Formation of TEMPOL-hydroxylamine during reaction between TEMPOL and hydroxyl radical: HPLC/ECD study

Nitroxyl radicals are important antioxidants that have been used to protect animal tissues from oxidative damage. Their reaction with hydroxyl radical ((OH)-O-center dot) is generally accepted to be the mechanism of antioxidant function. However, the direct interaction of nitroxyl radicals with (OH)-O-center dot does not always provide a satisfactory explanation in various pH, because the concentration of hydrogen ion may affect the generation of secondary (OH)-O-center dot-derived radicals. In the present study, it was confirmed that the reaction between 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) and (OH)-O-center dot generated TEMPOL-hydroxylamine, 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPON) and TEMPON-hydroxylamine using HPLC coupled with electrochemical detection. In the absence of NADH, TEMPOL-H may be generated by the reaction with secondary (OH)-O-center dot-derived radicals in acidic condition. In the presence of NADH, a large proportion of the nonparamagnetic products was TEMPOL-H. Finally, it was clarified that TEMPOL-H was generated during dopamine metabolism, which is believed to be one of the (OH)-O-center dot sources in pathological processes such as Parkinson's disease.