Oxidation of the enzymes involved in nitrogen assimilation plays an important role in the cadmium-induced toxicity in soybean plants

Cadmium causes oxidative damage and hence affects nitrogen assimilation. In the present work we tested the relationship between the inactivation of the enzymes involved in nitrogen assimilation pathway (glutamine synthetase (GS)/glutamate synthase (GOGAT)) and the protein oxidation in nodules of soybean (Glycine max L.) plants under Cd2+ stress. Therefore, the effect of Cd2+ and reduced gluthatione (GSH) on GS and GOGAT activities, and protein abundance and oxidation were analyzed. Under the metal treatment, amino acids oxidative modification occurred, evidenced by the accumulation of carbonylated proteins, especially those of high molecular weight. When Cd2+ was present in the nutrient solution, although a decrease in GS and GOGAT activities was observed (17 and 52%, respectively, compared to controls), the protein abundance of both enzymes remained similar to control nodules. When GSH was added together with Cd2+ in the nutrient medium, it protected the nodule against Cd2+ induced oxidative damage, maintaining GS and GOGAT activities close to control values. These results allow us to conclude that the inactivation of the nitrogen assimilation pathway by Cd2+ in soybean nodules is due to an increment in GS and GOGAT oxidation that can be prevented by the soluble antioxidant GSH.