Nitroxides block DNA scission and protect cells from oxidative damage

Abstract

The protective effect of cyclic stable nitroxide free radicals, having SOD-like activity, against oxidative damage was studied by using Escherichia coli xthA DNA repair-deficient mutant hypersensitive to H2O2. Oxidative damage induced by H2O2 was assayed by monitoring cell survival. The metal chelator 1,10-phenanthroline (OP), which readily intercalates into DNA, potentiated the H2O2-induced damage. The extent of in vivo DNA scission and degradation was studied and compared with the loss of cell viability. The extent of DNA breakage correlated with cell killing, supporting previous suggestions that DNA is the crucial cellular target of H2O2 cytotoxicity. The xthA cells were protected by catalase but not by superoxide dismutase (SOD). Both five- and six-membered ring nitroxides, having SOD-like activity, protected growing and resting cells from H2O2 toxicity, without lowering H2O2 concentration. To check whether nitroxides protect against O2.(-)-independent injury also, experiments were repeated under hypoxia. These nitroxides also protected hypoxic cells against H2O2, suggesting alternative modes of protection. Since nitroxides were found to reoxidize DNA-bound iron(II), the present results suggest that nitroxides protect by oxidizing reduced transition metals, thus interfering with the Fenton reaction.

Cardiac reperfusion damage prevented by a nitroxide free radical.

“…The TEMPO-dependent protection of isolated rat hearts from free radical damage extends the reported effect of nitroxides in bacteria and mammalian cell cultures (18,19,21,24) to whole organs. The dramatic protective activity, high efficiency, and apparent rapid cell penetration of TEMPO, along with low toxicity, suggest that a clinical application of nitroxides may be feasible.…”

“…Furthermore, the low toxicity of most nitroxides (23) would make them good candidates for in vivo use. So far nitroxide spin labels have been shown to prevent free radical damage dramatically in cell cultures (18,19,21,24). However, protective effects of the nitroxide radicals have not been investigated in intact organs or in whole animals.…”

A novel antiulcerogenic stable radical prevents gastric mucosal lesions in rats.

“…As cellular transition metals do not exist as 'free' ions, but rather coordinated to cellular components, deleterious species, such as authentic secondary *OH (reaction 5a) and hypervalent metal (reaction 5b), are formed 'site specifically' in a close proximity to critical biological targets. This site specific formation is responsible for damage potentiation and accounts for the frequent failures of radical scavengers to provide protection.42 43 The reactions of nitroxides with transition metals have been previously studied and reported.8 19 These results suggested that nitroxides can compete with H202 and oxidise the reduced metal ions19:…”

Purine metabolism and the microaerophily of Helicobacter pylori

“…Lactobionate inhibits the formation of ț OH by complexation of ferrous ions (Miller et al 1990); at concentrations below 0.5 mM it decreases by less than 20% the production of hydroxyl radicals; at 12 mM concentration the decrease is 75% (Chalroux et al 1995). Tempol is a superoxide radical scavenger (Samuni et al 1991); at concentrations below 0.5 mM it reduces by less than 10% the production of ț OH by the Fenton reaction; at 12 mM concentration the decrease is 95% (Chalroux et al 1995). The relative ability to limit radical generation at low inhibitor concentrations is: allopurinol > lactobionate ~ adenine > tempol.…”