Selenium interaction with mercury

Suzuki, K. T., Ogra, Y. Metabohsm of selenium audits interaction with mercury mechanisms by a speciation study. Phosphorus, Sulfur Silicon Relat Elem 2001, 171-172, 135-169. 

A factor, which has been identified as interacting with mercury kinetics, is selenium intake. Selenium seems to interact with the transport and distribution of most mercury compounds. This has been demonstrated for inorganic mercury (Parizek et al., 1971 a and b) and for methyl mercury (Iwata et al., 1973 Chen et al., 1975). However, the dose-response relationships for the interaction with selenium, as well as the mechanism involved, are still under debate. It is likely that the formation of a mercury-selenium complex is one of the explanations of the observed interaction. 

Selenium is readily available in a variety of foods including shrimp, meat, dairy products, and grains, with a recommended daily intake of 55 to 70 jug. It occurs in several forms with Se+6 being biologically most important. Selenium is readily absorbed by the intestine and is widely distributed throughout the tissues of the body, with the highest levels in the liver and kidney. It is active in a variety of cellular functions and interacts with vitamin E. Selenium appears to reduce the toxic effects of metals such as cadmium and mercury and to have anticarcinogenic activity. Selenium produces notable adverse effects both in deficiency and excess thus recommended daily intake for adults is approximately 70 Jg/day but should not exceed 200 pg/day. 

Ganther HE. 1980. Interactions of vitamin E and selenium with mercury and silver. Ann NY Acad Sci 355 212-226. 

Ridlington JW, Whanger PD. 1981. Interactions of selenium and antioxidants with mercury, cadmium and silver. Fundam Appl Toxicol 1 368-375. 

In the diet and at the tissue level, ascorbic acid can interact with mineral nutrients. In the intestine, ascorbic acid enhances the absorption of dietary iron and selenium reduces the absorption of copper, nickel, and manganese but apparently has little effect on zinc or cobalt. Ascorbic acid fails to affect the intestinal absorption of two toxic minerals studied, cadmium and mercury. At the tissue level, iron overload enhances the oxidative catabolism of ascorbic acid. Thus, the level of dietary vitamin C can have important nutritional consequences through a wide range of inhibitory and enhancing interactions with mineral nutrients. 

Concerning interactions with other chemicals, there is an ongoing debate about the value of fish in the diet versus the risk from increased exposure to methylmercury that may be in the fish. One recent study reported a beneficial effect from increased fish consumption even though mercury body burdens were increased to some extent (Davidson et al. 1998). One possible factor in the fish that could improve health is omega 3-fatty acid. Children and adults both benefit from a healthy diet, but there may more emphasis on the benefits to growing children. Other interactions for mercury include the effect of various substances on its gastrointestinal absorption (e.g., iron, zinc) or possibly protective effects from prevention or repair of mercury related oxidative damage (e.g., interactions with selenium as an antioxidant). No information was identified that specifically addresses differences in these interactions for children compared to adults. 

Copper interacts with numerous compounds normally found in natural waters. The amounts of the various copper compounds and complexes present in solution depend on water pH, temperature, and alkalinity and on the concentrations of bicarbonate, sulfide, and organic ligands. In animals, copper interacts with essential trace elements such as iron, zinc, molybdenum, manganese, nickel, and selenium and also with nonessential elements including silver, cadmium, mercury, and lead interactions may be either beneficial or harmful to the organism. The patterns of copper accumulation, metabolism, and toxicity from these interactions frequently differ from those produced by copper alone. Acknowledgment of these interactions is essential for understanding copper toxicokinetics. 

Mercury-selenium interactions are significant in marine mammals and seem to be a factor in loons however, this is not the case in marine birds. Mercury concentrations in oceanic birds were not correlated with selenium concentrations, as evidenced by values in livers of murres, Uria spp. and razorbills Alca torda, and in breast muscle of sooty terns Sterna fuscata. 

Mercury is a soft metal and as such it is expected to form secondary bonds, most readily with sulfur, selenium, and other heavy non-metals. The situation is, however, more complex and secondary interactions with other electronegative atoms have also been observed in the solid state. Interatomic distances longer than the expected van der Waals distances are, moreover, sometimes observed between molecules orientated such that weak interactions lead to particular arrangements in the crystal [69]. The are numerous examples of secondary bonds in organomercury chemistry although most are intramolecular there are several examples of inter-molecular secondary bonds leading to supramolecular self assembly. A review has been published on this subject [70] and many new examples have subsequently been reported. 

Various proposals have been presented to explain the interaction of selenium with heavy metals. However, no single one appears to explain the mechanism of interaction with all heavy metals. It appears that there are several mechanisms involved in this interaction and that more than one could be involved with a particular metal. It is clear that selenium does not protect animals against heavy metal toxicity by increasing their excretion instead, it causes an increased retention of metals (Parizek et al, 1971 Wagner et al., 1975 Diplock, 1976 Ganther, 1978 Whanger, 1981). A summary of the proposed interactions of selenium with cadmium, mercury, and silver is presented in Fig. 1. 

Parizek, J., Kalouskova, J., Babicky, A., Benes, J., and Pavlik, L., 1974, Interaction of selenium with mercury, cadmium and other toxic metals, in Trace Elements Metabolism in Animals, Vol. 2 (W. G. Hoekstra, J. W. Suttie, H. E. Ganther, and W. Mertz, eds.), pp. 119-131, University Park Press, Baltimore, Maryland. 

Whanger PD. 1985. Metabolic interactions of selenium with cadmium, mercury, and silver. Adv Nutr Res 7 221-250. 

Interactions Overabundance of one trace element can interfere with the metabolic use of another element available at normal levels. For example, addition of large amounts of zinc to a diet interferes with (antagonizes) intestinal copper absorption, resulting in copper deficiency from a diet with adequate copper content. Copper deficiency can provoke iron deficiency and anaemia. Molybdenum deficiency in animals can be induced by co-administration of large amounts of the similar element tungsten. Iron deficiency can also increase retention of cadmium and lead, and selenium has been proposed to protect against cadmium and mercury toxicity. 

Biological, chemical, and physical effects of airborne metals are a direct function of particle size, concentration, and composition. The major parameter governing the significance of natural and anthropogenic emissions of environmentally important metals is particle size. Metals associated with fine particulates are of concern particles larger than about 3-fjim aerodynamic equivalent diameter are minimally respirable, are ineffective in atmospheric interactions, and have a short air residence time. Seventeen environmentally important metals are identified arsenic, beryllium, cadmium, chromium, copper, iron, mercury, magnesium, manganese, nickel, lead, antimony, selenium, tin, vanadium, and zinc. This report reviews the major sources of these metals with emphasis on fine particulate emissions. 

The environmental scientist has at his disposal a variety of sensitive, multi-elemental analytical methods that can lead to a massive amount of data on airborne metals. Optimum use of these tools for environmental monitoring calls for focusing resources only on those metals that are environmentally important. Considerations of toxicity along with their ability to interact in the air, leading to the formation of secondary pollutants, and their presence in air have led to the identification of 17 environmentally important metals nickel, beryllium, cadmium, tin, antimony, lead, vanadium, mercury, selenium, arsenic, copper, iron, magnesium, manganese, titanium, chromium, and zinc. In addition to the airborne concentration, the particle size of environmentally important metals is perhaps the major consideration in assessing their importance. 

Magos, L., and M. Webb. 1980. The interaction of selenium with cadmium and mercury. Crit. Rev. Toxicol. 8(1) 1-42. 

A wide variety of interactions of selenium with essential and nonessential elements, vitamins, xenobiotics, and sulfur-containing amino acids have been demonstrated in numerous studies. Selenium has been reported to reduce the toxicity of many metals including mercury, cadmium, lead, silver, and to some extent, copper (Frost 1972). Most forms of selenium and arsenic interact to reduce the toxicity of both elements (Levander 1977). Because of selenium s role in the antioxidant glutathione peroxidase enzymes, selenium also reduces the toxicity of metals in vitamin E-deficient animals (Diplock et al. 1967). 

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