The antioxidants of legume nodule mitochondria

The mitochondria of legume root nodules are critical to sustain the energy- intensive process of nitrogen fixation. They also generate reactive oxygen species at high rates and thus require the protection of antioxidant enzyme s and metabolites. We show here that highly purified mitochondria from bean nodules (Phaseolus vulgaris L. cv. Contender x Rhizobium leguminosarum bv. phaseoli strain 3622) contain ascorbate peroxidase primarily in the inner membrane (with lesser amounts detected occasionally in the matrix), guaiaco l peroxidases in the outer membrane and matrix, and manganese superoxide di smutase (MnSOD) and an ascorbate-regenerating system in the matrix. This re generating system relies on homoglutathione (instead of glutathione) and py ridine nucleotides as electron donors and involves the enzymes monodehydroa scorbate reductase, dehydroascorbate reductase, and homoglutathione reducta se. Homoglutathione is synthesized in the cytosol and taken up by the mitoc hondria and bacteroids. Although bacteroids synthesize glutathione, it is n ot exported to the plant in significant amounts. We propose a model for the detoxification of peroxides in nodule mitochondria in which membrane-bound ascorbate peroxidase scavenges the peroxide formed by the electron transpo rt chain using ascorbate provided by L-galactono-1,4-lactone dehydrogenase in the inner membrane. The resulting monodehydroascorbate and dehydroascorb ate can be recycled in the matrix or cytosol. In the matrix, the peroxides formed by oxidative reactions and by MnSOD may be scavenged by specific iso zymes of guaiacol peroxidase, ascorbate peroxidase, and catalase.