Characterization of cadmium- and lead-phytochelatin complexes formed in a marine microalga in response to metal exposure

Phytochelatins (PCn) are thiol-containing peptides with general structure (g-Glu-Cys)n-Gly enzymatically synthesized by plants and algae in response to metal exposure. They are involved in the cellular detoxification mechanism for their capability to form stable metal-phytochelatin complexes. The speciation of Cd and Pb complexes with phytochelatins has been studied in laboratory cultures of the marine diatom Phaeodactylum tricornutum. An approach based on size-exclusion chromatography (SEC) with off-line detection of phytochelatins, by reverse-phase HPLC, and metal ion, by atomic absorption spectrometry, has been used. The formation of Cd- and Pb-PCn complexes with n value from 3 to 6 was demonstrated. The metal-PCn complexes formed with Cd appear to be different from those formed with Pb for the number of molecules of peptide involved in the complex and for the amount of the metal ion bound. The chromatographic behaviour of metal-PCn complexes is consistent with Pb-PCn complexes in which only a molecule of peptide binds the metal ion, and with Cd-PCn complexes containing two or more molecules of peptide. The metal/peptide molar ratio in Cd-PCn complexes was higher that in Pb-PCn complexes. The formation of Cd- or Pb-PC2 complexes was not demonstrated, probably for a dissociation during the cellular extract preparation. The effectiveness of phytochelatins in the detoxification of these two metal ions in this alga is discussed.