Superoxide Reaction with Nitroxides

Abstract

Stable, free radical nitroxides are commonly used ESR spectroscopy tools. However, it has recently been found that ESR observable signal from 5-membered ring spin-adducts or stable label nitroxides is lost or diminished by reaction with superoxide. A similar radical-radical annihilation was not found for six membered ring nitroxide radicals. To discern why six-membered ring nitroxides are not reduced under superoxide flux generated by hypoxanthine/xanthine oxidase, spectrophoprmetric (Cyt C) and chemilu-minescence (lucigenin) and ESR assays were used to follow the reactions. Spectrophotometry and chemi-luminescence clearly demonstrated that the six-membered piperidine-I-oxyl compounds (TEMPO, TEMPOL, and TEMPAMIN) rapidly react with superoxide: rate constants at pH 7.8 ranging from 7 × 10⁴ to 1.2 × 10⁻⁵M⁻¹s^−l. The absence of detectable ESR signal loss results from facile re-oxidation of the corresponding hydroxylamine by superoxide. To fully corroborate the efficiency of the 6-membered nitroxide superoxide dismutase activity, they were shown to protect fully mammalian cells from oxidative damage resulting from exposure to the superoxide and hydrogen peroxide generating system hypoxanthine/ xanthine oxidase. Since six-membered cyclic nitroxides react with superoxide about 2 orders of magnitude faster than the corresponding 5-membered ring nitroxides. they may ultimately be more useful as superoxide oxide dismutase mimetic agents.