HYDROGEN-PEROXIDE SCAVENGING SYSTEMS IN VACUOLES OF MESOPHYLL-CELLS OF VICIA-FABA

Vacuoles isolated from mesophyll protoplasts of Vicia faba contained ascorbate in addition to 3,4-dihydroxyphenylalanine (DOPA) and peroxidase. When a low concentration of H2O2(0.25 mM) was added to the vacuole fractions, a slow decrease in the level of ascorbate was observed. When 2.5 mH H2O2 was added to the vacuole fractions, a rapid decrease in the levels of ascorbate and formation of dopachrome were observed. The oxidation of ascorbate and the formation of dopachrome were inhibited by NaN3 (10 mM), KCN (1 mM) and stimulated by tropolone (5 mM), suggesting the participation of peroxidase but not phenol oxidase in the reactions. Vacuolar peroxidase isolated from leaves of V. faba oxidized DOPA and ascorbate in the presence of H2O2. The oxidation of DOPA was nearly completely inhibited by ascorbate but the oxidation of ascorbate was stimulated by DOPA, suggesting the reduction of an oxidized intermediate of DOPA to DOPA by ascorbate. In fact, ascorbate reduced dopaquinone, a two electron oxidized DOPA.