Catalase-like activity of horseradish peroxidase: relationship to enzyme inactivation by H2O2

H2O2 is the usual oxidizing substrate of horseradish peroxidase C (HRP-C). In the absence in the reaction medium of a one-electron donor substrate, H2 O2 is able to act as both oxidizing and reducing substrate. However, under these conditions the enzyme also undergoes a progressive loss of activity. There are several pathways that maintain the activity of the enzyme by reco vering the ferric form, one of which is the decomposition of H2O2 to molecu lar oxygen in a similar way to the action of catalase. This production of o xygen has been kinetically characterized with a Clark-type electrode couple d to an oxygraph. HRP-C exhibits a weak catalase-like activity, the initial reaction rate of which is hyperbolically dependent on the H2O2 concentrati on, with values for K-2 (affinity of the first intermediate, compound I, fo r H2O2) and k(3) (apparent rate constant controlling catalase activity) of 4.0 +/- 0.6 mM and 1.78 +/- 0.12 s(-1) respectively. Oxygen production by H RP-C is favoured at pH values greater than approx. 6.5; under similar condi tions HRP C is also much less sensitive to inactivation during incubations with H2O2. We therefore suggest that this pathway is a major protective mec hanism of HRP-C against such inactivation.