Demethylation of Methylmercury in Bird, Fish, and Earthworm

Abstract

Toxicity of methylmercury (MeHg) to wildlife and humans results from its binding to cysteine residues of proteins, forming MeHg-cysteinate (MeHgCys) complexes that hinder biological functions. MeHgCys complexes can be detoxified in vivo, yet how this occurs is unknown. We report that MeHgCys complexes are transformed into selenocysteinate (Hg(Sec)4) complexes in multiple animals from two phyla (a waterbird, freshwater fish, and earthworms) sampled in different geographical areas and contaminated by different Hg sources. In addition, high energy-resolution Xray absorption spectroscopy (HR-XAS) and chromatography-ICP mass spectrometry of the waterbird liver support the binding of Hg(Sec)4 to selenoprotein P and biomineralization of Hg(Sec)4 to chemically inert nanoparticulate mercury selenide (HgSe). The results provide a foundation for understanding mercury detoxification in higher organisms, and suggest that the identified MeHgCys to Hg(Sec)4 demethylation pathway is common in nature.All data supporting the findings of this study are available within the paper and have been deposited in the U.S. Geological Survey repository ScienceBase. 31 The deposit includes all HR-XANES spectra, the Hg L3-edge HR-EXAFS spectrum of the Clark's grebe liver, and the Cartesian coordinates of the Hg(selenoneine)4 complex and the Hg10(SeMe)20 cluster.

Chemical Forms of Mercury in Blue Marlin Billfish: Implications for Human Exposure

“…Although the marlin spectra are well reconstructed with the Hg(Sec) 4 and HgSe references, the fit results are compatible with the occurrence of multinuclear Hg x (Se,Sec) y clusters (y < 4x), as HR-XANES cannot differentiate Hg x (Se,Sec) y from a mixture of Hg(Sec) 4 and HgSe, and even Hg x (Se,Sec) y from Hg(Sec) 4 if the structure of the nanoclusters is highly disordered (Figure 2e). 33 The biomineralization of MeHgCys as HgSe implies demethylation capability of the muscle tissue of marlin via the formation of Hg(Sec) 4 complex that was only known to exist in the liver of freshwater fish, 32 in liver, kidneys, muscle, and brain of giant petrel seabirds, 33 and in liver, kidneys, and muscle of the Clark's grebe water bird. 32 Toxicological risk.…”

“…33 The biomineralization of MeHgCys as HgSe implies demethylation capability of the muscle tissue of marlin via the formation of Hg(Sec) 4 complex that was only known to exist in the liver of freshwater fish, 32 in liver, kidneys, muscle, and brain of giant petrel seabirds, 33 and in liver, kidneys, and muscle of the Clark's grebe water bird. 32 Toxicological risk. The speciation results provide a more precise evaluation of the toxicological risk resulting from the consumption of marlin.…”

“…Mercury exerts its toxicity via complexation to cysteinyl (Cys) and selenocysteinyl (Sec) metal-binding sites in proteins. 32,47 [Se] bio is negative for the two fish specimens, meaning that the muscle tissues are depleted in bioavailable Se. Since…”

“…25,30 XANES spectra measured at high energy-resolution (HR) provide increased chemical and structural sensitivity on Hg speciation in biomolecules at natural concentration (Figure S1). [31][32][33][34][35]…”

Indices of the DNA repair system in the brain of fish as a biomarker of inorganic mercury burden

“…Незважаючи на добре відому нейротоксичність органічної ртуті, цитотоксичні ефекти неорганічної ртуті погано вивчені щодо клітин нервової тканини. Важливим відкриттям для розуміння високої цитотоксичності ртуті були результати вивчення деметилювання ртуті в кишковій системі риб та інших організмів (Wang et al, 2017;Manceau et al, 2021).…”