Expression of Hepatic Metallothionein Messenger RNA in Feral and Caged Fish Species Correlates with Muscle Mercury Levels

Metallothioneins (MTs) are low-molecular-weight cytosolic proteins that are induced by cellular stress as well as exposure to various heavy metals including mercury. Excessive residues of mercury have recently been identified in various fish species of the lower Quachita River system in Arkansas. Fillets of mature largemouth bass (Micropterus salmoides) collected from Woodard Lake, an ox-bow lake of the Ouachita River, possessed muscle residues of mercury ranging from 0.3 to 1.0 ppm (μg/g). To assess the usefulness of using MT expression as a biomarker of mercury exposure, livers and filets were obtained from feral bass of Woodard Lake. Ouachita served as a control site having mercury residues below detection. Analyses using a ribonuclease protection assay with winter flounder MT cDNA revealed that bass had significantly elevated levels of MT mRNA which correlated (r2 = 0.756) with the levels of mercury in muscle fillets. To further explore the water quality of Woodard Lake, 10 juvenile channel catfish were housed in cages and placed where feral collections were made in both sites for 2 weeks. Mercury was not detected in muscle or liver and no significant difference in hepatic MT mRNA was observed. These data demonstrate that MT mRNA expression can be used as a tool to assess exposure to heavy metals and suggest that the elevated levels of mercury in large predatory fish may be due to trophic magnification rather than a single point-source exposure.