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Thallium bioaccumulation and cytosolic distribution among biomolecules of different molecular masses in the liver of brown trout (Salmo trutta Linnaeus, 1758) from the Croatian river Krka

Thallium is generally present in the freshwater ecosystems in very low levels (Dragun et al., 2011 ; Dragun et al., 2017). Due to its toxicity in very low concentrations, higher water solubility, mobility, and bioavailability compared to the other metals, as well as tendency to bioaccumulate in living organisms (Karbowska, 2016), Tl was included in the list of priority pollutants by the US Environmental Protection Agency (Das et al., 2006). Thus it should be regularly monitored in the natural waters and organisms. We have studied Tl bioaccumulation in the liver of brown trout (Salmo trutta Linnaeus, 1758) from Croatian river Krka at two differently contaminated sites in two seasons. The measurements performed by high resolution inductively coupled plasma mass spectrometry (HR ICP-MS) in the hepatic cytosols indicated higher Tl hepatic accumulation at the spring of the Krka River compared to the sampling site downstream of the Knin town (Dragun et al., 2017). Thallium cytosolic concentrations in S. trutta liver at the Krka River spring were 228.5±128.0 and 306.2±124.9 ng/g in October 2015 and May 2016, respectively, whereas near Knin town they were 86.4±48.1 and 130.6±66.9 ng/g in these two seasons, respectively (Dragun et al., 2017). Although bioaccumulation analyses revealed higher Tl levels at the upstream site, it was not sufficient for understanding of Tl metabolism and toxicity in hepatic cells of S. trutta. Therefore, we have further studied Tl distribution among cytosolic biomolecules in the liver of S. trutta. Size-exclusion high performance liquid chromatography (SEC-HPLC) with Superdex 200 chromatographic column (separation range from 10 to 600 kDa) was applied for separation of cytosolic biomolecules according to their molecular masses, whereas HR ICP-MS coupled offline was used for Tl measurement in obtained chromatographic fractions. The major quantity of cytosolic Tl from S. trutta liver, in both seasons and at both sites, was eluted at the retention times of cytosolic biomolecules of molecular masses in the range from 66 to 299 kDa, with the maxima at 141 kDa. The only difference that was observed between sites was in the peak heights, which increased following the increase of cytosolic Tl concentrations. Namely, the peak heights, i.e. the quantities of eluted Tl, were generally higher at the spring of the Krka River. Occurrence of a small peak in the range of molecular masses from 4 to 11 kDa, which was more expressed in the samples with higher cytosolic Tl concentrations, indicated that a small proportion of cytosolic Tl was bound to low molecular mass compounds. The information on cytosolic distribution of Tl among biomolecules of different molecular masses can be of a great importance as a first step in the identification of Tl-binding compounds in S. trutta liver, which will contribute to better understanding of S. trutta detoxification strategies for Tl and of possible Tl toxicity.

thallium; liver; cytosol; biomolecules; brown trout; hplc

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