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  • Title: Redox cycling of polycyclic aromatic hydrocarbon o-quinones: reversal of superoxide dismutase inhibition by ascorbate.
    Author: Jarabak R, Harvey RG, Jarabak J.
    Journal: Arch Biochem Biophys; 1997 Mar 01; 339(1):92-8. PubMed ID: 9056238.
    Abstract:
    When redox cycling of four polycyclic aromatic hydrocarbon o-quinones is catalyzed by the 17 beta-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.
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