Scavenging of Hydrogen Peroxide in the Endosperm of Ricinus communis by Ascorbate Peroxidase

The fat-storing endosperm of Ricinus communis L. was found to contain an ascorbate peroxidase (EC 1.11.1.11), which is nearly as active as catalase (EC 1.11.1.6) in degradation of hydrogen peroxide (H2O2) at its physiological concentrations. This ascorbate peroxidase probably functions together with monodehydroascorbate reductase (EC 1.6.5.4) or dehydroascorbate reductase (EC 1.8.5.1) and glutathione reductase (EC 1.6.4.2) to remove the H2O2 produced during the transformation of fat to carbohydrate in the glyoxysomes. The activities of these enzymes as well as the content of ascorbate and glutathione increase parallel to the activities of glyoxysomal marker enzymes during the course of germination. Inhibition of catalase by aminotriazole results in increases of the ascorbate peroxidase activity and of the glutathione content. All four enzymes are predominantly localized in the cytosol of the Ricinus endosperm with low activities found in the plastids and the mitochondria. The results suggest, that the ascorbate-dependent H2O2 scavenging pathway, which has been shown to be responsible for the reduction of photosynthetically derived H2O2 in the chloroplasts, operates also in the Ricinus endosperm.