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Metabolism and Enzymology

Isoenzymes of Superoxide Dismutase in Nodules of Phaseolus vulgaris L., Pisum sativum L., and Vigna unguiculata (L.) Walp. ¹

Unidad de Biología Vegetal, IRNA, CSIC, Apdo. 257, 37080 Salamanca, Spain, Unidad de Bioquímica Vegetal, Estación Experimental del Zaidín, CSIC, Apdo. 419, 18080 Granada, Spain

The activity and isozymic composition of superoxide dismutase (SOD; EC 1.15.1.1) were determined in nodules of Phaseolus vulgaris L., Pisum sativum L., and Vigna unguiculata (L.) Walp. formed by Rhizobium phaseoll 3622, R. Ieguminosarum 3855, and Bradyrhizobium sp. BR7301, respectively. A Mn-SOD was present in Rhizobium and two in Bradyrhizobium and bacteroids. Nodule mitochondria from all three legume species had a single Mn-SOD with similar relative mobility, whereas the cytosol contained several CuZn-SODs: two in Phaseolus and Pisum, and four in Vigna. In the cytoplasm of V. unguiculata nodules, a Fe-containing SOD was also present, with an electrophoretic mobility between those of CuZn- and Mn-SODs, and an estimated molecular weight of 57,000. Total SOD activity of the soluble fraction of host cells, expressed on a nodule fresh weight basis, exceeded markedly that of bacteroids. Likewise, specific SOD activities of free-living bacteria were superior or equal to those of their symbiotic forms. Soluble extracts of bacteria and bacteroids did not show peroxidase activity (EC 1.11.1.7), but the nodule cell cytoplasm contained diverse peroxidase isozymes which were readily distinguishable from leghemoglobin components by electrophoresis. Data indicated that peroxidases and leghemoglobins did not significantly interfere with SOD localization on gels. Treatment with chloroform-ethanol scarcely affected the isozymic pattern of SODs and peroxidases, and had limited success in the removal of leghemoglobin.

¹ Supported by grants PBT/15 from the CAICYT (Spain) (M. B., F. J. P.) and PB040401 from the DIGICYT (Spain) (L. M. S., L. A. R.).

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