Phytochelatin-cadmium-sulfide high-molecular-mass complexes of Euglena gracilis

High-molecular-mass PC complexes (PC-HMWCs) constituted by phytochelatins (PCs), cadmium and sulfide are synthesized by several organisms after exposure to cadmium. In this study, PC-HMWCs were isolated from photoheterotrophic Euglena gracilis and purified to homogeneity, resulting in compounds of molecular mass 50-380 kDa depending on the CdCl2 and sulfate concentrations in the culture medium. In contrast with plants and some yeasts, PC-HMWCs from E. gracilis mainly comprise (57-75%) monothiol molecules (Cys, gamma-glutamylcysteine, GSH) and, to a lesser extent (25-43%), PCs. A similar acid-soluble thiol compound composition was found in whole cell extracts. The -SH/Cd2+ and S2-/Cd2+ ratios found in purified PC-HMWCs were 1.5 and 1.8, respectively; the (-SH + S2-)/Cd2+ ratio was 3.2. PC-HMWCs of molecular mass 60 and 100 kDa were also localized inside Percoll-purified chloroplasts, in which cadmium and PCs were mainly compartmentalized. Cadmium and sulfur-rich clusters with similar sulfur/cadmium stoichiometries to those of the purified PC-HMWCs were detected in the chloroplast and throughout the cell by energy dispersive microanalysis and atomic resolution electron microscopy. The presence of PC-HMWCs in primitive photosynthetic eukaryotes such as the protist, E. gracilis, suggests that their function as the final cadmium-storage-inactivation process is widespread. Their particular intracellular localization suggests that chloroplasts may play a major role in the cadmium-resistance mechanism in organisms lacking a plant-like vacuole.