Modification of horseradish peroxidase induced by hydroxyl radicals. The influence of oxygen

Reactions of hydroxyl radicals with horseradish peroxidase (HRP) have been studied by means of pulse radiolysis technique in the absence and presence of oxygen. Hydroxyl radicals, produced in excess towards enzyme, react exclusively with the protein part of HRP with the rate constant k = 1.1 x 10(11) M-1 s-1. Activity loss induced by OH. is connected with such an enzyme modification that causes both the interference with substrate binding and partial blocking of the channel used by peroxide. It is shown that in the presence of oxygen the loss of activity is ca 10% higher, mainly due to restrictions in the formation of compound I, ie ferryl [Fe(IV) = O] pi-radical cation.