, or as antioxidants.