EFFECTS OF NITROXIDE STABLE RADICALS ON JUGLONE CYTOTOXICITY

Nitroxides stable radicals are unreactive toward most diamagnetic mole cules, but readily undergo one-electron redox reactions with paramagne tic species such as free radicals and transition metals, thus serving as cell-permeable antioxidants. The cytotoxicity of juglone (5-hydroxy -1, 4-naphthoquinone), like that of other naphthoquinones, requires bi oreduction to yield the semiquinone which in turn reduces oxygen to O- 2(radical-anion). Therefore, nitroxides are expected to mitigate cytot oxicity of quinone-based xenobiotics, such as naphthoquinones. In the present study, in vitro scission of isolated DNA was induced upon jugl one reduction by glutathione and Fe(II) ions, however, not by xanthine oxidase or cytochrome c reductase. The DNA scission was inhibited by nitroxides, catalase and chelating agents, though not by superoxide di smutase. Juglone was more toxic toward bacterial cells under hypoxia t han under air. Nitroxides less than or equal to 2 mM protected bacteri al cells from juglone-induced toxicity under both aerobic and hypoxic conditions. The cytoprotective effect of lipophilic nitroxide was grea ter than that of hydrophilic ones. Catalase and metal chelating agents decreased juglone-induced cell killing, whereas H2O2 increased it. Th e mechanisms underlying the nitroxides protective effect involve (a) t he reoxidation of reduced transition metal ions, (b) the selective rad ical-radical reaction with juglone semiquinone, and possibly (c) under aerobic condition catalytic removal of extra- and intracellular O-2(r adical-anion). The present results suggest also that the cell membrane rather than DNA is the main target of juglone toxicity. (C) 1994 Acad emic Press, Inc.