Effect of superoxide dismutase on the autoxidation of substituted hydro- and semi-naphthoquinones

Abstract

The effect of superoxide dismutase on the autoxidation of hydro- and semi-1,4-naphthoquinones with different substitution pattern and covering a one-electron reduction potential range from -95 to -415 mV was examined. The naphthoquinone derivatives were reduced via one or two electrons by purified NADPH-cytochrome P-450 reductase or DT-diaphorase, respectively. Superoxide dismutase did not alter or slightly enhance the initial rates of enzymic reduction, whereas it affected in a different manner the following autoxidation of the semi- and hydroquinones formed. Autoxidation was assessed as NADPH oxidation in excess to the amounts required to reduce the quinone present, H2O2 formation, and the redox state of the quinones. Superoxide dismutase enhanced 2--8-fold the autoxidation of 1,4-naphthosemiquinones, following the reduction of the oxidized counterpart by NADPH-cytochrome P-450 reductase, except for the glutathionyl-substituted naphthosemiquinones, whose autoxidation was not affected by superoxide dismutase. Superoxide dismutase exerted two distinct effects on the autoxidation of naphthohydroquinones formed during DT-diaphorase catalysis: on the one hand, it enhanced slightly the autoxidation of 1,4-naphthohydroquinones with a hydroxyl substituent in the benzene ring: 5-hydroxy-1,4-naphthoquinone and the corresponding derivatives with methyl- and/or glutathionyl substituents at C2 and C3, respectively. On the other hand, superoxide dismutase inhibited the autoxidation of naphthohydroquinones that were either unsubstituted or with glutathionyl-, methyl-, methoxyl-, hydroxyl substituents (the latter in the quinoid ring). The inhibition of hydroquinone autoxidation was reflected as a decrease of NADPH oxidation, suppression of H2O2 production, and accumulation of the reduced form of the quinone. The enhancement of autoxidation of 1,4-naphthosemiquinones by superoxide dismutase has been previously rationalized in terms of the rapid removal of O2-. by the enzyme from the equilibrium of the autoxidation reaction (Q2-. + O2----Q + O2-.), thus displacing it towards the right. The superoxide dismutase-dependent inhibition of H2O2 formation as well as NADPH oxidation during the autoxidation of naphthohydroquinones--except those with a hydroxyl substituent in the benzene ring--seems to apply to those organic substrates which can break down with simultaneous formation of a semiquinone and O2-.. Inhibition of hydroquinone autoxidation by superoxide dismutase can be interpreted in terms of suppression by the enzyme of O2-.- dependent chain reactions or a direct catalytic interaction with the enzyme that might involve reduction of the semiquinone at expense of O2(-.).(ABSTRACT TRUNCATED AT 400 WORDS)