Purification and characterization of atypical cadmium-binding polypeptides from Zea mays

Exposure of corn seedlings to high concentrations of Cd ions induces formation of large quantities of polypeptidic compounds which are composed almost exclusively of Glu, Cys and Gly and which bind Cd through thiolate coordination. These polymers occur in classes with molecular weights of 4000 and 8000, respectively, and are made up of chains of about 2000 crosslinked by disulfide bridges. By derivatization and HPLC separation four different molecular species of similar amino acid composition were resolved. In the "native" state all Cys are either coordinated to Cd or linked in intra- and intermolecular disulfide bridges. The overall thiol-to-Cd ratio is close to 2. Judged from the refractoriness to endopeptidase digestion it is inferred that as in glutathione Glu is linked to the adjoining amino acid through its gamma-carboxyl group.