Superoxide dismutases in nodules of leguminous plants

7 Pags.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 or four in Lupinus, three in Phaseolus, two in Vigna, 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–isoenzymes are present in most bacteroids as well as in all slow-growing rhizobia, but just one was observed in fast-growing rhizobia. Fe–SOD has not been found in free-living or symbiotic rhizobia. A faint CuZn–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 cells 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.Peer reviewe

Flavin-mediated reduction of ferric leghemoglobin from soybean nodules

22 Pags. The definitive version, with Figs., is available at: http://link.springer.com/journal/425The reduction of ferric leghemoglobin (Lb3+) from soybean (Glycine max (L.) Merr.) nodules by riboflavin, FMN and FAD in the presence of NAD(P)H was studied in vitro. The system NAD(P)H + flavin reduced Lb3+ to oxyferrous (Lb2+ · O2) or deoxyferrous (Lb2+) leghemoglobin in aerobic or anaerobic conditions, respectively. In the absence of O2 the reaction was faster and more effective (i.e. less NAD(P)H oxidized per mole Lb3+ reduced) than in the presence of O2; this phenomenon was probably because O2 competes with Lb3+ for reductant, thus generating activated O2 species. The flavin-mediated reduction of Lb3+ did not entail production of superoxide or peroxide, indicating that NAD(P)H-reduced flavins were able to reduce Lb3+ directly. The NAD(P)H + flavin system also reduced the complexes Lb3+ · nicotinate and Lb3+ · acetate to Lb2+ · O2, Lb2+ or Lb2+ · nicotinate, depending on the concentrations of ligands and of O2. In the presence of 200 μM nitrite most Lb remained as Lb3+ in aerobic conditions but the nitrosyl complex (Lb2+ · NO) was generated in anaerobic conditions. The above-mentioned characteristics of the NAD(P)H + flavin system, coupled with its effectiveness in reducing Lb3+ at physiological levels of NAD(P)H and flavins in soybean nodules, indicate that this mechanism may be especially important for reducing Lb3+ in vivo.M. Becana thanks the Spanish Ministry of Education and Science/Fulbright Commission for financial support.Peer reviewe