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

Abstract

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, R_MAX, 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 R_MAX ([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.