Heme-protein covalent bonds in peroxidases and resistance to heme modification during halide oxidation

Plant peroxidases, as typified by horseradish peroxidase (HRP), primarily catalyze the one-electron oxidation of phenols and other low oxidation potential substrates. In contrast, the mammalian homologues such as lactoperoxidase (LPO) and myeloperoxidase primarily oxidize halides and pseudohalides to the corresponding hypohalides (e.g., Br− to HOBr, Cl− to HOCl). A further feature that distinguishes the mammalian from the plant and fungal enzymes is the presence of two or more covalent bonds between the heme and the protein only in the mammalian enzymes. The functional roles of these covalent links in mammalian peroxidases remain uncertain. We have previously reported that HRP can oxidize chloride and bromide ions, but during oxidation of these ions undergoes autocatalytic modification of its heme vinyl groups that virtually inactivates the enzyme. We report here that autocatalytic heme modification during halide oxidation is not unique to HRP but is a general feature of the oxidation of halide ions by fungal and plant peroxidases, as illustrated by studies with Arthromyces ramosus and soybean peroxidases. In contrast, LPO, a prototypical mammalian peroxidase, is protected from heme modification and its heme remains intact during the oxidation of halide ions. These results support the hypothesis that the covalent heme-protein links in the mammalian peroxidases protect the heme from modification during the oxidation of halide ions.