Metal Detoxification in Freshwater Animals. Roles of Metallothioneins
In laboratory experiments with a variety of aquatic animals, the toxicity of non-essential metals normally exhibits a threshold response. At low exposure concentrations (low internal doses), the organism can detoxify the incoming metal and thus tolerate the exposure, whereas at higher concentrations, i.e., above some threshold, the detoxification mechanism is no longer able to protect the organism completely, the incoming metal binds to metal-sensitive sites within the cell, and deleterious effects begin to occur. Several metal-detoxification strategies have been identified in laboratory experiments, including metal sequestration in insoluble granules and metal complexation by metallothionein or metallothionein-like peptides (MTLP), but the ability of these mechanisms to prevent metals from binding to metal-sensitive sites in the intracellular environment has not been rigorously tested in field situations.
In this chapter we briefly summarize the laboratory evidence supporting the threshold model for metal toxicity and then present the results of our field studies on chronically exposed freshwater animals (