Chloroperoxidase-Catalyzed Oxidation of Aminopyrine

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Although in the absence of halide ion chloroperoxidase did not catalyze the ethylhydroperoxide (EHP)-supported oxidation of aminopyrine, in the presence of Br^- or Cl^-, chloroperoxidase did catalyze the oxidation of aminopyrine, generating the aminopyrine cation radical (AP⁺). The initial rate of AP⁺ formation was determined by monitoring the absorbance at 565 nm. The pH optimum of the reaction was centered around 5.0. The rate of AP⁺ formation showed typical Michaelis-Menten saturation kinetics with respect to EHP, aminopyrine and Br^-. The rate of formation of bromine in the chloroperoxidase-EHP-Br^- system was also determined by measuring the changein absorbance at 267 nm. In the system containing 1 mM EHP and 0.2 M KBr at pH 5.0, the rate was 1.8 nmol of bromine/s/μg of chloroperoxidase, which was slower than that of AP⁺ formation under the same conditions. The present results suggest that tha formation of AP⁺ is initiated by the halogenation of the N, N-dimethylamino group followed by the homolysis of the haloammonium cation, and that tha most likely halogenating reagent is an enzyme-bound halogenating intermediate.