Speciation of cadmium-gamma-glutamyl peptides complexes in cells of the marine microalga Paeodactylum tricornutum

Phaeodactylum tricornutum responds to Cd exposure by synthesizing phytochelatins (PCn) [(g-Glu-Cys)n-Gly] with n values from 2 to 6. Size exclusion chromatography was coupled with reverse-phase HPLC to follow the formation of complexes between Cd and g-glutamyl peptides in cells exposed to the metal ion during the growth cycle. Five stable complexes (Cd-PCn n = 2 to 6), in which peptides of unique chain length chelate Cd by formation of thiolate bonds, were identified. The formation of Cd-PCn complexes in which acid labile sulfide is incorporated (PC-coated CdS crystallites) was also demonstrated. These complexes exhibited luminescence and UV transition at 280 nm. The apparent MW was 8-10 kDa and the S2-/Cd ratio 0.4. The 66% of the total PC g-Glu-Cys units in these complexes was polymerized as PC4. The incorporation of sulfide into Cd-PCn complexes enhances the Cd/SCys ratio from 0.6 to 1.6. The kinetics of formation of Cd-thiols complexes is consistent with an involvement of the glutathione (GSH) in the Cd sequestration mechanism of P. tricornutum through the initial formation of Cd-GSH complexes. The metal ion is subsequently transferred from glutathione to Cd-induced phytochelatins to form more stable Cd-PCn complexes.