Distribution of selected essential (Co, Cu, Fe, Mn, Mo, Se, and Zn) and nonessential (Cd, Pb) trace elements among protein fractions from hepatic cytosol of European chub (Squalius cephalus L.)

Association of selected essential (Co, Cu, Fe, Mn, Mo, Se, and Zn) and nonessential (Cd, Pb) trace elements with cytosolic proteins of different molecular masses was described for the liver of European chub (Squalius cephalus) from weakly contaminated Sutla River in Croatia. The principal aim was to establish basic trace element distributions among protein fractions characteristic for the fish living in the conditions of low metal exposure in the water. The fractionation of chub hepatic cytosols was carried out by size exclusion high performance liquid chromatography (SE-HPLC; Superdex™ 200 10/300 GL column), and measurements were performed by high resolution inductively coupled plasma mass spectrometry (HR ICP-MS). Elution profiles of essential elements were mostly characterized by broad peaks covering wide range of molecular masses, as a sign of incorporation of essential elements in various proteins within hepatic cytosol. Exceptions were Cu and Fe, with elution profiles characterized by sharp, narrow peaks indicating their probable association with specific proteins, metallothionein (MT), and ferritin, respectively. The main feature of the elution profile of nonessential metal Cd was also single sharp, narrow peak, coinciding with MT elution time, and indicating almost complete Cd detoxification by MT under the conditions of weak metal exposure in the water (dissolved Cd concentration ≤0.3 μg L⁻¹). Contrary, nonessential metal Pb was observed to bind to wide spectrum of proteins, mostly of medium molecular masses (30–100 kDa), after exposure to dissolved Pb concentration of ~1 μg L⁻¹. The obtained information within this study presents the starting point for identification and characterization of specific metal/metalloid-binding proteins in chub hepatic cytosol, which could be further used as markers of metal/metalloid exposure or effect on fish.