Superoxide dismutases in nodules of leguminous plants

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Abstract
Isoenzymic composition of superoxide dismutases (SODs; EC 1.15.1.1) of legume nodules has been examined by using polyacrylamide gel electrophoresis. The study reveals that Cu plus Zn-SODs and Mn-SODs are widespread in the plant and bacteroidal fractions of nodules, respectively. The number of CuZn-isoenzymes, however, depends on the legume species: three of four in Lupinus, three in Phaseolus, two in Vignus, and one in Glycine, Trifolium, Pisum, and Medicago. The nodule plant fraction also exhibits Mn-SOD activity, which is, at least in Medicago, of plant origin. Two Mn.sbd.isoenzymes are present in most bacteroids as well as in all slow-growing rhizobia, but just one was observed in fast-growing rhizobia. Fe.sbd.SOD has not been found in free-living or symbiotic rhizobia. A faint CuZn.sbd.SOD activity was detected in the bacteroid fraction of Phaseolus, Trifolium, Lupinus, and Vigna. The high content and complex pattern of SOD isoenzymes in the host cell and bacteroids (despite their relatively anaerobic environment) indicate a substantial production of O2.- in nodules in vivo, and the necessity for nitrogenase and leghemoglobin protection.