Reactions of horseradish peroxidase on a platinum cathode

Abstract

Thin layer spectroelectrochemistry was used to observe the electrochemical reactions of horseradish peroxidase on a platinum cathode. When there is no oxygen dissolved in the solution, the reduction of ferriperoxidase into ferroperoxidase is very slow, but it can be accelerated by using flavin adenine dinucleotide (FAD) as redox mediator. The electrochemical re-oxidation of ferroperoxidase is much more rapid, even in the absence of mediator. Constant potential electrolysis makes it possible to prepare ferri- and ferroperoxidase mixtures and to determine the apparent standard potentials of the system at different pHs. The measurements were made for peroxidase and for two isoenzymes. With dissolved oxygen and depending on the potential sweep range, linear sweep voltammetry resulted in the appearance of compound III alone or mixed with compound I or II, depending on the pH of the solution. At more negative potentials, all the compounds were de-oxygenated electrochemically to return to the native peroxidase. In the presence of peroxidase, the electrochemical reduction of oxygen produced, depending on the potential, the superoxide anion or hydrogen peroxide that were responsible for the formation of compound III and of compounds I and II, respectively.