Detection of Monodehydroascorbic Acid Radical in Sulfite-Treated Leaves and Mechanism of its Formation

The aim of the present study is to detect the monodehydroascorbic acid (MDA) radical in broad bean (Vicia faba L.) leaves which were treated by vacuum-infiltration in Na2SO3 solution and subsequent centrifugation (sulfitetreated leaves). When sulfite-treated leaves were illuminated with white light, the electron spin resonance (ESR) signal of MDA radical was observed. The level of the MDA radical depended on the concentration of sulfite that was used for vacuum-infiltration and on the light intensity of illumination. The formation of the MDA radical in sulfitetreated leaves was inhibited by DCMU or by replacement of air with N2. Glycolaldehyde also inhibited the formation of MDA radical in sulfite-treated leaves. Catalase activity was decreased by the sulfite treatment without affecting significantly the activities of ascorbate peroxidase (AA-POX) and of peroxidase which preferentially oxidizes phenolics (PhOH-POX). From these results, we conclude that the formation of the MDA radical in sulfite-treated leaves is catalyzed by peroxidases using the H2O2 which is generated by photorespiration and the Mehler reaction.