Effects of cinnamic acid derivatives on indole acetic acid oxidation by peroxidase

The influence of cinnamic acid derivatives on the H₂O₂-independent oxidation of indole acetic acid by horseradish peroxidase was examined. Cinnamic acid derivatives show a sharp increase from a slight stimulation of the oxidative reaction to a complete inhibition in a very narrow concentration range. This threshold effect occurs not only for the diphenols caffeic and dihydrocaffeic acids but also for the monophenols ferulic and sinapic acids. The latter phenols are not demethylated to caffeic acid by horseradish peroxidase during the reaction. They are both inhibitors on their own account. We, therefore, conclude that the inhibition is not only due to the radical scavenging properties of the inhibitors; direct interactions between the inhibitors and peroxidase also play a role in the unusual behaviour of the inhibitors.