Binding Of Methylmercury Research Articles

Evidence on Neurotoxicity after Intrauterine and Childhood Exposure to Organomercurials.

Although the molecular mechanisms underlying methylmercury toxicity are not entirely understood, the observed neurotoxicity in early-life is attributed to the covalent binding of methylmercury to sulfhydryl (thiol) groups of proteins and other molecules being able to affect protein post-translational modifications from numerous molecular pathways, such as glutamate signaling, heat-shock chaperones and the antioxidant glutaredoxin/glutathione system. However, for other organomercurials such as ethylmercury or thimerosal, there is not much information available. Therefore, this review critically discusses current knowledge about organomercurials neurotoxicity-both methylmercury and ethylmercury-following intrauterine and childhood exposure, as well as the prospects and future needs for research in this area. Contrasting with the amount of epidemiological evidence available for methylmercury, there are only a few in vivo studies reporting neurotoxic outcomes and mechanisms of toxicity for ethylmercury or thimerosal. There is also a lack of studies on mechanistic approaches to better investigate the pathways involved in the potential neurotoxicity caused by both organomercurials. More impactful follow-up studies, especially following intrauterine and childhood exposure to ethylmercury, are necessary. Childhood vaccination is critically important for controlling infectious diseases; however, the safety of mercury-containing thimerosal and, notably, its effectiveness as preservative in vaccines are still under debate regarding its potential dose-response effects to the central nervous system.

Binding of methylmercury to humic acids (HA): Influence of solar radiation and sulfide addition reaction of HA

Methylmercury (MeHg) is a neurotoxin that bioaccumulates in organisms and it forms strong complexes with reduced sulfur-containing ligands in dissolved organic matter (DOM). In the present study, the influences of solar radiation and sulfide addition reaction of humic acids (HA) on MeHg binding to HA were investigated using synchronous fluorescence and FT-IR two-dimensional correlation spectroscopic (2DCOS) analysis. Results showed that the complexation of fluorescent fractions of HA and sulfur-reacted HA (S-HA) with MeHg was not significantly affected by photoreaction treatments and the affinity of fluorescent fractions followed the order of protein-like fractions > humic-like fractions > fulvic-like fractions for both HA and S-HA. FT-IR 2DCOS analysis showed that the affinity of various binding sites in DOM for MeHg changed under different photoreaction treatments. Under dark treatment, small molecular compounds with low humification degree such as aromatic amino acids may be the site with the strongest binding ability to MeHg in HA, whereas aliphatic amino acids and sulfur-containing groups from sulfide addition reactions play a role in complexing of S-HA and MeHg. Under BS treatment (irradiation of DOM before MeHg binding), aliphatic compounds in HA preferentially bind to MeHg and aliphatic amino acids are the components with the strongest complexing ability; but for S-HA binding to MeHg, unsaturated functional groups and aromatic groups are more sensitive (alkenes > alkanes, phenols > alcohols). Under AS treatment (irradiation of DOM after MeHg binding), unsaturated bonds and aromatic compounds in HA preferentially bind to MeHg and aromatic amino acids show the strongest complexing ability; but for S-HA binding to MeHg, aliphatic groups show the strongest complexing ability (alkanes, alkenes > aromatics). These findings help us to better understand the complexation mechanisms between MeHg and DOM.

Effect of Methylmercury Binding on the Peroxide-Reducing Potential of Cysteine and Selenocysteine.

Methylmercury (CH3Hg+) binding to catalytically fundamental cysteine and selenocysteine of peroxide-reducing enzymes has long been postulated as the origin of its toxicological activity. Only very recently, CH3Hg+ binding to the selenocysteine of thioredoxin reductase has been directly observed [PickeringI. J.Inorg. Chem., 2020, 59, 2711−271832049511], but the precise influence of the toxicant on the peroxide-reducing potential of such a residue has never been investigated. In this work, we employ state-of-the-art density functional theory calculations to study the reactivity of molecular models of the free and toxified enzymes. Trends in activation energies are discussed with attention to the biological consequences and are rationalized within the chemically intuitive framework provided by the activation strain model. With respect to the free, protonated amino acids, CH3Hg+ binding promotes oxidation of the S or Se nucleus, suggesting that chalcogenoxide formation might occur in the toxified enzyme, even if the actual rate of peroxide reduction is almost certainly lowered as suggested by comparison with fully deprotonated amino acids models.

Effect of hemoglobin adjustment on the precision of mercury concentrations in maternal and cord blood

The cord-blood mercury concentration is usually considered the best biomarker in regard to developmental methylmercury neurotoxicity. However, the mercury concentration may be affected by the binding of methylmercury to hemoglobin and perhaps also selenium. As cord-blood mercury analyses appear to be less precise than suggested by laboratory quality data, we studied the interrelationships of mercury concentrations with hemoglobin in paired maternal and cord blood samples from a Faroese birth cohort (N=514) and the Mothers and Children׳s Environmental Health study in Korea (n=797). Linear regression and structural equation model (SEM) analyses were used to ascertain interrelationships between the exposure biomarkers and the possible impact of hemoglobin as well as selenium. Both methods showed a significant dependence of the cord-blood concentration on hemoglobin, also after adjustment for other exposure biomarkers. In the SEM, the cord blood measurement was a less imprecise indicator of the latent methylmercury exposure variable than other exposure biomarkers available, and the maternal hair concentration had the largest imprecision. Adjustment of mercury concentrations both in maternal and cord blood for hemoglobin improved their precision, while no significant effect of the selenium concentration in maternal blood was found. Adjustment of blood–mercury concentrations for hemoglobin is therefore recommended.

Methylmercury Targets Photoreceptor Outer Segments

Human populations experience widespread low level exposure to organometallic methylmercury compounds through consumption of fish and other seafood. At higher levels, methylmercury compounds specifically target nervous systems, and among the many effects of their exposure are visual disturbances, including blindness, which previously were thought to be due to methylmercury-induced damage to the visual cortex. Here, we employ high-resolution X-ray fluorescence imaging using beam sizes of 500 × 500 and 250 × 250 nm(2) to investigate the localization of mercury at unprecedented resolution in sections of zebrafish larvae ( Danio rerio ), a model developing vertebrate. We demonstrate that methylmercury specifically targets the outer segments of photoreceptor cells in both the retina and pineal gland. Methylmercury distribution in both tissues was correlated with that of sulfur, which, together with methylmercury's affinity for thiolate donors, suggests involvement of protein cysteine residues in methylmercury binding. In contrast, in the lens, the mercury distribution was different from that of sulfur, with methylmercury specifically accumulating in the secondary fiber cells immediately underlying the lens epithelial cells rather than in the lens epithelial cells themselves. Since methylmercury targets two main eye tissues (lens and photoreceptors) that are directly involved in visual perception, it now seems likely that the visual disruption associated with methylmercury exposure in higher animals including humans may arise from direct damage to photoreceptors, in addition to injury of the visual cortex.

PH에 따른 부식유기산의 메틸수은 결합 리간드 변화: X-선 흡수분석

Mercury accumulates in biota mainly as methylmercury. In nature, methylmercury shows high affinity to organic matter and <TEX>$CH_3Hg^+$</TEX>-organic matter complexation affects the mobility and bioavailabiity of methylmercury. In this study, we examined the methylmercury binding sites in an aquatic humic acid as affected by the pH condition using Hg <TEX>$L_{III}$</TEX>-edge extended X-ray absorption fine structure (EXAFS). We evaluated methylmercury binding humic ligands using methylmercury-thiol, methylmerury-carboxyl, and methylmercury-amine complexation models. When <TEX>$CH_3Hg^+$</TEX>-to-humic reduced sulfur ratio is 0.3, we found that most of <TEX>$CH_3Hg^+$</TEX> binds to thiol ligands at pH 5 and 7. At pH 7, however, some carboxyl or amine ligand contribution is observed, unlike at pH 5 where <TEX>$CH_3Hg^+$</TEX> almost exclusively binds to thiol ligands. The carboxyl or amine ligand contribution may indicate that some types of thiol ligands in the natural organic matter have relatively low complexation constants or acid dissociation constants compared to those of some carboxyl or amine ligands. Analysis results indicate that ~0.2 fraction of methylmercury binds to amine or carboxyl ligands and ~0.8 to thiol ligands at pH 7.

Bonding of methyl mercury to reduced sulfur groups in soil and stream organic matter as determined by x-ray absorption spectroscopy and binding affinity studies

We combined synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and binding affinity studies to determine the coordination, geometry, and strength of methyl mercury, CH 3Hg (II), bonding in soil and stream organic matter. Samples of organic soil (OS), potentially soluble organic substances (PSOS) from the soil, and organic substances from a stream (SOS) draining the soil were taken along a short “hydrological transect.” We determined the sum of concentrations of highly reduced organic S groups (designated Org-S RED), such as thiol (RSH), disulfane (RSSH), sulfide (RSR), and disulfide (RSSR), using sulfur K-edge XANES. Org-S RED varied between 27% and 64% of total S in our samples. Hg L III-edge EXAFS analysis were determined on samples added CH 3Hg (II) to yield CH 3Hg (II)/Org-S RED ratios in the range 0.01–1.62. At low ratios, Hg was associated to one C atom (the methyl group) at an average distance of 2.03 ± 0.02 Å and to one S atom at an average distance of 2.34 ± 0.03 Å, in the first coordination shell. At calculated CH 3Hg(II)/Org-S RED ratios above 0.37 in OS, 0.32 in PSOS, and 0.24 in SOS, the organic S sites were saturated by CH 3Hg +, and O (and/or N) atoms were found in the first coordination shell of Hg at an average distance of 2.09 ± 0.01 Å. Based on the assumption that RSH (and possibly RSSH) groups take part in the complexation of CH 3Hg +, whereas RSSR and RSR groups do not, approximately 17% of total organic S consisted of RSH (+ RSSH) functionalities in the organic soil. Corresponding figures for samples PSOS and SOS were 14% and 9%, respectively. Competitive complexation of CH 3Hg + by halide ions was used to determine the average binding strength of native concentrations of CH 3Hg (II) in the OS sample. Using data for Org-S RED, calculated surface complexation constants were in the range from 10 16.3 to 10 16.7 for a model RSH site having an acidity constant of mercaptoacetic acid. These values compare favorably with identically defined stability constants (log K 1) for the binding of methyl mercury to thiol groups in well-defined organic compounds.

Relationship between semen parameters and mercury concentrations in blood and in seminal fluid from subfertile males in Hong Kong

Relationship between semen parameters and mercury concentrations in blood and in seminal fluid from subfertile males in Hong Kong

Analysis of Methyl Mercury Binding to Freshwater Humic and Fulvic Acids by Gel Permeation Chromatography/Hydride Generation ICP-MS

This study examined variation in MeHg (methyl mercury(II)) binding to humic and fulvic acids using GPC/HG-ICP-MS (gel permeation chromatography/hydride generation-inductively coupled plasma-mass spectrometry). Binding capacities of extracted fulvic acids for MeHg ranged between 6.65 × 10-11 and 2.37 × 10-10 mol/mg of DOC (dissolved organic carbon) as compared to between 2.96 × 10-10 and 9.59 × 10-10 mol/mg of DOC for extracted humic acids (i.e., 2−14 times higher). MeHg binding to fulvic acids increased at higher molecular sizes, but binding to humic acids was not related to molecular size. In the past, some mercury cycling models have used total sulfur as a proxy for thiol groups (the principal sites for MeHg binding). We show that total sulfur is not a good estimate of MeHg−DOC binding and that its use would result in underestimates of mercury biomagnification in food chains.

The detection of mercury, lead, and methylmercury binding sites on lysozyme by carbon-13 NMR chemical shifts of the carboxylate groups

The chemical shift changes observed in the carbon-13 NMR spectra of the carboxylate region in conjunction with previous fluorescence data indicate that mercury and lead bind to both disulfide bridge 64–80 and to Asp-52 in hen egg-white lysozyme. Methylmercury appears to bind primarily to the disulfide linkage. The carbon-13 NMR results support previous investigations by halide ion NMR and x-ray crystallography. The combined information from all of these techniques is useful for providing a clearer description of heavy metal binding to proteins.

Binding of methylmercury and methylmercury-thiol complexes by myelin isolated from mice of differing selenium status.

The chemical form of methylmercury found in the body provides evidence of the intimate involvement of glutathione and glutathione metabolites with methylmercury. The involvement of glutathione in converting protein bound methylmercury to low molecular weight methylmercury-selenium compounds was shown earlier. The effectiveness of selenium in modifying mercury metabolism has attracted increasing attention. After finding that concurrent equimolar selenite injections increased the uptake of mercury but did not alter the mercury distribution in the brain and that selenium reversed the effect of mercury upon glutathione peroxidase, Prohaska and Ganther suggested a complex of mercury and selenium may be involved in the interaction of mercury and selenium. However, considering that the protective effect of selenium is exerted within the tissue without decreasing the concentration or amount of mercury in the tissue, selenium is probably not simply chelating the bulk of brain mercury. The purpose of the following experiments was to determine the binding characteristics of thiol-metabolites of methylmercury to myelin isolated from the central nervous system. The possible alteration of such binding by dietary selenium was also investigated.

The effects of methyl mercury binding to microtubules

The effects of methyl mercury hydroxide (MeHg) on the in vitro polymerization and depolymerization of microtubules were studied. Polymerization was totally inhibited at 3.0 × 10 −5 m MeHg and depolymerization occurred at concentrations above 1.0 × 10 −5 m MeHg, reaching a maximal rate of −0.33%/min at 5.0 × 10 −5 m MeHg. At or above 1.0 × 10 −4 m MeHg, a mercury-protein aggregate formed in both the polymerization and depolymerization systems. Fifteen free sulfhydryl groups per tubulin dimer were determined, and MeHg bound to all 15. When MeHg bound to only 2 free sulfhydryl groups per dimer, it inhibited polymerization. MeHg bound to free sulfhydryl groups exposed uniquely on the surface of microtubules, as well as those free sulfhydryl groups exposed on the ends. These results show MeHg in vitro to be a potent microtubule assembly inhibitor at ratios stoichiometric with the tubulin dimer. The effects of MeHg on microtubules are presumably mediated through MeHg binding to free sulfhydryl groups both on the ends and on the surface of microtubules. The presence of binding sites (free sulfhydryl groups) on the microtubule surface suggests multiple classes of binding sites for MeHg.

Effects of coadministered sodium selenite on short-term distribution of methyl mercury in the rat

Adult male Sprague-Dawley rats received iv injections of 1 μmole of methyl mercury/kg alone or coadministered with 5 μmole of sodium selenite/kg. Tissue concentrations of methyl mercury were determined at 5, 20, and 60 min after treatment. Selenite treatment produced a significant increase in cerebral methyl mercury concentrations and a significant decrease in kidney methyl mercury concentrations at all time points. The concentration of methyl mercury in liver was significantly increased by selenite coadministration at 5 and 20 min but at 60 min after injection the concentration was not significantly different from that found in rats receiving methyl mercury alone. Selenite treatment also significantly lowered blood methyl mercury concentrations at all time points. This decrease was associated with a significant decrease in the concentration of methyl mercury in erythrocytes at 5, 20, and 60 min. Plasma methyl mercury levels at 5 min postinjection were slightly higher in selenite-treated rats but were significantly lower in treated animals at 20 and 60 min. Treatment of rats with selenite did not specifically alter the extent of methyl mercury binding to glutathione in the 108,000 g supernatant of cerebrum or in erythrocyte hemolysates. In rats receiving either methyl mercury alone or with selenite low-molecular-weight methyl mercury complexes could not be detected in plasma 5 min after iv injection. These results suggested that selenite exerted immediate effects on the distribution of methyl mercury in the rat but did not alter the binding of methyl mercury to glutathione in the soluble components of erythrocytes and cerebrum or lead to formation of a persistent low-molecular-weight methyl mercury complex in plasma.

A proton nuclear magnetic resonance study of the binding of methylmercury in human erythrocytes

The binding of methylmercury, CH 3Hg(II), by small molecules in the intracellular region of human erythrocytes has been studied by 1H-NMR spectroscopy. To suppress or completely eliminate interfering resonances from the much more abundant hemoglobin protons, spectra were measured by a technique based on the transfer of saturation throughout the envelope of hemoglobin resonances following a selective presaturation pulse or by the spin-echo Fourier transform method. With these techniques, 1H-NMR spectra were measured for the more abundant intracellular small molecules, including glycine, alanine, creatine, lactic acid, ergothioneine and glutathione, in both intact and hemolyzed erythrocytes to which CH 3Hg(II) had been added. The results for intact erythrocytes indicate that part of the CH 3Hg(II) is complexed by intracellular glutathione. These results also indicate that exchange of CH 3Hg(II) among glutathione molecules is fast, with the average lifetime of a CH 3Hg(II)-glutathione complex estimated to be less than 0.01 s. From exchange-averaged chemical shifts of the resonance for the proton on the α-carbon of the cysteine residue of glutathione, it is shown that, in hemolyzed erythrocytes, the sulfhydryl group of glutathione binds CH 3Hg(II) more strongly than the sulfhydryl groups of hemoglobin.

Binding of methylmercury(II) by HeLa S3 suspension-culture cells: Intracellular methylmercury levels and their effect on DNA replication and protein synthesis

HeLa S3 cells were exposed to varied concentrations of methylmercury over varied periods of time and its binding by the cells was studied using 203Hg-labeled methylmercuric chloride as radioactive marker. Also studied was the effect of cell-bound methylmercury on DNA replication and protein synthesis and on the growth rate of the cells. The results show that methylmercury binding is a rapid process, with much of the organomercurial bound within the the first 60 min of incubation, and that considerable quantities of organic mercury become affixed to the cells. The amounts of bound methylmercury, [CH 3Hg(II)] bound , given in mol/cell, range from 2 × 10 −16 (at 1 h of incubation and at 1 μM CH 3Hg(II) in the medium) to almost 4 × 10 −14 (at 24 h of incubation and at 100 μM CH 3Hg(II) in the medium). A [CH 3Hg(II)] bound value of about 30 × 10 −16 mol/cell appears to be the threshold below which cells display a normal growth pattern and below which metabolic events such as DNA replication or protein synthesis are affected only to a minor degree but above which major changes in cell metabolism and cell growth take place. Methylmercury binding by the cells is tight so that only 20% of the bound material is released from the cells over a 3-h incubation period when the cells are placed into fresh, methylmercury-free growth medium. Analysis of the binding data in terms of binding to identical and completely independent sites yields an association constant K of 7.92 × 10 4 l/mol and for the maximum concentration of cellular binding sites the value 2.40 × 10 −14 mol/cell or 1.45 × 10 10 sites/cell. Evidence is presented which shows that cellular sulfhydryl groups do not suffice to provide all the sites taken up by methylmercury and that binding, in all likelihood, involves basic nitrogen, too. The levels of cell-bound methylmercury are such that binding to HeLa DNA and HeLa chromatin, for instance, can readily take place. Methylmercury binding data obtained by using the technique of particle-induced X-ray emission (PIXE) are in good agreement with the data obtained via isotope dilution.

Evaluation of methyl mercury chelating agents using red blood cells and isolated hepatocytes

The relative efficacy of thiol-containing mercurial scavengers was assayed by using cellular suspensions of erythrocytes or isolated hepatocytes. The blood cells incubated in a buffer (pH 7.4) containing 1 mM glucose (10% hematocrit) were exposed to 5 μM methyl mercuric chloride. In the absence of extracellular thiols the red blood cells took up more than 90% of methyl mercury from the surrounding medium during 5–10 min. This uptake was almost completely inhibited by dimercaptosuccinic acid (DMSA) (1 mM) and the same chelant could rapidly remove 80% of the mercury from ‘pre-loaded’ erythrocytes. Hepatocytes prepared according to the method of Seglen [11] in a suspension of 10 6 cells/ml in a buffer containing 5 mM glucose and 5 mg/ml of bovine serum albumin were also exposed to methyl mercuric chloride (4 μM). Almost 50% of the mercurial was taken up by the cells slowly during the incubation period of 240 min. DMSA (1 mM) almost completely blocked the methyl mercury binding by the hepatocytes. 2-Mercaptopropionylglycin (Thiola) or mercaptosuccinic acid (MSA) was almost as effective mercurial scavengers as DMSA in hepatocytes and in red blood cells. Diethyldithiocarbamate (DDC) and dimercaptopropanol (BAL) were considerably less effective than DMSA to inhibit the mercurial binding to hepatocytes. Experiments in vivo have shown that DMSA is a better mercurial chelator than Thiola or MSA, whereas DDC and BAL may both be considered to be inapplicable in methyl mercury poisonings. Our cellular assay provides preliminary information of the efficiency of chelating thiols and may serve as a useful first approximation when planning further experiments.

Methyl mercury binding substances from the brain of experimentally exposed squirrel monkeys (Saimiri sciureus).

Squirrel monkeys were given a single subtoxic dose of 203Hg-labelled methyl mercuric hydroxide. After three weeks the brains were dissected out, homogenized and separated into particulate and soluble fractions by ultracentrifugation. The soluble fractions were further separated into high molecular weight and low molecular weight components by ultrafiltration. The major part (75%) of the radioactivity was associated with the particulate fraction while high molecular weight compounds in the soluble fraction accounted for 16%. The remainder (9%) was bound to glutathione.

Binding of methylmercury to ovalbumin as methylmercuric cysteine.

Binding of methylmercury to ovalbumin as methylmercuric cysteine.

Quantitative aspects of mercurial-agarose gel electrophoresis

Single-stranded nucleic acids are capable of extensive intramolecular base pairing as well as intermolecular aggregation. Consequently, electrophoretic studies of single-stranded nucleic acids are most effective when conducted under denaturing conditions. A number of techniques are available for nucleic acid denaturing gel electrophoresis (1–3). In this paper we describe certain quantitative features of one of these techniques, mercurial-agarose gel electrophoresis (4–7). Specifically, we address the questions of resolution and base composition dependence and we introduce a new mercurial for agarose gel electrophoresis, p-chloromercuriphenyl-sulfonic acid. In a previous publication we demonstrated that methylmercury was an effective denaturant in an agarose gel (4). The mechanism of denaturation is presumably the disruption of hydrogen bonding by the reversible binding of methylmercury to uridine and guanosine imino nitrogens. At saturating mercurial concentrations accurate molecular weights can be determined, free of conformation effects. The presence of methylmercury has no observable effect on the mechanical properties of the gel. Hence the denaturing power of the gel can be readily varied. The strength and rigidity of agarose gels make them considerably easier to handle than acrylamide gels, and the large pore size ensures a system compatible with high molecular weight RNA.

Tissue Uptake, Subcellular Distribution, and Metabolism of 14CH3HgCl and CH3203HgCl by Rainbow Trout, Salmo gairdneri

Uptake, whole tissue, and subcellular distribution of methylmercury (MeHg) by rainbow trout, Salmo gairdneri, was studied during, and in three 2-wk intervals following, a single 24-h exposure to 14CH3HgCl and CH3203HgCl. Gel permeation chromatography of cytosol fractions was performed on select tissues. Gills contained approximately 10 times as much MeHg as any other tissue after 24-h exposure whereas concentrations were lowest for brain and skeletal muscle. In the 2-wk period following exposure, gill MeHg concentrations decreased to levels comparable with most other tissues; methylmercury in most other tissues increased and then decreased during the subsequent 4-wk period. Methylmercury levels in brain, skeletal muscle, and gonad were highest at 6 wk. During the 6-wk interval following MeHg exposure, the percent of mercury as inorganic mercury (Hg++) increased in gill, kidney, and liver and to a lesser extent in all other tissues except skeletal muscle. Total MeHg was greatest in cytosol fractions and usually made up from 50 to 80% of the total tissue mercury. In liver cytosol a methylmercury binding metallothionein-like species accounted for up to 40% of the total methylmercury bound. Inorganic mercury (Hg++), probably derived from demethylation of methylmercury, was associated with a metallothionein-like protein detected in gill tissue but no more than 6% of the total mercury in the soluble fraction was bound to this protein(s). Mercury binding to metallothionein-like proteins in kidney and splenic fractions was minimal in spite of relatively large amounts of mercury in the cytosol from these tissues. Metallothionein may act as a binding scavenger for methylmercury and inorganic mercury in fish. Key words: methylmercury, uptake by trout, tissue distribution, subcellular distribution, metallothionein, detoxification