The oxidation of manganese by plant extracts in the presence of hydrogen peroxide

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Abstract
A system which oxidizes Mn in the presence of H2O2 was demonstrated in horseradish root extracts. This system also exists in other root extracts. Under suitable conditions (i.e. in pyrophosphate or citrate at pH 7) the oxidation product accumulates as a colored manganic complex. Using horseradish root extract, MnO2 was isolated by the dismutation of manganipyrophosphate at weakly alkaline reaction. The oxidation product decomposes hydrazine and can be estimated manometrically by means of this reaction. An increase in the rate of decomposition of H2O2 was observed on adding small amts. of manganous sulphate to horseradish extracts. This effect was attributed to Mn oxidation. The Mn-oxidizing system in horseradish extract consists of a thermolabile and a thermostable factor together with H2O2. Partially purified peroxidase prepns., in the presence of certain peroxidase substrates and H2O2, oxidize Mn++. It is suggested that the thermolabile and thermostable factors in the extract are peroxidase and peroxidase substrate respectively. The hypothesis is advanced that the Mn reduces the oxidized peroxidase substrate and thereby is itself oxidized. This involves the substrate in a cycle of oxidation and reduction. At low H2O2 concns., the Mn oxidation product may react with plant metabolites other than H2O2; this would involve the Mn in an oxidation-reduction cycle, which may explain its effect on plant respiration.