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

The kinetics of the autoxidation of sixteen hydroquinones (QH(2)) (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 degrees C both with and without added superoxide dismutase (SOD). QH(2) oxidation occurs typically with a self-acceleration. A maximum rate of oxidation, R-MAX, was found to be the most indicative parameter characterizing QH(2) oxidizability. A kinetic scheme of QH(2) autoxidation was developed; computer simulations carried out on the basis of this scheme reproduce the main kinetic features of the studied process. QH(2) autoxidation is suggested to be a free-radical chain process with semiquinone (Q(.-)) and superoxide (O-2(.-)) as chain-carrying species. The oxidation is initiated by reaction (1) Q+QH(2)--> 2Q(.-)+2H(+). The addition of SOD results in two main effects: shifting the equilibrium (2) Q(.-)+O(2)reversible arrow Q+O-2(.-) (K-2) to the right and suppressing reaction (3) QH(2)+O-2(.-)--> Q(.-)+H2O2. The net effect of SOD depends basically on K-2. When K-2< 0.1, the addition of SOD results in stimulation of the oxidation; when K-2> 0.1, the more SOD inhibits the oxidation, the higher K-2. The concentration of SOD causing the 50%-effect on R-MAX ([SOD](50)), both inhibitory and stimulatory, decreases dramatically when K-2 increases. At [SOD]>>[SOD](50) the rate of QH(2) autoxidation is definitively determined by the rate of reaction (1). For the majority of QH(2), [SOD](50) is significantly less than the physiological values of [SOD] and thus QH(2) autoxidation in biological environment is expected to occur in the above kinetically simple mode.