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The bioelectrocatalytic function of two plant peroxidases, horseradish peroxidase (HRP) and a newly purified royal palm tree-leaf peroxidase (RPTP), was studied on graphite electrodes in aqueous buffer solutions, over the pH range 7.0 -3.0, and in aqueous buffer solutions, at pH 6.0, containing different amounts of a polar organic cosolvent, i.e., ethanol and acetonitrile. The kinetics of direct and of mediated (using catechol as mediator) reduction of H 2 O 2 at the HRP-and RPTP-modified graphite electrodes were analyzed amperometrically at À 50 mV(vs. Ag j AgCl j 0.1 M KCl) in a flow-through wall-jet electrochemical system. Values of the apparent rate constant of the heterogeneous electron transfer between the enzyme and graphite, k s , the rate constant for enzymatic reduction of H 2 O 2 , k 1 , and the rate constant of mediated reduction of H 2 O 2 , k 3, were determined using the modified Koutecky-Levich approach. Analysis of the variation of the rate constants and the response of the peroxidase-modified electrodes to H 2 O 2 with pH and content of the organic cosolvent demonstrated that maximal peroxidase bioelectrocatalytic activity occurred at pH 5.0 -6.0 and at concentrations of ethanol of 10 -20% v/v. At a lower pH, higher concentrations of ethanol, and in aqueous solutions of acetonitrile, the bioelectrocatalytic activity of the peroxidase-modified electrodes drastically decreased. However, contrary to the data for homogeneous catalysis, both peroxidases were bioelectrocatalytically active even in 95% organic co-solvents, thus demonstrating a stabilizing effect of the enzyme immobilization on the bioelectrocatalytic performance of the peroxidases. RPTP immobilized on graphite demonstrated lower overall activity but a higher pH-and organic cosolvent-stability than HRP.

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