The intermediate free radicals generated from phenols, naphthols and benzoate, in the peroxidase- and oxidase-reactions of horse radish peroxidase and in the peroxidase-reaction of methemoglobin, were studied by electron spin resonance spectroscopy. The difference between the peroxidase- and oxidase-reactions of HRP are demonstrated, i.e., the ferro horse radish peroxidase-O2 system attacks both phenols and benzoate yielding unidentified radicals, which may be hydroxy-cyclohexadienyl radicals, while the horse radish peroxidase-H2O2 system reacts only with phenols and naphthols producing the phenoxy-and naphthoxy-radicals. Phenoxy-radical formation from phenols, in the reactions horse radish peroxidase-H2O2 and methemoglobin-H2O2, occurs independently of the molecular sizes of phenols but dependently on their redox-potentials. On the basis of kinetic studies on methemoglobin-H2O2 system, the existence of a reactive intermediate complex between methemoglobin and H2O2 is proposed, which may be similar to compound-I or -II of horse radish peroxidase and which further degenerates to MetHb radical. The oxidation of phenols and naphthols takes place outside of the hemepocket of methemoglobin.
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