Metallothionein-Like Protein: Is It an Efficient Biomarker of Metal Contamination? A Case Study Based on Fish From the Tunisian Coast

The aim of this work was to assess the relative importance of natural fluctuations in metallothionein-like protein (MTLP) levels associated with the sexual status of fish versus fluctuations due to metal exposure. In order to see fluctuations due to metal exposure, comparisons were made on the same fish species Scorpaena porcus sampled in polluted and unpolluted sites. The hermaphrodite fish Serranus scriba and Scorpaena porcus, in which sexes are separate, were compared at the unpolluted site to see fluctuations caused by the sexual status. In both species, metals and the MTLP levels were determined in the gills and liver. In these organs, Cd, Cu, and Zn distributions were examined in different fractions: the insoluble fraction (IF) and the cytosol divided into thermolabile compounds (HDF) and the heat stable compounds including MTLP. MTLP levels were higher in the liver (3.09 mg/g in S. porcus, 1.59 mg/g in S. scriba) than in gills (0.13 mg/g in S. porcus, 0.40 mg/g in S. scriba). For Scorpaena porcus, metals and MTLP levels varied with sex, whereas in Serranus scriba, which is a hermaphrodite species, inherent variations were also observed. At the polluted site, MTLP, Cd, and Cu concentrations in the gills of S. porcus increased but the supplementary metals were not associated with the heat stable compounds including MTLP. At this site, hepatic MTLP bound more metals than at the unpolluted site, but its binding capacity was not sufficient to avoid the binding of metals to the insoluble and the heat denaturable fractions. In light of these results and in spite of its hermaphrodism, it is questionable whether to consider S. scriba as a good candidate for biomonitoring based on MTLP. S. porcus could be useful for this purpose only if the MTLP capacity in binding metals is not exceeded. The MTLP could be considered as a biomarker only if it is investigated in relatively unpolluted sites.