Superoxide dismutase enhances chain-breaking antioxidant capability of hydroquinones

2-tert-butyl- (1), 2,6-dimethyl- (2), 2,5-dimethyl- (3), trimethyl- (4), and 2,3-dimethoxy-5-methyl- (5) substituted p-hydroquinones (QH(2)) were tested as a chain-breaking antioxidant during the oxidation of methyl linoleate (ML) in dodecyl sulfate micellar solution, pH 7.40, at 37 degreesC. In the absence of superoxide dismutase (SOD), all the studied QH(2) displayed very moderate if any antioxidant capability. When 5-25 U/ml SOD was added, QH(2) showed a pronounced ability to inhibit NE oxidation. The stoichiometric factor of inhibition was found to be about one for all the tested QH(2) in the presence of SOD. The reactivities of QH(2) to the NIL peroxy radical increase in the order QH(2)5<QH(2) 3 < QH(2) 1 approximate to QH(2) 2 < QH(2) 4; reactivity of QH(2) 4 exceeds that reported for the majority of phenolic antioxidants. The features of QH(2) as an antioxidant in aqueous environments is likely associated with the reactivity of semiquinone (O radical anion) formed due to attack of the peroxy radical to QH(2). O radical anion reacts readily with molecular oxygen with formation of superoxide (O(2)radical anion); in turn, O(2)radical anion attacks both to QH(2) and NIL (likely, as HO(2)radical anion) that results in fast depleting QH(2) and chain propagation, respectively. The addition of SOD results in purging a reaction mixture from O(2)radical anion and, as a corollary, in depressing undesirable reactions with the participation of O(2)radical anion. Under these conditions, QH(2) displays the theoretically highest inhibitory activity which is determined solely by the reactivity of QH(2) to the peroxy radical.