Inertness of horseradish peroxidase to 2,4‐dichlorophenoxyacetic acid

Abstract The possibility of mutual effects of 2,4‐D and horseradish (Armoracia lapathifolia L.) peroxidase on each other has been explored by four procedures. (i) Compounds I, II, and III of horseradish peroxidase (HRP) and H2O2 were exposed to 2,4‐D. (ii) Extracts from batchwise operations of HRP + H2O2 and 2,4‐D were analyzed for oxidation products by means of thin layer chromatography. (iii) The velocity of the IAA oxidase reaction with HRP as catalyst, and (iv) Km and Vs of the overall peroxidation of guaiacol by HRP + H2O2, were determined in the absence and presence of 2,4‐D. The results failed to show any effect of 2,4‐D; only at very high concentrations did 2,4‐D slightly inhibit the oxidation of IAA by one isoperoxidase. It is concluded that 2,4‐D does not promote growth in plants by hampering a peroxidase‐catalyzed IAA oxidation. It seems probable that 2,4‐D perturbs the isoperoxidase pattern by acting at some step prior to the release of the enzyme from its site of synthesis.