Redox Cycling of Polycyclic Aromatic Hydrocarbono-Quinones: Reversal of Superoxide Dismutase Inhibition by Ascorbate

When redox cycling of four polycyclic aromatic hydrocarbono-quinones is catalyzed by the 17β-hydroxysteroid dehydrogenase, autooxidation of the hydroquinone is a free radical chain reaction in which superoxide anion is the propagating species. Superoxide dismutase inhibits the redox cycling of these quinones, and ascorbate reverses this inhibition. Studies of the mechanism, using 9,10-phenanthrenequinone, show that ascorbate competes with superoxide dismutase for the superoxide anion; the ascorbyl radical formed then oxidizes the hydroquinone. In this mechanism, ascorbyl radical participates in chain propagation. The reversal of superoxide dismutase inhibition by ascorbate is observed when other two-electron reductases catalyze the cycling, and it occurs in the absence of metal ions. Although ascorbate is generally thought to be an antioxidant, it behaves as a prooxidant in the experiments reported here.