Strong induction of phytochelatin synthesis by zinc in marine green alga, Dunaliella tertiolecta

Synthesis of phytochelatins (PCs), heavy-metal-sequestering peptides, in th e marine green alga, Dunaliella tertiolecta, was evaluated under various co nditions of exposure to heavy metals. To investigate the effect of heavy me tals on both PC synthesis and their upstream biosynthetic reactions, an ion -pair-HPLC system was developed in this study, by which PCs and their biosy nthetic intermediates, cysteine (Cys), gamma -glutamylcysteine (gamma EC) a nd glutathione (GSH), could be determined simultaneously with high sensitiv ity. When the cells were exposed to Zn2+, the level of PCs was maximal at 2 00 muM and significantly higher than that obtained after exposure to 400 mu M Cd2+, which is the strongest inducer of PC synthesis in higher plants in vivo and in vitro as well as in microalgae. The predominant PC subtype was PC4, followed by PC3 and PC5, whereas PC2, which is generally abundant in h igher plants, has the lowest level among PC2 to PC5. These results suggest that the characteristics of PC synthase in D. tertiolecta including the req uirement of heavy metals for its catalysis and substrate specificity toward s GSH and PCn are considerably different from those in higher plants and ot her algae. While PC synthesis proceeded in the heavy-metal-treated cells, t he level of GSH did not appreciably change. To maintain the same size of th e GSH pool, GSH must be newly synthesized to balance the amount consumed fo r PC synthesis.