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Autoxidation of naphthohydroquinones: Effects of pH, naphthoquinones and superoxide dismutase

The rates of autoxidation of a number of pure naphthohydroquinones have been determined, and the effects of pH, superoxide dismutase (SOD) and of the parent naphthoquinone on the oxidation rates have been investigated. Most compounds were slowly oxidised in acid solution with the rates increasing with increasing pH, although 2-hydroxy-, 2-hydroxy-3-methyl- and 2-amino-1,4-naphthohydroquinone were rapidly oxidised at pH 5 and the rates of oxidation of these substances were comparatively unresponsive to changes in pH. At pH 7.4, autoxidation rates decreased in the order 2,3-dichloro-1,4-naphthohydroquinone > 5-hydroxy > 2-bromo > 2-hydroxy-3-methyl > 2-amino > 2-hydroxy > 2-methoxy > 2,3-dimethoxy > 2,3-dimethyl > 2-methyl > unsubstituted hydroquinone. The autoxidation rates of the alkyl, alkoxy, hydroxy and amino derivatives were decreased in the presence of SOD, but this enzyme had no effect on the rate of autoxidation of the 2,3-dichloro and 2-bromo derivatives while that of the 5-hydroxy derivative was increased. The rates of autoxidation of all compounds except the halogen derivatives and 5-hydroxy-1,4-naphthohydroquinone were increased by addition of the parent naphthoquinone, and quinone addition partially or completely overcame the inhibitory effect of SOD. There is evidence that the reduction of quinones to hydroquinones in vivo may lead either to detoxification or to activation. This may be due to differences in the rate or mechanism of autoxidation of the hydroquinones that are formed, and the data gained in this study will provide a framework for testing this possibility.