An international team of researchers has studied the in vivo detoxification of methylmercury in multiple animals from two phyla (a waterbird, freshwater fish, and earthworms). They report that the methylmercury complex with cysteine is transformed into selenocysteinate [Hg(Sec)4], followed by biomineralization to chemically inert nanoparticulate mercury selenide (HgSe).
Background:Mercury (Hg) enters ecosystems as inorganic forms through atmospheric deposition at the global scale through anthropogenic points sources such as coal power plants, waste incinerators or cement kilns. Inorganic divalent mercury [Hg(II)] is subject to methylation by microorganisms, and the resulting methylmercury (MeHg) is taken up by multicellular organisms and bioaccumulated to relatively high concentrations. The toxicity of methylmercury results from its binding to cysteine residues of proteins, forming MeHg-cysteinate (MeHgCys) complexes that hinder biological functions of the protein. MeHgCys complexes can be detoxified in vivo via metabolic reactions within the organism, however, how this is done is unknown.
The new study:
Photo: Clark's grebe (Aechmophorus clarkii) |
To address this question, an international team of researchers investigated chemical forms of Hg in the liver and extrahepatic tissue of the waterbird Clark's grebe (Aechmorphus clarkii) and several species of freshwater fish and in whole bodies of earthworms. All animals were collected from contaminated sites, resulting in high mercury concentrations.
Mercury speciation analysis was performed by using high energy resolution X-ray absorption spectroscopy (HR-XAS). Additionally, the speciation of Hg and Se in Clark’s grebe liver and muscle was also investigated by high-performance liquid chromatography coupled to inductively coupled mass spectrometry making use of double affinity chromatography for separation (AF HPLC-ICP MS).
All samples were measured for total Hg and Se concentrations by chemical analysis and for Hg speciation by HR-XANES spectroscopy. Only the Clark’s grebe liver tissue contained high enough Hg (43.1 ± 4.3 mg kg−1) for HR-EXAFS measurement. Two dominant Hg species occur in the Clark’s grebe tissues as indicated by isosbestic points in the HR-XANES spectra, points where all spectra have the same normalized absorption. When comparing the spectra for the different tissues of the bird, one can observe the peak for the MeHgCys species at fixed energy but different intensity, while the liver spectrum shows a shift of the top edge to lower energy. Such shift is characteristic of a shift in Hg coordination from linear in feathers and brain to tetrahedral in liver.
Muscle and kidney spectra showed variable proportions of these two species. The Hg species in the liver spectrum does best but not exactly match to nanoparticulate tiemannite (HgSeNP). The seleniol nature of the new tetrahedral Hg species was confirmed by HR-EXAFS spectroscopy, indicating Hg-Se and Hg-S pairs in the first coordination shell and Hg-Hg pairs in the second coordination shell. The minor contribution of MeHgCys was confirmed at 18% by chemical analysis. The authors conclude that the inorganic Hg species is a tetrahedral selenolate complex with selenocysteine residues. This Hg(Sec)4 complex was observed in the two animal phyla with different proportions of Hg species, MeHgCys, Hg(Sec)4 and a Hg dithiolate complex Hg(SR)2.
The authors interpret their results as a strong evidence for demethylation of MeHg as a tetraselenolate complex in the two animal phyla. This species can be formed with low-molecular-weight (LMW) seleniol molecules and with selenoproteins. Selenoneine is a LMW molecule small enough to form a Hg(Sec)4 complex and its presence has been reported in fish. With respect to selenoproteins, only selenoprotein P (SelP) contains at least four selenocysteine residues. Mercury-bound SelP was identified in the plasma of mercury exposed humans (Inuit adults and miners). The authors conclude, that the results presented here fuel the idea that the C-terminal domain of SelP acts as a nucleation centre for the formation of HgSe in vertebrates, driving in vivo MeHgCys detoxification. The binding of Hg to SelP may, however, deplete neuronal and glial cells in organo-Se essential to selenoenzyme synthesis and activity. It may also alter the antioxidant function of SelP.
The Original study
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Demethylation of Methylmercury in Bird, Fish, and Earthworm, Environ. Sci., 55/3 (2021) 1527-1534.
DOI: 10.1021/acs.est.0c04948 Related studies (newest first)
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