Antioxidant responses and bioaccumulation in Ictalurus melas under mercury exposure
Introduction
Among the heavy metals, mercury (Hg), which is largely unstudied physiologically, is highly toxic when found at metabolically active sites, even at relatively low amounts. Given its toxicity and capacity for biomagnification along the food chain, mercury often poses a high ecotoxicological risk for aquatic organisms (Pelletier, 1995). This metal is a well-known pro-oxidant that exerts oxidative stress via H2O2 production, induces a decrease in glutathione (GSH) levels, and causes lipid peroxidation (Stohs and Bagchi, 1995). Reduced GSH is involved in the formation of GSH-S conjugates with the ionic forms of Hg, forming linear II covalent complexes (Rabenstein, 1989). It acts as an intracellular chelator, preventing the nucleophilic interaction of metals with the main cellular structures (Maracine and Segner, 1998). Depletion of GSH may reduce the cellular ability to destroy free radicals and reactive oxygen species, so that it raises the general oxidative potential in the cells. Thus, GSH and its associated enzymes play a protective role within the cell. In fact, the conjugation of GSH with a xenobiotic, either spontaneously, or catalyzed by GSH-S-transferases (GST), decreases xenobiotic reactivity and makes these molecules more soluble in water, and they can be more readily eliminated (Boyland and Chasseaud, 1969). This thiol is synthesized by cells by γ-glutamyl cycle or recycled through the action of glyoxalase I (GI) and glyoxalase II (GII). GSH reductase (GR) cooperates with GSH peroxidases (Se-GPx and GPx) in regeneration of GSH.
Variations in GSH levels and the associated enzyme activities associated with mercury exposure have been studied, but the responses were largely variable depending on species, experimental times, and metal concentrations (Chatterjee and Bhattacharya, 1984; Heisinger and Scott, 1985; Di Simplicio and Leonzio, 1989; Maracine and Segner, 1998; Canesi et al., 1999; Elia et al., 2000). In our preliminary studies, it was evidenced that GSH and several GSH-dependent enzymes in liver of Ictalurus melas were affected by mercury (100, 200, and 400 μg/L Hg2+) after 96 h of exposure (Elia et al., 2000). We suggested that those high metal concentrations were responsible for inhibition of GST and GPx and for the increase of total GSH content.
Thus, for a better understanding of the antioxidative defense mechanisms of fish, the present research examines the changes of total GSH and its associated enzymes (GST, GSH peroxidases, GR, and GI and GII) in liver of catfish under 10 days of mercury exposure at lower doses than those previously utilized (35, 70, and 140 μg/L Hg2+). In addition the distribution of mercury in tissues is reported.
Section snippets
Animals
Twenty catfish (mean total length 23.9±2.7 cm; mean weight 171.10±51.19 g) were purchased from a local fish farm of Lake Trasimeno (central Italy). Before use, fish were kept for 15 days’ acclimation time in a 500 L aerated flow-through holding tank filled with dechlorinated, carbon-filtered city tap water (pH 7.6, hardness 21.01°F, dissolved oxygen 8.5 mg/L, conductivity 298 μS/cm, and temperature 13°C). Concentrations of mercury that show acute and subacute toxicity in I. melas have been already
Results
The mean mercury concentrations and relative standard deviation in tissues of treated and untreated catfish after 10 days Hg2+ exposure are reported in Table 1. As evidenced, the accumulation of metal by I. melas resulted in a net increase of the total mercury content in all tissues with respect to the control. At the lowest mercury concentration, the metal content was similar in gills, liver, and kidneys and they were all statistically different from muscle. At 70 μg/L and 140 μg/L of metal,
Discussion
The accumulation of mercury in treated I. melas resulted in a significant increase of the total metal level in all tissues with respect to the control fish. These results, according to those of other studies carried out on rainbow trout (Walczak et al., 1986), carp (Yediler and Jacobs, 1995), and catfish (Elia et al., 2000) exposed directly to mercury chloride solubilized salt under different experimental conditions, exhibited the highest metal concentrations in the kidneys and gills followed
Conclusions
The differences observed of the effects of mercury on GSH metabolism in liver of fish can obviously depend on the heavy metal concentrations and on the time of exposure. These results point out that the enhancement of the activities of catfish enzymes involved in the inactivation of reactive molecules formed during oxidative stress (i.e., 2-ketoaldehydes) could provide an additional protection against the oxidative damage induced by mercury at these doses. However, to make more clear the role
Acknowledgements
The research presented in this article was supported in part by a grant from the Ministero della Sanità, Italy (Ricerca Corrente No. 9/1996).
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