Lignin synthesis: The generation of hydrogen peroxide and superoxide by horseradish peroxidase and its stimulation by manganese (II) and phenols

The enzyme horseradish peroxidase (EC 1.11.1.7) catalyses oxidation of NADH. NADH oxidation is prevented by addition of the enzyme superoxide dismutase (EC 1.15.1.1) to the reaction mixture before adding peroxidase but addition of dismutase after peroxidase has little inhibitory effect. Catalase (EC 1.11.1.6) inhibits peroxidase-catalysed NADH oxidation when added at any time during the reaction. Apparently the peroxidase uses hydrogen peroxide (H₂O₂) generated by non-enzymic breakdown of NADH to catalyse oxidation of NADH to a free-radical, NAD., which reduces oxygen to the superoxide free-radical ion, O₂ ^.-. Some of the O₂ ^.- reacts with peroxidase to give peroxidase compound III, which is catalytically inactive in NADH oxidation. The remaining O₂ ^.- undergoes dismutation to O₂ and H₂O₂. O₂ ^.- does not react with NADH at significant rates. Mn²⁺ or lactate dehydrogenase stimulate NADH oxidation by peroxidase because they mediate a reaction between O₂ ^.- and NADH. 2,4-Dichlorophenol, p-cresol and 4-hydroxycinnamic acid stimulate NADH oxidation by peroxidase, probably by breaking down compound III and so increasing the amount of active peroxidase in the reaction mixture. Oxidation in the presence of these phenols is greatly increased by adding H₂O₂. The rate of NADH oxidation by peroxidase is greatest in the presence of both Mn²⁺ and those phenols which interact with compound III. Both O₂ ^.- and H₂O₂ are involved in this oxidation, which plays an important role in lignin synthesis.