Inhibitory effect of superoxide-generating quinones on superoxide dismutase.

The one-electron reduction of quinones to semiquinones and the subsequent autooxldatlon of the semlqulnone back to the quinone can generate large quantities of superoxide (0~) (1,2). This reductlon-oxldation process, known as redox cycling, is anion radical catalyzed by a variety of cellular flavoproteins and is thought to be responsible for many of the therapeutic and toxic effects of quinonoid compounds. Since quinones have been shown previously to inhibit a variety of enzymatic processes (3-5), we have investigated the effect of various quihones on the activity of superoxide dlsmutase (SOD), the key enzyme involved in protecting cells against the toxic effects of 0~. Here we report that certain qulnones inhibit SOD activity in vitro and may, therefore, potentiate their own toxicity by this effect. Quinones were preincubated with bovine blood SOD (50 units/ml) in 196 ~i of 50 mM phosphate buffer, pH 7.8, at 25 °. The quinones were added in 4 ~i dimethyl sulfoxide to give a maximal final concentration of I00 ~M. Control SOD was preincubated with 4 ~i dlmethyl sulfoxide alone. After a 30-min prelncubation, the activity of the SOD was determined by dual wavelength spectrophotometry (550-557 nm), using a hypoxanthine/xanthlne oxldase O~ generating system with cytochrome ~ (40 ~M) as the indicating scavenger (6). The final c~ncentratlon of qulnone in the reaction euvette never exceeded 0.5 ~M. None of the quinones tested altered the rate of O~-dependent cytochrome c reduction at this concentration. The percent inhibition of SOD was determined by subtracting the actiVity of SOD incubated with qulnone from the control SOD activity anddlviding this number by the control activity. 1,2and 1,4Naphthoquinone, menadione, and 9,10-phenanthrene qulnone all inhibited SOD activity to some extent (Table i). These four qulnones are potent redox cyclers with