Cadmium Effects on Enzymes of Ammonia Assimilation in Excised Etiolated Maize Leaf Segments during Greening: A Mechanistic Approach

Supply of CdCl2 in the presence of NH4NO3 to excised etiolated maize leaf segments during greening decreased the glutamine synthetase and nicotinamide adenine dinucleotide reduced (NADH) dependent glutamate synthase activities, while the ferredoxin (Fd) dependent glutamate synthase and glutamate dehydrogenase activities were increased. Inclusion of inorganic nitrogen, metabolites and the inhibitor influenced the effect of Cd on glutamine synthetase activity. The % inhibition of activity caused by Cd was higher with NO3<sup style='margin-left:-6px;'>- but lower with NH4<sup style='margin-left:-6px;'>+. Glutamine, 2-oxoglutarate, glutathione and sucrose decreased the % inhibition by Cd with the more prominent effect with glutamine and sucrose. Methionine sulfoximine exerted a more prominent effect for + Cd enzyme at lower concentration. The results indicate the involvement of reciprocal effects of Cd on glutamine synthetase and glutamate dehydrogenase activities and also on NADH- and Fd-glutamate synthase activities. For the inhibitory effect of Cd on glutamine synthetase activity, NH4<sup style='margin-left:-6px;'>+, glutamine, 2-oxoglutarate, glutathione and sucrose exerted a protective effect with the sucrose being most effective.