Oxidation of vacuolar and apoplastic phenolic substrates by peroxidase: Physiological significance of the oxidation reactions

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Phenolic components and peroxidases are localized in vacuoles. Vacuolar peroxidase can oxidize phenolics when H2O2 is formed in vacuoles or tonoplasts, or when H2O2 formed outside of vacuoles is diffused into the organelles. In a mixture of phenolics containing a good and a poor substrate for peroxidase, a radical transfer reaction is possible from the radicals of the good substrate to the poor substrate, resulting in the enhancement of oxidation of the poor substrate. Phenoxyl radicals formed by peroxidase-dependent reactions are reduced by ascorbate in vacuoles. So, as long as ascorbate is present in vacuoles, the accumulation of oxidation products of phenolics is not significant. This suggests that ascorbate/phenolics/peroxidase systems in the vacuoles can scavenge H2O2. During aging, some phenolics are accumulated in vacuoles and the apoplast, and the accumulated phenolics are oxidized to brown components by peroxidase-dependent reactions. The brown components can produced O2- and H2O2 by autooxidation. The significance and the mechanisms of browning are discussed in tobacco leaves and onion scales.

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