Do selenium and glutathione inhibit the toxic effects of mercury in marine lamellibranchs?

The effect of selenium (SeO2) and glutathione (GSH) on the bioaccumulation of mercury (HgCl2) and on the activities of lysosomal enzymes in four species of tropical estuarine lamellibranchs is reported. A definite correlation between mercury levels in the external medium and tissue uptake and physiological behaviour--opening and closing of shell valves, response to mechanical stimulus, mucus secretion, and incidence of bleeding--was evident. In the clams exposed to Hg (range 0.1-5.0 mg l-1), bioaccumulation was dependent on the ambient concentration of Hg. The highest bioaccumulation of Hg occurred during the initial 24 h exposure period. Further exposure of up to 7 days did not increase the body burden of Hg. Of the four bivalve species exposed to 0.1 mg Hg l-1, Perna viridis showed the highest levels of Hg (approximately 47 ppm) followed by Anadara granosa, A. rhombea (approximately 25 ppm) and Meretrix casta (approximately 9 ppm). The uptake of Hg by A. granosa was greatly reduced by GSH, whereas Se enhanced it by 50% when administered in combination with Hg. However, the presence of Hg did not influence the uptake of Se. Exposure to combined GSH and Hg resulted in almost complete inhibition of Hg uptake in all four bivalve species. Prior exposure to GSH, however, did not have the same influence on their uptake of Hg. Nevertheless, exposure of clams to GSH following initial exposure to Hg resulted in complete depuration of accumulated Hg. The activities of lysosomal enzymes--arylsulfatase, acid phosphatase, beta-galactosidase and beta-glucuronidase--varied considerably. Treatment with Hg and GSH, separately and in combination, significantly enhanced the levels of beta-galactosidase (P less than 0.05) and beta-glucuronidase (P less than 0.001) in the digestive gland after 96 h exposure. Although Se increased beta-glucuronidase activity (P less than 0.001), it had no effect on beta-galactosidase. On exposure to Hg + Se the activity of both enzymes decreased, except in P. viridis where it increased by 39%. The results show unequivocally that Se does not offer any protection against the toxic effects of mercury in marine lamellibranchs, whereas in many marine vertebrates it does. GSH, a thiol-rich tripeptide, on the other hand, completely nullifies the toxic effects of Hg, both in vivo and in vitro.