Decay of the HO2 and O−2 Radicals Catalyzed by Superoxide Dismutase. A Pulse Radiolytic Investigation

Abstract

The pulse radiolysis technique has been employed in the investigation of the dismutation of superoxide radicals, O⁻₂ and HO₂, in the presence of superoxide dismutase in aqueous solutions. The decay of superoxide radicals in the presence of the enzyme was found to be first order in both enzyme and superoxide concentrations. An apparent second order reaction rate constant was found to be about 2 × 10⁹ M⁻¹ sec⁻¹, decreasing slightly as the pH is increased from 5 to 9.5. A mechanism which accounts for all our observations is proposed. It includes two steps: (1) formation of a product (EO⁻₂ or E⁻) from one enzyme (E) molecule and one O⁻₂ radical ion; (2) regeneration of E by a reaction of this product with an additional O⁻₂ ion radical. The reaction rate constants k = (1.4 ± 0.2) × 10⁹ and k = (1.9 ± 0.6) × 10⁹ M⁻¹ sec⁻¹ were measured at pH = 7 in an oxygenated 0.16 M sodium formate solution.