INFLUENCE OF CADMIUM ON THE PRODUCTION OF GAMMA-GLUTAMYLCYSTEINE PEPTIDES AND ENZYMES OF NITROGEN ASSIMILATION IN ZEA-MAYS SEEDLINGS

We examined the effect of cadmium (Cd) additions on a GDH1-null line o f maize and its wild-type isogenic sibling. Addition of Cd increases t he synthesis of metallothioneines which are glutamate-and cysteine-ric h peptides. We predicted a reduced synthesis of gamma-glutamylcysteine (gamma EC) peptides in the mutant relative to the wild type if glutam ate dehydrogenase (GDH) was limiting the drainage of carbon from the t ricarboxylic acid cycle (TCAC). In our experiments there were similar increases in levels of gamma EC peptides in both mutant and wild-type seedlings in response to Cd. There was a marked increase in the phosph oenolpyruvate carboxylase (PEPcase) polypeptide and in one of the poly peptide bands of glutamine synthetase in both mutant and wildtype seed lings. However, no change was seen in the polypeptide levels of GDH or glutamate synthase (GOGAT). Thus, in contrast to PEPcase, an enhanced carbon drain from the TCAC in response to Cd exposure does not requir e enhanced levels of either GDH or GOGAT polypeptides.