Abstract
Methylmercury (MeHg) is one of the most potent neurotoxins. It is produced in nature through the methylation of inorganic divalent mercury (HgII) by phylogenetically diverse anaerobic microbes. The mechanistic understanding of the processes that govern the extent of bacterial export of MeHg, its bioaccumulation, and bio-toxicity depends on accurate quantification of its species, especially its complexation with low molecular mass thiols; organometallic complexes that are difficult to detect and measure in natural conditions. Here, we report the development of a novel analytical method based on liquid chromatography tandem mass spectrometry (LC-MS/MS) to determine 13 MeHg complexes with important thiol compounds which have been observed in the environment and in biological systems. By using online preconcentration via solid phase extraction (SPE), the method offers picomolar (12–530 pM) detection limits, the lowest reported so far for the determination of MeHg compounds. Among three different SPE materials, a weak cation exchange phase showed the best efficiency at a low pH of 2.5. We further report the presence of MeHg-cysteine, MeHg-cysteamine, MeHg-penicillamine, MeHg-cysteinylglycine, and MeHg-glutamylcysteine as the predominant MeHg–thiol complexes in the extracellular milieu of an important HgII methylating bacterium, Geobacter sulfurreducens PCA, exposed to 100 nM of HgII.
Highlights
Methylmercury (MeHg) is ingested by humans mainly via rice and fish consumption [1, 2] and can damage the central nervous system and kidney and is especially dangerous for fetuses and infants [3]
Significant increases in the HgII methylation rate have been reported for bacteria culture assays amended with specific low molecular mass (LMM) thiols [20, 21] and subsequent cellular export of MeHg has been observed to be facilitated by addition of cysteine [22]
We present a novel methodology for the determination of a comparatively large number of MeHg complexes (13 complexes) with LMM thiol ligands which has been frequently found in bacteria culture assays and in terrestrial and aquatic ecosystems [5, 11, 12, 21, 35]
Summary
Methylmercury (MeHg) is ingested by humans mainly via rice and fish consumption [1, 2] and can damage the central nervous system and kidney and is especially dangerous for fetuses and infants [3]. The sensitivity of LC-ICPMS methods is generally high with respect to the Hg signal but different MeHg complexes are identified by chromatographic retention time only This causes a relatively large risk for peak overlaps and misidentification in samples with many thiol compounds, in particular if the sample contains a substantial concentration of HgII forming HgII-thiol complexes [12, 35]. In bacteria culture and environmental systems, many different LMM thiol compound are often present [5, 11] and the development of a highly selective and more robust method is essential to characterize MeHg–thiol complexes at concentrations relevant for culture systems with HgII methylating bacteria and for the environment. Three different SPE cartridges of dimensions 2.1 × 20 mm, 15 μm were investigated at different pH: Hydrophilic-Lipophilic-Balanced (HLB) Oasis, Weak-Cation-Exchange (WCX) Oasis, and Weak-AnionExchange (WAX) Oasis, Water Scientific
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