Phenol-dependent H2O2 Breakdown by Soybean Root Plasma Membrane-bound Peroxidase is Regulated by Ascorbate and Thiols

In the presence of hydrogen peroxide, soybean root plasma membranes oxidize caffeic acid, ferulic acid and coniferyl alcohol, exhibiting a pattern very similar to that described for horseradish peroxidase (EC 1.11.1.7). Ascorbic acid, rysteine, and dithioerythritol, added during the course of the reaction, caused inhibition that, in particular for ascorbic acid and cysteine, had a transient effect, being followed by a recovery of activity, as observed for cell wall-bound guaiacol peroxidase. This transient effect is paralleled by ascorbate oxidation: the oxidation of phenolics could restart only when ascorbate was completely oxidized. A similar delay of phenolic oxidation was also observed in the presence of rysteine or dithioerythritol. The reduction of hydrogen peroxide to water by horseradish peroxidase or soybean root plasma membrane, in the presence of phenolics, was enhanced by the addition of ascorbic acid, rysteine or dithioerythritol. At pH 7.0, horseradish peroxidase was reduced to Compound III by thiols but not by ascorbic acid, while at pH 5.5 ascorbate, rysteine or dithioerythritol did not exert any effect. These results confirm that plasma membranes possess peroxidase(s) similar to the well-known horseradish peroxidase, probably localized on the apoplastic side of the membrane, which is regulated by ascorbic acid availability. This enzyme could have a dual role in the generation/breakdown of H2O2 depending on the physiological circumstances.