The importance of metallothionein for the accumulation of copper, zinc and cadmium in environmentally exposed perch, Perca fluviatilis

A field study of the role of metallothionein (MT) in the binding of heavy metals in perch (Perca fluviatilis), exposed to moderate levels of copper, zinc and cadmium, was performed. Perch were sampled at four sites in Sweden in September during two consecutive years. Two sites were located in the vicinity of a brassworks and two outside the emission range. The first year, fish from the two brassworks sites and from one of the uncontaminated sites were collected. The second year, fish from the most contaminated site and from the two uncontaminated sites were caught. The levels of hepatic copper, zinc and cadmium reflected the concentrations of these metals in water and were increased in fish from the two contaminated sites. The level of cadmium in liver was relatively low. MT was induced in liver of perch caught at the most contaminated site. The hepatic MT content in individual livers correlated well to the accumulation of copper (r = 0.85, P less than 0.001) and zinc (r = 0.75, P less than 0.001). There was a low but significant correlation between the levels of MT and cadmium in the liver (r = 0.48, P less than 0.001). The relationship between MT and metals was very similar both years. Subcellular fractionation of the metals in the liver revealed that an induction of MT was followed by an increased amount of copper, zinc and cadmium bound to the protein. The relative fraction of the total hepatic copper and cadmium bound to MT was increased at the most contaminated site, whereas there was no difference in subcellular distribution of zinc between the sites. In perch from the most contaminated site, the portions of hepatic copper, zinc and cadmium found in the cytosolic fraction were 70, 57 and 81%, respectively. Seventy-one % of the copper, 29% of the zinc and 84% of the cadmium found in hepatic cytosol of fish from the same site, eluted together with MT after gel filtration chromatography. The analysis of the subcellular distribution of copper, zinc and cadmium demonstrates that MT is responsible for the binding of a large amount of the total hepatic cellular content of copper and cadmium in perch.