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. Mn(2+) 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 Mn(2+) 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.