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The living organisms resist/tolerate heavy metal stress by several mechanisms. These include efflux of metal ions, redox changes, binding to cell wall, extracellular or intracellular sequestration by peptides/ proteins or other biomolecules and finally storage within vesicular compartments (for reviews, see Baker & Walker, 1990; Brown & Hall, 1990; Shaw, 1990; Verkleij & Schat, 1990; Silver, 1992; Stillman et al., 1992; Winklemann & Winge, 1994). Eukaryotes limit the concentrations of reactive free metal ions by intracellular sequestration. The chelated metal ions may be internalized within intracellular compartments such as vacuoles. Glutathione (GSH) (Singhal et al., 1987), GSH-related phytochelatins (PCs) and cysteine-rich metallothioneins (MTs) (Stillman et al., 1992; Winklemann & Winge, 1994) are the main metal sequestering molecules. GSH appears to be the primary line of defense against metal toxicity in most organisms. The inhibition of GSH synthesis by chemical or genetic means sensitizes the cells to Cd and possibly other metal ions (Singhal et al., 1987; Coblentz & Wolf 1994). A second line of defense against toxic metals is provided by MTs in animals and some yeasts (Stillman et al., 1992; Winklemann & Winge, 1994). The animals lacking functional MT genes are sensitized to Cd toxicity (Masters et al., 1994). |
