Molecular characterization of homo- and heterodimeric mercury(II)-bis-thiolates of some biologically relevant thiols by electrospray ionization and triple quadrupole tandem mass spectrometry

Integration of proteomics and network toxicology reveals the mechanism of mercury chloride induced hepatotoxicity, in mice and HepG2 cells

Mercury is one heavy metal toxin that could cause severe health impairments. Mercury exposure has become a global environmental issue. Mercury chloride (HgCl₂) is one of mercury's main chemical forms, but it lacks detailed hepatotoxicity data. The present study aimed to investigate the mechanism of hepatotoxicity induced by HgCl₂ through proteomics and network toxicology at the animal and cellular levels. HgCl₂ showed apparent hepatotoxicity after being administrated with C57BL/6 mice (16 mg/kg.bw, oral once a day, 28 days) and HepG2 cells (100 μmol/L, 12 h). Otherwise, oxidative stress, mitochondrial dysfunction and inflammatory infiltration play an important role in HgCl₂-induced hepatotoxicity. The differentially expressed proteins (DEPs) after HgCl₂ treatment and enriched pathways were obtained through proteomics and network toxicology. Western blot and RT-qPCR results showed Acyl-CoA thioesterase 1 (ACOT1), Acyl-CoA synthetase short chain family member 3 (ACSS3), Epidermal growth factor receptor (EGFR), Apolipoprotein B (APOB), Signal transducer and activator of transcription 3 (STAT3), Alanine--glyoxylate aminotransferase (AGXT), cytochrome P450 3A5(CYP3A5), CYP2E1 and CYP1A2 may be the major biomarkers for HgCl₂-induced hepatotoxicity, which involved chemical carcinogenesis, fatty acid metabolism, CYPs-mediated metabolism, GSH metabolism and others. Therefore, this study can provide scientific evidence for the biomarkers and mechanism of HgCl₂-induced hepatotoxicity.

Methylmercury's chemistry: From the environment to the mammalian brain

Methylmercury is a neurotoxicant that is found in fish and rice. MeHg's toxicity is mediated by blockage of -SH and -SeH groups of proteins. However, the identification of MeHg's targets is elusive. Here we focus on the chemistry of MeHg in the abiotic and biotic environment. The toxicological chemistry of MeHg is complex in metazoans, but at the atomic level it can be explained by exchange reactions of MeHg bound to -S(e)H with another free -S(e)H group (R¹S(e)-HgMe + R²-S(e)H ↔ R¹S(e)H + R²-S(e)-HgMe). This reaction was first studied by professor Rabenstein and here it is referred as the "Rabenstein's Reaction". The absorption, distribution, and excretion of MeHg in the environment and in the body of animals will be dictated by Rabenstein's reactions. The affinity of MeHg by thiol and selenol groups and the exchange of MeHg by Rabenstein's Reaction (which is a diffusion controlled reaction) dictates MeHg's neurotoxicity. However, it is important to emphasize that the MeHg exchange reaction velocity with different types of thiol- and selenol-containing proteins will also depend on protein-specific structural and thermodynamical factors. New experimental approaches and detailed studies about the Rabenstein's reaction between MeHg with low molecular mass thiol (LMM-SH) molecules (cysteine, GSH, acetyl-CoA, lipoate, homocysteine) with abundant high molecular mass thiol (HMM-SH) molecules (albumin, hemoglobin) and HMM-SeH (GPxs, Selenoprotein P, TrxR1-3) are needed. The study of MeHg migration from -S(e)-Hg- bonds to free -S(e)H groups (Rabenstein's Reaction) in pure chemical systems and neural cells (with special emphasis to the LMM-SH and HMM-S(e)H molecules cited above) will be critical to developing realistic constants to be used in silico models that will predict the distribution of MeHg in humans.

Chronic Kidney Disease and Exposure to Nephrotoxic Metals

Chronic kidney disease (CKD) is a common progressive disease that is typically characterized by the permanent loss of functional nephrons. As injured nephrons become sclerotic and die, the remaining healthy nephrons undergo numerous structural, molecular, and functional changes in an attempt to compensate for the loss of diseased nephrons. These compensatory changes enable the kidney to maintain fluid and solute homeostasis until approximately 75% of nephrons are lost. As CKD continues to progress, glomerular filtration rate decreases, and remaining nephrons are unable to effectively eliminate metabolic wastes and environmental toxicants from the body. This inability may enhance mortality and/or morbidity of an individual. Environmental toxicants of particular concern are arsenic, cadmium, lead, and mercury. Since these metals are present throughout the environment and exposure to one or more of these metals is unavoidable, it is important that the way in which these metals are handled by target organs in normal and disease states is understood completely.

The aging kidney and the nephrotoxic effects of mercury

Owing to advances in modern medicine, life expectancies are lengthening and leading to an increase in the population of older individuals. The aging process leads to significant alterations in many organ systems, with the kidney being particularly susceptible to age-related changes. Within the kidney, aging leads to ultrastructural changes such as glomerular and tubular hypertrophy, glomerulosclerosis, and tubulointerstitial fibrosis, which may compromise renal plasma flow (RPF) and glomerular filtration rate (GFR). These alterations may reduce the functional reserve of the kidneys, making them more susceptible to pathological events when challenged or stressed, such as following exposure to nephrotoxicants. An important and prevalent environmental toxicant that induces nephrotoxic effects is mercury (Hg). Since exposure of normal kidneys to mercuric ions might induce glomerular and tubular injury, aged kidneys, which may not be functioning at full capacity, may be more sensitive to the effects of Hg than normal kidneys. Age-related renal changes and the effects of Hg in the kidney have been characterized separately. However, little is known regarding the influence of nephrotoxicants, such as Hg, on aged kidneys. The purpose of this review was to summarize known findings related to exposure of aged and diseased kidneys to the environmentally relevant nephrotoxicant Hg.

Mechanisms involved in the transport of mercuric ions in target tissues

Mercury exists in the environment in various forms, all of which pose a risk to human health. Despite guidelines regulating the industrial release of mercury into the environment, humans continue to be exposed regularly to various forms of this metal via inhalation or ingestion. Following exposure, mercuric ions are taken up by and accumulate in numerous organs, including brain, intestine, kidney, liver, and placenta. In order to understand the toxicological effects of exposure to mercury, a thorough understanding of the mechanisms that facilitate entry of mercuric ions into target cells must first be obtained. A number of mechanisms for the transport of mercuric ions into target cells and organs have been proposed in recent years. However, the ability of these mechanisms to transport mercuric ions and the regulatory features of these carriers have not been characterized completely. The purpose of this review is to summarize the current findings related to the mechanisms that may be involved in the transport of inorganic and organic forms of mercury in target tissues and organs. This review will describe mechanisms known to be involved in the transport of mercury and will also propose additional mechanisms that may potentially be involved in the transport of mercuric ions into target cells.

A Study of the Complexation of Mercury(II) with Dicysteinyl Tetrapeptides by Electrospray Ionization Mass Spectrometry

In this study we evaluated a method for the characterization of complexes, formed in different relative ratios of mercury(II) to dicysteinyl tetrapeptide, by electrospray ionization orbitrap mass spectrometry. This strategy is based on previous successful characterization of mercury-dicysteinyl complexes involving tripeptides by utilizing mass spectrometry among other techniques. Mercury(II) chloride and a dicysteinyl tetrapeptide were incubated in a degassed buffered medium at varying stoichiometric ratios. The complexes formed were subsequently analyzed on an electrospray mass spectrometer consisting of a hybrid linear ion- and orbi- trap mass analyzer. The electrospray ionization mass spectrometry (ESI-MS) spectra were acquired in the positive mode and the observed peaks were then analyzed for distinct mercury isotopic distribution patterns and associated monoisotopic peak. This work demonstrates that an accurate stoichiometry of mercury and peptide in the complexes formed under specified electrospray ionization conditions can be determined by using high resolution ESI MS based on distinct mercury isotopic distribution patterns.

Toxicity of Glutathione-Binding Metals: A Review of Targets and Mechanisms

Mercury, cadmium, arsenic and lead are among priority metals for toxicological studies due to the frequent human exposure and to the significant burden of disease following acute and chronic intoxication. Among their common characteristics is chemical affinity to proteins and non-protein thiols and their ability to generate cellular oxidative stress by the best-known Fenton mechanism. Their health effects are however diverse: kidney and liver damage, cancer at specific sites, irreversible neurological damages with metal-specific features. Mechanisms for the induction of oxidative stress by interaction with the cell thiolome will be presented, based on literature evidence and of experimental findings.

Novel biomarkers of mercury-induced autoimmune dysfunction: a cross-sectional study in Amazonian Brazil

Mercury is a ubiquitous environmental contaminant, causing both neurotoxicity and immunotoxicity. Given its ability to amalgamate gold, mercury is frequently used in small-scale artisanal gold mining. We have previously reported that elevated serum titers of antinuclear autoantibodies (ANA) are associated with mercury exposures of miners in gold mining. The goal of this project was to identify novel serum biomarkers of mercury-induced immunotoxicity and autoimmune dysregulation. We conducted an analysis of serum samples from a cross-sectional epidemiological study on miners working in Amazonian Brazil. In proteomic screening analyses, samples were stratified based on mercury concentrations and ANA titer and a subset of serum samples (N=12) were profiled using Immune Response Biomarker Profiling ProtoArray protein microarray for elevated autoantibodies. Of the up-regulated autoantibodies in the mercury-exposed cohort, potential target autoantibodies were selected based on relevance to pro-inflammatory and macrophage activation pathways. ELISAs were developed to test the entire sample cohort (N=371) for serum titers to the highest of these autoantibodies (anti-glutathione S-transferase alpha, GSTA1) identified in the high mercury/high ANA group. We found positive associations between elevated mercury exposure and up-regulated serum titers of 3760 autoantibodies as identified by ProtoArray. Autoantibodies identified as potential novel biomarkers of mercury-induced immunotoxicity include antibodies to the following proteins: GSTA1, tumor necrosis factor ligand superfamily member 13, linker for activation of T cells, signal peptide peptidase like 2B, stimulated by retinoic acid 13, and interferon induced transmembrane protein. ELISA analyses confirmed that mercury-exposed gold miners had significantly higher serum titers of anti-GSTA1 autoantibody [unadjusted odds ratio=89.6; 95% confidence interval: 27.2, 294.6] compared to emerald miners (referent population). Mercury exposure was associated with increased titers of several autoantibodies in serum including anti-GSTA1. These proteins play a wide variety of roles, including as antioxidants, in the regulation of pro- and anti-inflammatory cytokines, as well as danger and oxidative stress signaling. Dysregulation of these proteins and pathways is believed to play a role in autoimmune diseases such as rheumatoid arthritis, Sjögren׳s syndrome, and multiple sclerosis. Taken together, these results suggest that mercury exposure can induce complex autoimmune dysfunction and the immunotoxic effects of this dysfunction may be measured by serum titers to autoantibodies such as anti-GSTA1.

Synthesis and ESI mass spectrometric analysis of the association of mercury(II) with multi-cysteinyl peptides

In order to gain more insight into the associations of mercury(II) with cysteinyl peptides, we investigated the effect of increasing cysteinyl residues on complex type formations. Three series of di-, tri-, and tetra-cysteinyl peptides, D[CGD]nCG (CP 2A, CP 3A, and CP 4A), E[CEG]nCG (CP 2B, CP 3B, and CP 4B) and E[CDG]nCG (CP 2C, CP 3C, and CP 4C), where n=1, 2, or 3, were prepared by microwave-assisted solid phase peptide synthesis. Complexes formed in different relative ratios of mercury(II) to cysteinyl peptides were characterized by electrospray orbitrap mass spectrometry utilizing complex specific mercury isotopic patterns. In equimolar mercury(II) to peptide ratio, all three series of di-, tri-, and tetra-cysteinyl peptides form predominantly the 1:1Hg(peptide) complex type, indicating that the intervening amino acid residues do not elicit preferential complex type formation. However, in non-equivalent mercury(II) to peptide ratio, the number of cysteinyl residues has a significant effect on the Hg:peptide stoichiometry in the complex formed. For example, in four times excess peptide, the 1:2Hg(peptide)2 and 1:1Hg(peptide) complexes are formed for di-cysteinyl peptides but not for the tri- and tetra-cysteinyl peptides. In contrast, the 2:1Hg2(peptide) and 1:1Hg(peptide) complexes are formed for the tri- and tetra-cysteinyl peptides. In excess mercury(II), CP 4C formed exclusively the 2:1Hg2(peptide) complex. The exact number of deprotonations observed for each complex could be derived from its signature mercury isotope pattern and monoisotopic peak mass. These multi-cysteinyl peptides present an attractive option for mercury chelation or environmental heavy metal remediation.

Formation of Mercury(II)-Glutathione Conjugates Examined Using High Mass Accuracy Mass Spectrometry

Maternal exposure to Hg(II) during pregnancy has been identified as a potential causal factor in the development of severe neurobehavioral disorders. Children with autism have been identified with lower reduced glutathione (GSH)/oxidized glutathione (GSSG) ratios, and GSH is known to strongly bind Hg(II). In order to gain insight into the mechanism by which GSH binds Hg(II), high resolution mass spectrometry coupled with tandem mass spectrometry was utilized to examine the conjugation process. While the 1:1 Hg(II):GSH conjugate is not formed immediately upon mixing aqueous solutions of Hg(II) and GSH, two species containing Hg(II) are observed: the 1:2 Hg(II):GSH conjugate, [(GS)₂Hg + H⁺], and a second Hg(II)-containing species around m/z 544. Interestingly, this species at m/z 544 decreases in time while the presence of the 1:1 Hg(II):GSH conjugate increases, suggesting that m/z 544 is an intermediate in the formation of the 1:1 conjugate. Experiments using the high mass accuracy capability of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry coupled to an electrospray ionization source indicate that the intermediate species is [GSH + HgCl]⁺, and not the 1:1 conjugate [Hg(GSH) − H + 2H₂O]⁺ postulated in previous literature. Further confirmation of [GSH + HgCl]⁺ is supported by collision of induced dissociation experiments, which show neutral loss of HCl from the intermediate and loss of the N- and C-terminal amino acids, indicating binding of Hg(II) at the Cys residue.

Evaluation of the association of mercury(II) with some dicysteinyl tripeptides

The present study was undertaken to gain insight into the associations of mercury(II) with dicysteinyl tripeptides in buffered media at pH 7.4. We investigated the effects of increasing the distance between cysteinyl residues on mercury(II) associations and complex formations. The peptide-mercury(II) formation constants and their associated thermodynamic parameters in 3-(N-morpholino)propanesulfonic acid (MOPS) buffered solutions were evaluated by isothermal titration calorimetry. Complexes formed in different relative ratios of mercury(II) to cysteinyl peptides in ammonium formate buffered solutions were characterized by LTQ Orbitrap mass spectrometry. The results from these studies show that n-alkyl dicysteinyl peptides (CP 1-4), and an aryl dicysteinyl peptide (CP 5) can serve as effective "double anchors" to accommodate the coordination sites of mercury(II) to form predominantly one-to-one Hg(peptide) complexes. The aryl dicysteinyl peptide (CP 5) also forms the two-to-two Hg(2)(peptide)(2) complex. In the presence of excess peptide, Hg(peptide)(2) complexes are also detected. Notably, increasing the distance between the ligating groups or "anchor points" in CP 1-5 does not significantly affect their affinity for mercury(II). However, the enthalpy change (ΔH) values (ΔH(1) ~ -91 kJ mol(-1) and ΔH(2) ~ -66 kJ mol(-1)) for complex formation between CP 4 and 5 with mercury(II) are about one and a half times larger than the related values for CP 1, 2 and 3 (ΔH(1) ~ -66 kJ mol(-1) and ΔH(2) ~ 46 kJ mol(-1)). The corresponding entropy change (ΔS) values (ΔS(1) ~ -129 J K(-1) mol(-1) and ΔS(2) ~ -116 J K(-1) mol(-1)) of the structurally larger dicysteinyl peptides CP 4 and 5 are less entropically favorable than for CP 1, 2 and 3 (ΔS(1) ~ -48 J K(-1) mol(-1) and ΔS(2) ~ -44 J K(-1) mol(-1)). Generally, these associations result in a decrease in entropy, indicating that these peptide-mercury complexes potentially form highly ordered structures. The results from this study show that dicysteinyl tripeptides are effective in binding mercury(II) and they are promising motifs for the design of multi-cysteinyl peptides for binding more than one mercury(II) ion per peptide.

Discovering mercury protein modifications in whole proteomes using natural isotope distributions observed in liquid chromatography-tandem mass spectrometry

The identification of peptides that result from post-translational modifications is critical for understanding normal pathways of cellular regulation as well as identifying damage from, or exposures to xenobiotics, i.e. the exposome. However, because of their low abundance in proteomes, effective detection of modified peptides by mass spectrometry (MS) typically requires enrichment to eliminate false identifications. We present a new method for confidently identifying peptides with mercury (Hg)-containing adducts that is based on the influence of mercury's seven stable isotopes on peptide isotope distributions detected by high-resolution MS. Using a pure protein and E. coli cultures exposed to phenyl mercuric acetate, we show the pattern of peak heights in isotope distributions from primary MS single scans efficiently identified Hg adducts in data from chromatographic separation coupled with tandem mass spectrometry with sensitivity and specificity greater than 90%. Isotope distributions are independent of peptide identifications based on peptide fragmentation (e.g. by SEQUEST), so both methods can be combined to eliminate false positives. Summing peptide isotope distributions across multiple scans improved specificity to 99.4% and sensitivity above 95%, affording identification of an unexpected Hg modification. We also illustrate the theoretical applicability of the method for detection of several less common elements including the essential element, selenium, as selenocysteine in peptides.

Glutathione complex formation with mercury(II) in aqueous solution at physiological pH

The mercury(II) complexes formed in neutral aqueous solution with glutathione (GSH, here denoted AH(3) in its triprotonated form) were studied using Hg L(III)-edge extended X-ray absorption fine structure (EXAFS) and (199)Hg NMR spectroscopy, complemented with electrospray ionization mass spectrometric (ESI-MS) analyses. The [Hg(AH)(2)](2-) complex, with the Hg-S bond distances at 2.325 ± 0.01 Å in linear S-Hg-S coordination, and the (199)Hg NMR chemical shift at -984 ppm, dominates except at high excess of glutathione. In a series of solutions with C(Hg(II)) ∼17 mM and GSH/Hg(II) mole ratios rising from 2.4 to 11.8, the gradually increasing mean Hg-S bond distance corresponds to an increasing amount of the [Hg(AH)(3)](4-) complex. ESI-MS peaks appear at -m/z values of 1208 and 1230 corresponding to the [Na(4)Hg(AH)(2)(A)](-) and [Na(5)Hg(AH)(A)(2)](-) species, respectively. In another series of solutions at pH 7.0 with C(Hg(II)) ∼50 mM and GSH/Hg(II) ratios from 2.0 to 10.0, the Hg L(III)-edge EXAFS and (199)Hg NMR spectra show that at high excess of glutathione (∼0.35 M) about ∼70% of the total mercury(II) concentration is present as the [Hg(AH)(3)](4-) complex, with the average Hg-S bond distance 2.42 ± 0.02 Å in trigonal HgS(3) coordination. The proportions of HgS(n) species, n = 2, 3, and 4, quantified by fitting linear combinations of model EXAFS oscillations to the experimental EXAFS data in our present and previous studies were used to obtain stability constants for the [Hg(AH)(3)](4-) complex and also for the [Hg(A)(4)](10-) complex that is present at high pH. For Hg(II) in low concentration at physiological conditions (pH 7.4, C(GSH) = 2.2 mM), the relative amounts of the HgS(2) species [Hg(AH)(2)](2-), [Hg(AH)(A)](3-), and the HgS(3) complex [Hg(AH)(3)](4-) were calculated to be 95:2:3. Our results are not consistent with the formation of dimeric Hg(II)-GSH complexes proposed in a recent EXAFS study.

Transport of inorganic mercury and methylmercury in target tissues and organs

Owing to the prevalence of mercury in the environment, the risk of human exposure to this toxic metal continues to increase. Following exposure to mercury, this metal accumulates in numerous organs, including brain, intestine, kidneys, liver, and placenta. Although a number of mechanisms for the transport of mercuric ions into target organs were proposed in recent years, these mechanisms have not been characterized completely. This review summarizes the current literature related to the transport of inorganic and organic forms of mercury in various tissues and organs. This review identifies known mechanisms of mercury transport and provides information on additional mechanisms that may potentially play a role in the transport of mercuric ions into target cells.

Mercury(II) complex formation with glutathione in alkaline aqueous solution

The structure and speciation of the complexes formed between mercury(II) ions and glutathione (GSH = L-glutamyl-L-cysteinyl-glycine) have been studied for a series of alkaline aqueous solutions (C(Hg2+) approximately 18 mmol dm(-3) and C(GSH) = 40-200 mmol dm(-3) at pH approximately 10.5) by means of extended X-ray absorption fine structure (EXAFS) and 199Hg NMR spectroscopy at ambient temperature. The dominant complexes are [Hg(GS)2](4-) and [Hg(GS)3](7-), with mean Hg-S bond distances of 2.32(1) and 2.42(2) angstroms observed in digonal and trigonal Hg-S coordination, respectively. The proportions of the Hg(2+)-glutathione complexes were evaluated by fitting linear combinations of model EXAFS oscillations representing each species to the experimental EXAFS spectra. The [Hg(GS)4](10-) complex, with four sulfur atoms coordinated at a mean Hg-S bond distance of 2.52(2) angstroms, is present in minor amounts (< 30%) in solutions containing a large excess of glutathione (C(GSH) > or = 160 mmol dm(-3)). Comparable alkaline mercury(II) cysteine (H2Cys) solutions were also investigated and a reduced tendency to form higher complexes was observed, because the deprotonated amino group of Cys(2-) allows the stable [Hg(S,N-Cys)2](2-) chelate to form. The effect of temperature on the distribution of the Hg(2+)-glutathione complexes was studied by comparing the EXAFS spectra at ambient temperature and at 25 K of a series of glycerol/water (33/67, v/v) frozen glasses with C(Hg2+) approximately 7 mmol dm(-3) and C(GSH) = 16-81 mmol dm(-3). Complexes with high Hg-S coordination numbers, [Hg(GS)3](7-) and [Hg(GS)4](10-), became strongly favored when just a moderate excess of glutathione (C(GSH) > or = 28 mmol dm(-3)) was used in the glassy samples, as expected for a stepwise exothermic bond formation. Addition of glycerol had no effect on the Hg(II)-glutathione speciation, as shown by the similarity of the EXAFS spectra obtained at room temperature for two parallel series of Hg(II)-glutathione solutions with C(Hg2+) approximately 7 mmol dm(-3), with and without 33% glycerol. Also, the 199Hg NMR chemical shifts of a series of 18 mmol dm(-3) mercury(II) glutathione solutions with 33% glycerol were not significantly different from those of the corresponding series in aqueous solution.

Counting sulfhydryls and disulfide bonds in peptides and proteins using mercurial ions as an MS-tag

Organic mercurial compounds are the most specific and sensitive reagents for reaction with the sulfhydryl groups (SHs) in peptides and proteins because of the strong mercury-sulfur affinity. Using the monofunctional organic mercury ion RHg(+) as a mass spectrometry (MS)-tag has the advantages of reacting with one sulfhydryl group, offering definite mass shift, and especially stable and characteristic nonradioactive isotopic distribution. Mass spectrometric analysis of derivatized sulfhydryls in peptides/proteins is thus an alternative for precisely counting the number of sulfhydryl groups and disulfide bonds (SS). Here the tags used include monomethylmercury chloride, monoethylmercury chloride, and 4-(hydroxymercuri) benzoic acid. The feasibility of this strategy is demonstrated using HPLC/ESI-MS to count SHs and SS in model peptides/proteins, i.e., glutathione, phytochelatins, lysozyme and beta-lactoglobulin, which contain increasing SHs and various SS linkages.

MRP2 and the DMPS- and DMSA-mediated elimination of mercury in TR(-) and control rats exposed to thiol S-conjugates of inorganic mercury

Cysteine (Cys) and homocysteine (Hcy)-S-conjugates of inorganic mercury (Hg2+) are transportable species of Hg2+ that are taken up readily by proximal tubular cells. The metal chelators, 2,3-dimercaptopropane-1-sulfonic acid (DMPS) and meso-2,3-dimercaptosuccinic acid (DMSA), have been used successfully to extract Hg2+ from these cells, presumably via the multidrug resistance protein (Mrp2). In the current study, we tested the hypothesis that Mrp2 is involved in the DMPS- and DMSA-mediated extraction of Hg2+ following administration of Hg2+ as an S-conjugate of Cys or Hcy. To test this hypothesis, control and TR(-) (Mrp2-deficient) rats were injected with 0.5 micromol/kg HgCl2 (containing 203Hg2+) conjugated to 1.25 micromol/kg Cys or Hcy. After 24 and 28 h, rats were treated with saline or 100 mg/kg DMPS or DMSA. Tissues were harvested 48 h after Hg2+ exposure. The renal and hepatic burden of Hg2+ was greater in saline-injected TR- rats than in corresponding controls. Accordingly, the content of Hg2+ in the urine and feces was less in TR- rats than in controls. Following treatment with DMPS or DMSA, the renal content of Hg2+ in both groups of rats was reduced significantly and the urinary excretion of Hg2+ was increased. In liver, the effect of each chelator appeared to be dependent upon the form in which Hg2+ was administered. In vitro experiments provide direct evidence indicating that DMPS and DMSA-S-conjugates of Hg2+ are substrates for Mrp2. Overall, these data support our hypothesis that Mrp2 is involved in the DMPS and DMSA-mediated extraction of the body burden of Hg2+.

Mercury exposure and public health

Mercury is a metal that is a liquid at room temperature. Mercury has a long and interesting history deriving from its use in medicine and industry, with the resultant toxicity produced. In high enough doses, all forms of mercury can produce toxicity. The most devastating tragedies related to mercury toxicity in recent history include Minamata Bay and Niagata, Japan in the 1950s, and Iraq in the 1970s. More recent mercury toxicity issues include the extreme toxicity of the dimethylmercury compound noted in 1998, the possible toxicity related to dental amalgams, and the disproved relationship between vaccines and autism related to the presence of the mercury-containing preservative, thimerosal.

A study of the glutathione metaboloma peptides by energy-resolved mass spectrometry as a tool to investigate into the interference of toxic heavy metals with their metabolic processes

To better understand the fragmentation processes of the metal-biothiol conjugates and their possible significance in biological terms, an energy-resolved mass spectrometric study of the glutathione conjugates of heavy metals, of several thiols and disulfides of the glutathione metaboloma has been carried out. The main fragmentation process of gamma-glutamyl compounds, whether in the thiol, disulfide, thioether or metal-bis-thiolate form, is the loss of the gamma-glutamyl residue, a process which ERMS data showed to be hardly influenced by the sulfur substitution. However, loss of the gamma-glutamyl residue from the mono-S-glutathionyl-mercury (II) cation is a much more energetic process, possibly pointing at a strong coordination of the carboxylic group to the metal. Moreover, loss of neutral mercury from ions containing the gamma-glutamyl residue to yield a sulfenium cation was a much more energetic process than those not containing them, suggesting that the redox potential of the thiol/disulfide system plays a role in the formal reduction of the mercury dication in the gas phase. Occurrence of complementary sulfenium and protonated thiol fragments in the spectra of protonated disulfides of the glutathione metaboloma mirrors the thiol/disulfide redox process of biological importance. The intensity ratio of the fragments is proportional to the reduction potential in solution of the corresponding redox pairs. This finding has allowed the calculation of the previously unreported reduction potentials for the disulfide/thiol pair of cysteinylglycine, thereby confirming the decomposition scheme of bis- and mono-S-glutathionyl-mercury (II) ions. Finally, on the sole basis of the mass spectrometric fragmentation of the glutathione-mercury conjugates, and supported by independent literature evidence, an unprecedented mechanism for mercury ion-induced cellular oxidative stress could be proposed, based on the depletion of the glutathione pool by a catalytic mechanism acting on the metal (II)-thiol conjugates and involving as a necessary step the enzymatic removal of the glutamic acid residue to yield a mercury (II)-cysteinyl-glycine conjugate capable of regenerating neutral mercury through the oxidation of glutathione thiols to the corresponding disulfides.

Determination and characterization of phytochelatins by liquid chromatography coupled with on line chemical vapour generation and atomic fluorescence spectrometric detection

Liquid chromatography (LC) coupled on line with UV/visible diode array detector (DAD) and cold vapour generation atomic fluorescence spectrometry (CVGAFS) has been developed for the speciation, determination and characterization of phytochelatins (PCs). The method is based on a bidimensional approach, e.g. on the analysis of synthetic PC solutions (apo-PCs and Cd(2+)-complexed PCs) (i) by size exclusion chromatography coupled to UV diode array detector (SEC-DAD); (ii) by the derivatization of PC -SH groups in SEC fractions by p-hydroxymercurybenzoate (PHMB) and the indirect detection of PC-PHMB complexes by reversed phase liquid chromatography coupled to atomic fluorescence detector (RPLC-CVGAFS). MALDI-TOF/MS (matrix assisted laser desorption ionization time of flight mass spectrometry) analysis of underivatized synthetic PC samples was performed in order have a qualitative information of their composition. Quantitative analysis of synthetic PC solutions has been performed on the basis of peak area of PC-PHMB complexes of the mercury specific chromatogram and calibration curve of standard solution of glutathione (GSH) complexed to PHMB (GS-PHMB). The limit of quantitation (LOQ) in terms of GS-PHMB complex was 90 nM (CV 5%) with an injection volume of 35 microL, corresponding to 3.2 pmol (0.97 ng) of GSH. The method has been applied to analysis of extracts of cell cultures from Phaeodactylum tricornutum grown in Cd-containing nutrient solutions, analysed by SEC-DAD-CVGAFS and RPLC-DAD-CVGAFS.

Electrospray ionization and triple quadrupole tandem mass spectrometry study of some biologically relevant homo- and heterodimeric cadmium thiolate conjugates

A series of 19 compounds of general formula R1S-Cd-SR2, R1, and R2, being some biologically relevant thiol amino acids and peptides, were prepared by direct reaction of cadmium(II) ions and thiols in water at millimolar concentration. The obtained products were characterized by electrospray ionization and triple quadrupole tandem mass spectrometry. The source spectra of stoichiometric 1:2 Cd-thiol systems containing either an individual thiol or equimolar mixtures of two different thiols featured several Cd-containing signals, although at much lesser intensity than in the previously reported experiments with mercury(II) (J. Am. Soc. Mass Spectrom. 2004, 15, 288-300). Also, the relative intensity of the homo- and heterodimeric thiolates were significantly different from the theoretically expected 1:2:1 ratio, thus pointing at some degree of discrimination between the different thiols. In particular, homo-cysteine showed much less reactivity than cysteine, and penicillamine and cysteine methyl ester much less than the free amino acid. The fragment spectra show structure-specific ions for the different ligands bound to the metal ion and allow a stand-alone determination of the connectivity also of isomeric pairs. The fragmentation pathways are similar to those observed for the corresponding mercury(II) analogues, with the addition of further intense and specific fragments, one formally carrying a Cd-bound OH ligand and one connected as a five-membered oxazolone carrying a cadmium-bis-thiolate side chain, both formed with a high intensity. Energy-resolved fragmentation data show that metal-free ions can be generated from cysteine but not from glutathione conjugates and point to the possibility of unveiling differences in the biochemical behavior of the conjugates of different heavy metals through the detailed study of their mass spectrometric fragmentation.

Molecular and ionic mimicry and the transport of toxic metals

Despite many scientific advances, human exposure to, and intoxication by, toxic metal species continues to occur. Surprisingly, little is understood about the mechanisms by which certain metals and metal-containing species gain entry into target cells. Since there do not appear to be transporters designed specifically for the entry of most toxic metal species into mammalian cells, it has been postulated that some of these metals gain entry into target cells, through the mechanisms of ionic and/or molecular mimicry, at the site of transporters of essential elements and/or molecules. The primary purpose of this review is to discuss the transport of selective toxic metals in target organs and provide evidence supporting a role of ionic and/or molecular mimicry. In the context of this review, molecular mimicry refers to the ability of a metal ion to bond to an endogenous organic molecule to form an organic metal species that acts as a functional or structural mimic of essential molecules at the sites of transporters of those molecules. Ionic mimicry refers to the ability of a cationic form of a toxic metal to mimic an essential element or cationic species of an element at the site of a transporter of that element. Molecular and ionic mimics can also be sub-classified as structural or functional mimics. This review will present the established and putative roles of molecular and ionic mimicry in the transport of mercury, cadmium, lead, arsenic, selenium, and selected oxyanions in target organs and tissues.

Characterization of the disulfides of bio-thiols by electrospray ionization and triple-quadrupole tandem mass spectrometry

Glutathione and other intracellular low molecular mass thiols act both as the major endogenous antioxidant and redox buffer system and, as recently highlighted, as an important regulator of cellular homeostasis. Such cellular functions are mediated by protein thiolation, a newly recognized post-translational modification which involves the formation of mixed disulfides between GSH and key disulfide-linked Cys residues in the native protein structure. It is also well known that thiol-seeking heavy metals, such as mercury, cadmium and lead, may interfere in this regulatory system, thus disrupting the cellular functioning. To identify such mixed disulfides in order to investigate their biological role, 15 homo- and heterodimeric disulfides were prepared by air oxidation of binary mixtures containing cysteine, homocysteine, penicillamine, N-acetylcysteine, N-acetylpenicillamine and glutathione and their protonated molecules were characterized by mass spectrometry. Collisionally activated unimolecular decomposition of protonated homo- and heterodimeric disulfides generated by electrospray ionization gives rise to fission of the disulfide system both between the two sulfur atoms and across the C--S bonds, to yield structurally specific fragments which allow one to define the structure of the compounds and to discriminate between isomeric compounds. Fission between the sulfur atoms yields a pair of R--S(+) ions and, in some cases, also the complementary fragments corresponding to the protonated amino acids. Fission across the C--S bonds mainly occurs in the disulfides of N-acetylcysteine and N-acetylpenicillamine and gives rise to non-S-containing fragments formally similar to those obtained from some mercapturic acids. The complementary fragments, formally connected as R--S--S(+) ions are also observed. Fragmentation of glutathione disulfides mainly shows the characteristic loss of the terminal gamma-linked glutamic acid and little, if any, fragmentation of the disulfide system.