Involvement of malate, monophenols, and the superoxide radical in hydrogen peroxide formation by isolated cell walls from horseradish (Armoracia lapathifolia Gilib.)

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Abstract
Peroxidase associated with isolated horseradish cell walls catalyzes the formation of H2O2 in the presence of NADH. The reaction is stimulated by various monophenols, especially of coniferyl alcohol. NADH can be provided by a bound malate dehydrogenase. This system is capable of polymerizing coniferyl alcohol yielding an insoluble dehydrogenation polymer. NADH was found to be oxidized by two different mechanisms, one involving Mn2+, monophenol, and the superoxide radical O2 ·- in a reaction that is not affected by superoxide dismutase, and another one depending on the presence of free O2 ·- and probably of an enzyme-NADH complex. A scheme of these reaction chains, which are thought to be involved in the lignification process, is presented.