Metallomics of Mercury: The Role of Selenium

Abstract

Mercury-selenium (Hg-Se) interaction is perhaps the most documented bioantagonism. Since its discovery in the 1960s, extensive studies have been carried out on the wide occurrence, chemical mechanisms, and toxicological significance of this bioantagonism. However, major knowledge gaps exist in the underlying mechanism at the molecular level which is the objective of this present research. To study molecular level mechanism of this bioantagonism, four new MeHg-selenoamino acid namely, methylmercury-D,L-selenopenicillaminate, methylmercury-L-selenoglutathionate, and two methylmercury-L-selenomethioninate complexes (one via a Hg-Se bonding and the other via Hg-N bonding) were synthesized and characterized by NMR, FT-IR and mass spectrometry. Their structural and electronic properties were studied by X-ray crystallography and quantum mechanical calculations. These studies reveal that all four complexes chemically and structurally resemble their sulfur analogues. This suggests that mimicry could play a role in the MeHg-Se antagonism. Chemical coupling values from NMR suggest that MeHg+ has stronger affinity for Se than for S. It has long been proposed and analytically proven that mercury selenide, HgSe(s), is the end product of the Hg-Se bioantagonism. However, the pathway of its formation in biological systems was poorly understood. Experiments carried out in this study suggested that HgSe(s) could be formed from both inorganic Hg and MeHg in the presence of Se. In the case of MeHg, we found that its binding with selenoamino acids could result in the demethylation of MeHg and formation of HgSe nanoparticles. NMR and gas chromatography – mass spectrometry (GC-MS) studies confirmed the presence of bis(methylmercury) selenide (BMSe) and dimethylmercury as reaction intermediates based on which a demethylation pathway was proposed. Inorganic Hg interacts with selenite in presence of glutathione (GSH) and form HgSe1-xSx (0 < x < 1) nanoparticles via a black solution or precipitate. The dissolution/precipitation is reversible upon adjustment of pH. UV-visible spectra, TEM and XPS analyses revealed that the black solution is HgSe1-xSx nanoparticles with diameter < 5 nm which at high pH and upon separation becomes sparingly soluble. This study provides a new plausible explanation of tissue distribution patterns of HgSexS1-x in biological systems.