General background

Heavy metals cause an abundance of effects on all living animals. In fish there is a multitude of information on different alterations that are caused by cadmium, copper, zinc and mercury (Sorensen, 1991). Heavy metals affect every level of organization from the society (behaviour) to the organism (reproductive) and sub-cellular level (enzymatic and composition changes). The effects can be general to all of the metals or highly metal specific. Since the organism can not destroy the metals by metabolic degradation, the only way in which an organism can protect itself from heavy metal poisoning is by decreasing the rate of uptake, by binding the metals to a ligand that will hinder the metal from disrupting normal physiological processes or increase the rate of excretion.

Metallothionein with its exceptionally high content of thiol groups is extremely effective in binding heavy metal ions. Metallothioneins are widely distributed in nature and are generally considered to be involved in zinc and copper homeostasis, heavy metal detoxification and possibly as a free radical scavenger (Hamer, 1986). As well as being able to bind group IB and IIB heavy metals, metallothionein is also induced by addition of these metals (Zafarullah, Olsson & Gedamu, 19896). Studies on fish and fish cell lines have further shown that the hepatic metallothionein levels are elevated by glucocorticoids, noradrenalin and progesterone (Hyllner et ai, 1989; Olsson et al, 199(k, George et al, 1992). Metallothionein has further been shown to be endogenously regulated during different developmental life stages (Olsson et al, 1990b). An understanding of the complex and highly controlled regulation of metallothionein in fish is necessary if the system is to be used in environmental monitoring.