Physico-chemical forms of storage of metals (Cd, Cu and Zn) and metallothionein-like proteins in gills and liver of marine fish from the Tunisian coast: ecotoxicological consequences

Abstract

Three fish species ( Diplodus annularis, Scorpaena porcus and Scorpaena scrofa) were sampled off the coast of the industrialized area of Sfax. The distribution of Cd, Cu and Zn was examined in different fractions of both gills and liver: insoluble fraction and cytosol divided into thermolabile compounds and heat-stable compounds. In the latest fraction, quantitative analysis of metallothionein-like proteins (MTLP) was achieved by using differential pulse polarography. In the gills, the insoluble fraction was the major compartment for the storage of all three metals. In the liver, 50% Cd and Cu at least were also associated with this fraction whereas Zn was bound primarily with heat-stable compounds, a pattern which is probably due to the role of MTLP in Zn homeostasis and to the fact that Zn is a constituent element of metallothionein. According to the species, one eighth to a little more than one quarter of Cd, Cu or Zn was bound to the thermolabile fraction in the liver, probably due to the presence of numerous enzymes involved in the metabolic functions of this organ. In both gills and liver, Cd and Cu concentrations in the thermolabile fraction were increased with increasing organ burden of these metals. This observation suggests an enhancement of the toxic hazard since this fraction includes metabolic targets of metals. Statistically significant relationships have been shown between metals and MTLP concentrations in both organs. However, the use of MTLP as a biomarker has to be revised in the light of natural changes (according to age, sex, etc.) which could conceal variations due to metabolic pollution.