Synthesis and stability of phytochelatins induced by cadmium and lead in the marine diatom Phaeodactylum tricornutum

The synthesis of phytochelatins (PC), intracellular metal-binding polypepti des characterized by a repeating sequence of gamma -glutamic acid- cysteine (gamma -Glu-Cys) pairs, has been studied in laboratory cultures of the mar ine diatom Phaeodactylum tricornutum exposed to Cd, Pb or Zn, Cd and Pb wer e able to induce PC of different degree of polymerization. The accumulation of the peptides follows a direct relationship with the metal exposure. No PC induction was observed in Zn-treated cultures, although the intracellula r concentration of Zn increased during exposure. Both in short-term (7 h ex posure, 10 muM Cd or Pb) and 3-day experiments (metal concentration less th an 0.5 muM), the major fraction of total PC gamma -Glu-Cys subunits synthes ized was polymerized as PC2 when cells were exposed to Pb, but as PC4 when cells were exposed to Cd. In short-term experiments about 50% of the gamma -Glu-Cys residues of the cellular pool of glutathione was quickly and almos t quantitatively converted into PC. Recovery experiments, in which metal-st ressed cells are suspended in a metal-free medium, showed a decrease of the PC pool and a concomitant increase of glutathione, suggesting a mechanism of degradation and release of metal-phytochelatin complexes. (C) 2001 Elsev ier Science Ltd. All rights reserved.