Iodide as the mediator for the reductive reactions of peroxidases

Lignin peroxidase H2 (LiPH2) from the white rot fungus Phanerochaete chrysosporium catalyzed the reduction of cytochrome c, nitro blue tetrazolium, ferric iron, molecular oxygen, and triiodide in a reaction mixture containing LiPH2, H2O2, EDTA, and iodide. Activity followed first order kinetics with respect to EDTA concentration. The reductive activity observed with LiPH2 using iodide as the mediator was comparable to that obtained using a variety of other free radical mediators such as veratryl alcohol, 1,4-dimethoxybenzene, and 1,2,3- and 1,2,4-trimethoxybenzene. EDTA-derived radicals were detected by ESR spin trapping upon incubation of LiPH2 with H2O2, iodide, and EDTA. Reduction activity was also observed using other peroxidases such as lactoperoxidase, horseradish peroxidase, and myeloperoxidase. For the reduction activity of LiPH2, it is proposed that the oxidation of EDTA is mediated by the iodide radical, and the reduction of various electron acceptors is mediated by EDTA radicals. The inhibition of reduction activity at higher concentrations of iodide might be due to the combination of iodide radicals to form I2 which forms a stable triiodide complex by reacting with excess iodide.