Oxidation of Halides by Peroxidases and Their Subsequent Reductions

The iodide oxidase activity and iodide-dependent pseudocatalatic activity of lignin peroxidase H2, an extracellular enzyme of the white rot fungus Phanerochaete chrysosporium, was inhibited by EDTA. The inhibition of iodide oxidase activity by EDTA was reversed at higher concentrations of iodide. Similar results were observed with a number of peroxidases. On further investigation, it was found that EDTA was decarboxylated in a reaction mixture containing a peroxidase, iodide, H2O2, and EDTA. EDTA was also decarboxylated by hypoiodite, a possible intermediate during oxidation of iodide by peroxidases. Iodide-dependent pseudocatalatic activity was increased with an increase in the concentration of H2O2 and inhibited at higher concentrations of iodide. EDTA was also oxidized by horseradish peroxidase, lactoperoxidase, and myeloperoxidase using iodide or bromide as a mediator. However, only myeloperoxidase was able to decarboxylate EDTA using chloride as a mediator. It is proposed that halide is oxidized to hypohalite by peroxidases. The hypohalite is then reduced by EDTA, H2O2, or halide. Reduction is associated with the decarboxylation of EDTA, oxidation of H2O2 to molecular oxygen, or oxidation of halide.