Kinetics of oxidation of hydroquinones by molecular oxygen. Effect of superoxide dismutase

The kinetics of the autoxidation of sixteen hydroquinones (QH2) (substituted 1,4-hydroquinones and 1,4-dihydroxynaphthalenes as well as 9,10-dihydroxyphenanthrene) were studied using the Clark electrode technique in aqueous solution, pH 7.40, at 37 °C both with and without added superoxide dismutase (SOD). QH2 oxidation occurs typically with a self-acceleration. A maximum rate of oxidation, RMAX, was found to be the most indicative parameter characterizing QH2 oxidizability. A kinetic scheme of QH2 autoxidation was developed; computer simulations carried out on the basis of this scheme reproduce the main kinetic features of the studied process. QH2 autoxidation is suggested to be a free-radical chain process with semiquinone (Q˙−) and superoxide (O2˙−) as chain-carrying species. The oxidation is initiated by reaction (1) Q + QH2→2Q˙− + 2H+. The addition of SOD results in two main effects: shifting the equilibrium (2) Q˙− + O2⇆Q + O2˙− (K2) to the right and suppressing reaction (3) QH2 + O2˙−→Q˙− + H2O2. The net effect of SOD depends basically on K2. When K2 < 0.1, the addition of SOD results in stimulation of the oxidation; when K2 > 0.1, the more SOD inhibits the oxidation, the higher K2. The concentration of SOD causing the 50%-effect on RMAX ([SOD]50), both inhibitory and stimulatory, decreases dramatically when K2 increases. At [SOD] ≫ [SOD]50 the rate of QH2 autoxidation is definitively determined by the rate of reaction (1). For the majority of QH2, [SOD]50 is significantly less than the physiological values of [SOD] and thus QH2 autoxidation in biological environment is expected to occur in the above kinetically simple mode.