The work explains why thiol compounds such as cysteine (Cys) enhance Hg methylation.
Mercury cellular sorption by Desulfovibrio desulfuricans ND132 is found to be the main cause of limited Hg biological uptake and methylation, but addition of thiols such as Cys can liberate the sorbed Hg for enhanced MeHg production. Cells do not preferentially take up Hg-thiol complexes, but Hg binding and exchange between cells and complexing thiol ligands in solution is an important controlling mechanism for Hg uptake and methylation.
We explain a long-standing question: "Why do thiols such as Cys enhance Hg methylation?" We suggest a new mechanism of Hg biological uptake and methylation resulting from Hg ligand exchange between cell surface binding proteins or transporters and thiols and naturally dissolved organic matter in the environment.
Microbial conversion of inorganic mercury (IHg) to MeHg is a significant environmental concern because of the bioaccumulation and biomagnification of toxic MeHg in the food web. Laboratory incubation studies have shown that, despite the presence of large quantities of IHg in cell cultures, MeHg biosynthesis often reaches a plateau or a maximum within hours or a day by an as yet unexplained mechanism. Here we report that mercuric Hg(II) can be taken up rapidly by cells of D. desulfuricans ND132, but a large fraction of the Hg(II) is unavailable for methylation because of strong cellular sorption. Thiols, such as Cys, glutathione, and penicillamine, added either simultaneously with Hg(II) or after cells have been exposed to Hg(II), effectively desorb or mobilize the bound Hg(II), leading to a substantial increase in MeHg production. The amount of thiol-desorbed Hg(II) is strongly correlated to the amount of MeHg produced (r = 0.98). Cells do not preferentially take up Hg(II)-thiol complexes, but Hg(II)-ligand exchange between these complexes and the cell-associated proteins likely constrains Hg(II) uptake and methylation. We suggest that, aside from aqueous chemical speciation of Hg(II), binding and exchange of Hg(II) between cells and complexing ligands such as thiols and naturally dissolved organics in solution is an important controlling mechanism of Hg(II) bioavailability, which should be considered when predicting MeHg production in the environment.
Liu, Y.-R., X. Lu, L. Zhao, J. An, J.-Z. He, E.M. Pierce, A. Johs, and B. Gu. 2016. Effects of Cellular Sorption on Mercury Bioavailability and Methylmercury Production by Desulfovibrio desulfuricans ND132. Environmental Science and Technology 50 (24):13335–13341. DOI:10.1021/acs.est.6b04041.
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