Heavy metals and glutathione metabolism in mussel tissues

Abstract

Data are presented on the tissue glutathione content and on the activity of two of the main enzymes involved in GSH metabolism ( γ-glutamyl-cysteine synthetase (GCS), GSH transferase (GST)) in the gills and digestive gland of mussels ( Mytilus galloprovincialis Lam.) exposed for 1, 4 and 7 days to sublethal concentrations of copper and of mercury, both in the inorganic form of HgCl 2 and in the organic form of CH 3HgCl. The results indicate that Cu 2 + (0.6 μM) and CH 3Hg + (0.2 μM) can affect, although to a different extent, glutathione metabolism in mussel tissues; common traits were a decrease in tissue glutathione content, stimulation of GST activity and inhibition of GSH synthesis rate; on the other hand, inorganic Hg 2 + (0.2 μM) did not significantly affect glutathione content and metabolism in mussel tissues. Exposure to copper (0.6 μM) resulted in a decrease in total glutathione content in both gills and the digestive gland that was maximal after the first day, followed by a tendency to recover at longer exposure times. Such a decrease seemed to be mainly related to a stimulation of GST activity, which was transient in gills and persistent in the digestive gland. The tissue glutathione content was also decreased in mussel exposed to methylmercury (0.2 μM); however, such a decrease seemed to be mainly due to inhibition of the GSH synthesis rate and, to a lesser extent, to an increased GST activity. On the contrary, inorganic Hg 2 + (0.2 μM) did not significantly affect glutathione metabolism, although mussel exposure to the same concentration of organic and inorganic Hg 2 + resulted in similar total metal tissue load in mussel tissues. The role of glutathione as a potential defense against heavy metals is discussed in relation to different routes of metal toxicity and homeostasis in mussel tissues.