Academic journal article Environmental Health Perspectives

The Roles of Serum Selenium and Selenoproteins on Mercury Toxicity in Environmental and Occupational Exposure

Academic journal article Environmental Health Perspectives

The Roles of Serum Selenium and Selenoproteins on Mercury Toxicity in Environmental and Occupational Exposure

Article excerpt

Many studies have found that mercury (Hg) exposure is associated with selenium (Se) accumulation in vivo. However, human studies are limited. To study the interaction between Se and Hg, we investigated the total Se and Hg concentrations in body fluids and serum Se-containing proteins in individuals exposed to high concentrations of Hg. Our objective was to elucidate the possible roles of serum Se and selenoproteins in transporting and binding Hg in human populations. We collected data from 72 subjects: 35 had very low Hg exposure as evidenced by mean Hg concentrations of 0.91 and 1.25 ng/mL measured in serum and urine, respectively; 37 had high exposure (mean Hg concentrations of 38.5 and 86.8 hg/mL measured in serum and urine, respectively). An association between Se and Hg was found in urine (r = 0.625; p < 0.001) but not in serum. Hg exposure may affect Se concentrations and selenoprotein distribution in human serum. Expression of both selenoprotein P and glutathione peroxidase (GSH-Px) was greatly increased in Hg miners. These increases were accompanied by devated Se concentrations in serum. In addition, selenoprotein P bound more Hg at higher Hg exposure concentrations. Biochemical observations revealed that both GSH-Px activity and malondialdehyde concentrations increased in serum of the Hg-exposed group. This study aids in the understanding of the interaction between Se and Hg. Selenoproteins play two important roles in protecting against Hg toxicity. First, they may bind more Hg through their highly reactive selenol group, and second, their antioxidative properties help eliminate the reactive oxygen species induced by Hg in vivo. Key words: antagonism, Hg-exposed subjects, mercury, selenium, selenoproteins, serum. doi:10.1289/ehp.7861 available via http://dx.doi.org/[Online 18 January 2006]

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Mercury (Hg) is currently one of the most prevalent pollutants in the environment. It is highly bioconcentrated through the food chain and damages mainly nerves and immune systems. It is harmful both to humans and animals (Clarkson 1997).

Selenium (Se) is an essential micronutrient with important biological and biochemical functions in organisms because of its unique antioxidant properties and its ability to regulate thyroid gland metabolism. It is well known that Se is an antagonist that moderates the toxic effects of many heavy metals such as arsenic, cadmium, Hg, and lead in organisms. Although Se and Hg co-accumulation in humans and other mammals is well known (Falnoga et al. 2000; Kosta et al. 1975), the mechanism of interaction between Se and Hg is still not understood. It is thought to be attributed to the formation of biologically inert Hg-Se compounds. Burk et al. (1974) suggested that the Hg-Se-protein complex plays a role in restraining the acute toxicity of inorganic Hg by binding Hg to prevent it from reaching the target tissues. Recent in vitro studies suggest that Se and Hg could form Hg-Se complexes in a reducing environment and that this 1:1 complex is then bound with plasma selenoprotein P (SEP) (Suzuki et al. 1998; Yoneda and Suzuki 1997).

In mammalian serum, Se is incorporated mainly into three proteins--SeP, extracellular glutathione peroxidase (GSH-Px), and albumin. The first two are well-known selenoproteins. Naturally occurring selenoproteins such as thioredoxin reductases, GSH-Px, SeP, iodothyronine deiodinase types I, II, and III, and others with specific functions have also been identified (Kryukov et al. 2003). Among them, SeP is a unique selenoprotein and contains several selenocysteine (Sec) and cysteine (Cys) residues, indicating that it is capable of transporting Se and binding heavy metals. The sequence of the cloned DNA shows that SeP contains 10 Sec groups encoded by UGA stop codons in the open reading frame of its mRNA (Burk et al. 2001; Ma et al. 2002). But purified SeP from humans and rats was recently characterized by immunoaffinity chromatography and was found to contain 7-8 Se atoms per Sec molecule as Se attached to a Cys base. …

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