SCAVENGING OF OXYGEN RADICALS BY HEME PEROXIDASES

The reactions of two heme peroxidases, horseradish peroxidase and lact operoxidase and their compounds II (oxoferryl heme intermediates, Fe(I V) = O or ferric protein radical Fe(III)R(.)) and compounds III (reson ance hybrids [Fe(III)-O-2(-.) <----> Fe(II)-O-2] with superoxide radic al anion generated enzymatically or radiolytically, and with hydroxyl radicals generated radiolytically, were investigated. It is suggested that only the protein radical form of compound II of lactoperoxidase r eacts with superoxide, whereas compound II of horseradich peroxidase, which exists only in oxoferryl form, is unreactive towards superoxide. Compound III of the investigated peroxidases does not react with supe roxide. The lactoperoxidase activity loss induced by hydroxyl radicals is closely related to the loss of the ability to form compound I (oxo ferryl porphyrin pi-cation radical, Fe(IV) = O(Por(+.)) or oxoferryl p rotein radical Fe(IV) = O(R(.)))). On the other hand, the modification of horseradish peroxidase induced by hydroxyl radicals has been repor ted to cause also restrictions in substrate binding (Gebicka, L. & Geb icki, J.L., 1996, Biochimie 78, 62-65). Nevertheless, it has been foun d that only a small fraction of hydroxyl radicals generated homogeneou sly does inactivate the enzymes.