Mercury and Selenium - A Review on Aspects Related to the Health of Human Populations in the Amazon

Abstract

Mercury (Hg) toxicity is governed by cellular thiol compounds and its capacity to generate reactive oxygen radicals and oxidative stress. Selenium (Se) plays a key role in the prevention of the toxic effects of Hg by modulating the activity of several Se-dependent enzymes, including glutathione peroxidase (GSH-Px). In addition, dietary Se can reduce Hg toxicity by directly interacting with either Hg(II) or methylmercury (MeHg) to form inert products, such as HgSe complexes.. Although experimental and environmental data have indicated a protective role for selenium against Hg toxicity, human data are more limited and somewhat conroversial In the Amazon Region of Brazil, Hg pollution is rampant as a result of gold (Au) mining and other anthropogenic factors, leading to pervasive release of large quantities of metallic Hg⁰ into the environment. Exposure to Hg in this region is associated with direct occupational exposure in the gold mining industry, as well as consumption by in inhabitants of riverside communities of a diet rich in MeHg-contaminated fish. Human exposure to MeHg in the Amazon through the diet has been monitored by measuring Hg and MeHg in hair samples. In this paper, we review the environmental contamination of Hg in the Amazon and detail human exposures in populations of this region. We conclude with a brief synopsis on Se levels in the Amazon population and provide a brief review of data available on the interaction between Hg and Se in this region. Overall, the literature supports the notion that low environmental Se is linked to susceptibility to Hg toxicity and that Se levels could be used as a bioindicator to monitor the health of Hg exposed subjects. However, in light of the limited human data on this subject, further epidemiological studies are needed to clarify how changes in Se levels modify the toxicity of environmental Hg.

Keywords: Amazon; Tapajós river basin; exposure; mercury; methylmercury; selenium.

Figure 1.

Gold mines, called Garimpos in Portuguese, are located throughout several states of the Brazilian Amazon: Amapá (AP), Amazonas (AM), Maranhão (MA), Mato Grosso (MT), Pará (PA), Rondônia (RO), and Roraima (RR). In these locations, gold mining is very often performed using artisanal processes that release large amounts of elemental mercury (Hg⁰) in the environment.

Figure 2.

Interaction of inorganic selenium [Se(IV)] and selenomethionine with MeHg or Hg(II) via formation of H₂Se (selenidric acid) or “selenohydryl pathway”. Selenite [Se(IV)] and selenomethione can be metabolized to H₂Se, which can be incorporated into selenocysteine that is incorporated in selenoproteins. H₂Se can react with inorganic mercury forming HgSe. H₂Se can also react with methylmercury forming a complex that can be decomposed to form the insoluble mercury selenide salt (HgSe). Hypothetically, CH₃Se⁻ (monomethylselenolate) can also react with Hg(II) or methylmercury to form complexes that can decompose to form HgSe. Adapted and modified from (Farina et al. 2009 in Press).