Membrane lipid peroxidation, nitrogen fixation and leghemoglobin content in soybean root nodules

Membrane lipids in soybean nodules may undergo oxidative degradation result ing in the loss of membrane structural integrity and physiological activiti es. One of the final products of lipid peroxidation is malondialdehyde (MDA ), which can react with thiobarbituric acid (TBA) in vitro to form a chromo genic adduct, a Schiff base product that can be measured spectrophotometric ally. MDA formation was quantified in the nodules as well as in the adjacen t root tissue. Lipid peroxidation was initially high in soybean nodules ind uced by Bradyrhizobium japonicum, but sharply declined following an increas e in both leghemoglobin content and nitrogen fixation rate. Lipid peroxidat ion was 2 to 4 times higher in the nodules than in their corresponding adjo ining root tissue. Malondialdehyde levels in ineffective nodules were 1.5 t imes higher than those in effective nodules. MDA formation was also shown t o occur in the 'leghemoglobin-free' cytosolic fraction, the 'leghemoglobin' traction, and the nodule tissue pellet. Antioxidants, such as reduced asco rbic acid, glutathione, and 8-hydroxyquinoline, caused a partial suppressio n of lipid peroxidation, whereas ferrous sulfate, hydrogen peroxide, iron E DTA, disodium-EDTA, and beta -carotene induced MDA formation. In contrast, quenchers of oxy gen free radicals such as HEPES, MES, MOPS, PIPES, phenyla lanine, Tiron, thiourea, sodium azide, and sodium cyanide (uncouplers of ox idative phosphorylation) caused somewhere between a 12 to 70% reduction in MDA production. TEA-reactive products were formed despite the incorporation of superoxide dismutase, proxidase, and catalase into the reaction mixture .