RETENTION OF CADMIUM IN ROOTS OF MAIZE SEEDLINGS - ROLE OF COMPLEXATION BY PHYTOCHELATINS AND RELATED THIOL PEPTIDES

Cd from roots of maize was partitioned in seedlings exposed to 3 mu M CdSO4 for 1 to 7 d. Most of the root Cd (92-94%) was buffer soluble an d provided the classical metal-induced cysteine-rich, high-molecular-w eight Cd-binding complex. This complex, however, bound only part of th e Cd within the roots, from 19% after 1 d of exposure to 59% by d 7. T hree families of peptides formed the Cd-binding complex: (gamma-glutam ic acid-cysteine)(n)-glycine [(gamma-Glu-Cys)(n)-Gly], or phytochelati ns, (gamma-Glu-Cys)(n), and (gamma-Glu-Cys)(n)-Glu. The monothiols gam ma-Glu-Cys-Gly (glutathione), gamma-Glu-Cys, and gamma-Glu-Cys-Glu wer e absent from the complex. The n(2) oligomers of any peptide were the least concentrated, whereas the n(3) and n(4) oligomers increased in t he complex with exposure to Cd. By d 7, 75% of (gamma-Glu-Cys)(4)-Gly, 80% of (gamma-Glu-Cys)(4), and 73% of (gamma-GluCys)(3)-Glu were comp lexed with Cd. The peptide thiol:Cd molar ratio for the complexes was 1.01 +/- 0.07, as if the minimal amount of thiol was used to bind Cd. Acid-labile sulfide occurred in the complexes from d 1 onward at the l ow S2-:Cd molar ratio of 0.18 +/- 0.02.