Antagonistic interaction of mercury and selenium in a marine fish is dependent on their chemical species.

It is well-known that selenium (Se) shows protective effects against mercury (Hg) bioaccumulation and toxicity, but the underlying effects of Se chemical species, concentration, and administration method are poorly known. In this study, we conducted laboratory studies on a marine fish Terapon jurbua to explain why Hg accumulation is reduced in the presence of Se observed in field studies. When Se and Hg were administrated concurrently in the fish diets, different Se species including selenite, selenate, seleno-dl-cystine (SeCys), and seleno-dl-methionine (SeMet) affected Hg bioaccumulation differently. At high concentration in fish diet (20 µg g(-1) normally), selenate and SeCys significantly reduced the dietary Hg(II) assimilation efficiency (AE) from 38% to 26%. After the fish were pre-exposed to dietary selenite or SeMet (7 µg g(-1) normally) for 22 days with significantly elevated Se body concentrations, the Hg(II) AEs were pronouncedly reduced (from 41% to 15-26%), whereas the dissolved uptake rate constant and elimination rate constant were less affected. In contrast to Hg(II), all the MeHg biokinetic parameters remained relatively constant whether Se was administrated simultaneously with the fish diet or when the fish were pre-exposed to Se with elevated body concentrations. Basic biokinetic measurements thus revealed that Se had direct interaction with Hg(II) during dietary assimilation rather than with MeHg and that different Se species had variable effects on Hg assimilation.

Effects of selenite and chelating agents on mammalian thioredoxin reductase inhibited by mercury: implications for treatment of mercury poisoning.

Mercury toxicity is a highly interesting topic in biomedicine due to the severe endpoints and treatment limitations. Selenite serves as an antagonist of mercury toxicity, but the molecular mechanism of detoxification is not clear. Inhibition of the selenoenzyme thioredoxin reductase (TrxR) is a suggested mechanism of toxicity. Here, we demonstrated enhanced inhibition of activity by inorganic and organic mercury compounds in NADPH-reduced TrxR, consistent with binding of mercury also to the active site selenolthiol. On treatment with 5 µM selenite and NADPH, TrxR inactivated by HgCl(2) displayed almost full recovery of activity. Structural analysis indicated that mercury was complexed with TrxR, but enzyme-generated selenide removed mercury as mercury selenide, regenerating the active site selenocysteine and cysteine residues required for activity. The antagonistic effects on TrxR inhibition were extended to endogenous antioxidants, such as GSH, and clinically used exogenous chelating agents BAL, DMPS, DMSA, and α-lipoic acid. Consistent with the in vitro results, recovery of TrxR activity and cell viability by selenite was observed in HgCl(2)-treated HEK 293 cells. These results stress the role of TrxR as a target of mercurials and provide the mechanism of selenite as a detoxification agent for mercury poisoning.

The role of thiol-reducing agents on modulation of glutamate binding induced by heavy metals in platelets.

In the present study, we investigated if thiol-reducing agents are capable of altering mercury (Hg2+), lead (Pb2+) and cadmium (Cd2+) effects on platelet glutamatergic system. Dimercaprol (BAL), a dithiol chelating agent therapeutically used for the treatment of heavy metals poisoning, was capable of protecting the [3H]-glutamate binding against the effects caused by Pb2+ and Hg2+. 2,3-Dimercaptopropane-1-sulfonic acid (DMPS), another dithiol-reducing chelating agent, was capable of protecting the effect caused by Cd2+, Pb2+ and Hg2+. The similar effect was observed with addition of dithiothreitol (DTT) on [3H]-glutamate binding in human platelets. Dithiol-reducing agents (BAL, DMPS and DTT) alone did not alter [3H]-glutamate binding. In contrast, reduced glutathione (GSH), a monothiol-reducing agent, caused a significant inhibition on [3H]-glutamate binding at all concentrations tested. GSH did not modify heavy metal effects on [3H]-glutamate binding in platelets. The findings of the present investigation indicate that dithiol-reducing agents are capable of altering Hg2+, Pb2+ and Cd2+ effects on platelet glutamatergic system. In vitro data on chelating-metal interactions provide only an estimated guide to the treatment of heavy metal poisoning. Consequently, more studies in intoxicated patients are necessary to determine the precise use of the peripheral models and chelating agents.

A study of Se-Hg antagonism in Glycine max (soybean) roots by size exclusion and reversed phase HPLC-ICPMS.

An attempt was made to study selenium (Se) and mercury (Hg) interactions in plants, specifically soybean (Glycine max), by inductively coupled plasma mass spectrometric detection. Greenhouse-cultivated plants were subjected to treatment with different regimens of Se and Hg and analyzed for their metabolized species in roots, stems, leaves, pods and beans. Most of the water-soluble Hg was found to be localized in the roots in association with Se in a high molecular weight entity, as identified by size exclusion chromatography. This entity was also extracted in protein specific isolate, but it resisted enzymatic breakdown. Complete breakdown of this high molecular weight species was accomplished by acid hydrolysis. Optimization of the conditions for acid hydrolysis is discussed. Hg and Se species found in root extract were studied by ion-pairing chromatography. In a sub-study, the Se distribution pattern was found to be unaffected by the presence of Hg, but the amount of Se assimilated was found to be higher in plants coexposed to Hg.

Selenium long-term administration and its effect on mercury toxicity.

An in vivo experiment was conducted to assess selenium bioaccumulation and bioaccessibility through the food chain and its effect on Hg toxicity. For this purpose 72 chickens were fed under different controlled conditions. Chickens were exposed to a common basal diet or a diet supplemented with Hg(II), MeHg, and Se(IV). Enzymatic digestion (feed, chicken muscle, liver, and kidney) as well as simulated human gastric and intestinal digestion (chicken muscle) led to the identification of selenomethionine (SeMet) in all the samples analyzed. Therefore, although chickens have no efficient mechanism for SeMet synthesis they can be considered as a source of SeMet due to its diet and the plant-animal food chain. The kidneys were the target organ for both total Se and SeMet in chickens (1604 +/- 136 and 128 +/- 6 microg kg(-1), respectively), but the greatest body store, among the tissues studied, was the muscle in both cases (84-96% of total Se). Long-term administration of inorganic and organic mercury did not alter SeMet distribution significantly. The antagonistic effect of Se on Hg toxicity by favoring MeHg demethylation is discussed.

[Antagonistic effects of selenium on the expression of c-fos in central nerval system of rat included by mercury contaminated rice].

The objective of this paper is to study the antagonisms between selenium and mercury and the effect of different species mercury on the brain injury. The expression of c-fos mRNA and c-FOS protein in rat brain induced by Hg-contaminated rice was observed by using reverse transcriptions polymerase chain reaction (RT-PCR) and immunocytochemical methods. The results show the Hg-contaminated rice induced significantly the expression of c-fos mRNA and c-FOS protein; selenium could antagonize mercury accumulative level in brain. Antagonistic effects of selenium on the expression of c-fos included by mercury and the molecule mechanism of the antagonisms between selenium and mercury was probed, too.

2,3-Dimercapto-1-propanol does not alter the porphobilinogen synthase inhibition but decreases the mercury content in liver and kidney of suckling rats exposed to HgCl2.

Heavy metals have received great attention as environmental pollutants mainly because once introduced in the biological cycle they are incorporated in the food chain. Especially the mercury toxicity due to a diversity of effects caused by different chemical species should be emphasized. Heavy metal intoxication has been treated with chelating agents such as 2,3-dimercapto-1-propanol (BAL). However, the efficacy of this treatment is questionable due to the lack of specific effect on the toxic metal. The present study examined the effects of HgCl2 exposure (five doses of 5.0 mg/kg between ages 8 to 12 days) on physiological parameters, on porphobilinogen synthase activity, and on mercury content in liver, kidneys and brain from suckling rats. The effect of BAL (one dose of 12.5-75 mg/kg) applied 24 hr after mercury intoxication on these parameters was also investigated. The results demonstrate that HgCl2 intoxication induced a decrease of corporal weight gain as well as brain weight and an increase in renal weight. The inhibition of porphobilinogen synthase from liver and kidney, is still significant and was not modified by subsequent BAL treatment. However, BAL altered two effects induced by mercury: increase in death percentage and decrease in mercury contents in liver and kidney. The increase of mortality induced by mercury was not promoted by metal redistribution to brain nor by the increase of porphobilinogen synthase inhibition induced by metal. More investigations are necessary to determine if the different effects of BAL on intoxication by metals are possibly related to other tissues and/or if the probable metal-chelating complex formed is more toxic than the metal itself.

Hg2+ and small-sized polyethylene glycols have inverse effects on membrane permeability, while both impair neutrophil cell motility.

Toxic effects after exposure to mercury are well documented in human. Little is, however, known about how Hg(2+) affect host defense in general and neutrophil functions in particular. We show here that exposure of human neutrophils to HgCl(2) dose-dependently impairs chemoattractant-stimulated motility. Long-term exposure (5-10 min) to Hg(2+) yields a rapid influx of extracellular Ca(2+) followed by leakage of cytosolic fluorophores, as assessed using fura-2 and ratio imaging microscopy. The inhibition on motility was partly reversible, since pre-treated neutrophils placed in an Hg(2+)-free environment displayed higher migration rates. The Hg(2+)-induced fluxes were prevented by addition of small-sized polyethylene glycols (PEG 200-400), which also dose-dependently inhibited neutrophil transmigration. Localized, minute micropipette additions of Hg(2+) or PEG caused retraction of the leading edge and redirection of cell migration. Since Hg(2+) increases and PEGs decrease membrane permeability in a partially competitive manner, we suggest that the known aquaporin-inhibitor Hg(2+) alters membrane permeability by affecting the bidirectional flux through the leukocyte aquaporin-9 (AQP9) while small-sized PEGs yield decreased membrane permeability by becoming trapped in the promiscuous channel. The local additions of Hg(2+) or PEG probably force other cell regions to take over from those with blocked AQPs. Hence, the cells turn direction of motility away from the micromanipulator needle.

Modification of mercury toxicity by selenium: practical importance?

The interaction between mercury and selenium may involve a variety of toxicologically and biochemically distinct processes. In this paper, the interaction between inorganic mercury and sodium selenite, the interaction most extensively studied, as well as the interaction between methylmercury (MeHg) and selenium, the interaction perhaps most significant for non-occupational human populations, will be discussed. It has been shown that the former interaction can be understood as a modification of the kinetic behavior of inorganic mercury by selenite, but this interaction may occur only under very limited conditions. On the other hand, the mechanism of the latter interaction is largely unknown, and kinetic modification appears to play only a minor role. An interaction between MeHg and seleno-proteins or a possible interaction between the inorganic mercury, resulting from the demethylation of MeHg, and the selenium may be important. Compared to the experimental findings, little evidence of the toxicological modification of MeHg by selenium was obtained in epidemiological studies.

Effects of selenium and mercury on the enzymatic activities and lipid peroxidation in brain, liver, and blood of rats.

Recent studies have reported on the toxicity and related oxidative stress of selenium and mercury. The present study compares the effects of Se as sodium selenite (Na2SeO3) and Hg as mercuric chloride (HgCl2) separately and in combination. Rats received repeated oral doses of Se (0.5 micromol/ml), Hg (0.5 micromol/ml), or Se in combination with Hg (0.5 micromol/ml of each) for 5 consecutive days. Rat serum, brain and liver samples were collected for biochemical assays. The following biochemical alterations occurred in response to Hg treatment: protein content (brain and liver), acetylcholinesterase (AChE) (brain and serum), acid and alkaline (AcP and AlP) phosphatases (plasma and liver) and glutathione S-transferase (GST) (plasma and liver) activities were significantly (P<0.05) decreased, while lactate dehydrogenase (LDH) (plasma, brain and liver), aspartate and alanine aminotransferase (AST, ALT) (serum and liver) activities were significantly increased. Thiobarbituric acid reactive substances (TBARS) was significantly increased in brain and liver. Effect of Se alone included decreased AcP, AlP and GST (serum and liver) activities. However, LDH (serum, brain and liver) and AST (liver) and TBARS (brain and liver) increased. Selenium in combination with Hg partially or totally alleviated the toxic effects of Hg on different studied enzymes. It is concluded that Se could be able to antagonize the toxic effects of mercury.

Effects of mercury on the isolated heart muscle are prevented by DTT and cysteine.

The protective effects of dithiothreitol (DTT, 50 microM) and cysteine (CYS, 100 microM) against toxic effects of HgCl2 (1, 2.5, 5, and 10 microM) were studied in isolated, isometrically contracting rat papillary muscles. Force reduction promoted by Hg2+ was prevented by both DTT and CYS. Also, after both treatments, no significant changes in dF/dt were observed. A progressive reduction in the time to peak tension was observed when increased concentrations of HgCl2 were used after CYS and DTT treatment. This was an indication that the enhancement of calcium release from the sarcoplasmic reticulum produced by mercury was not affected by DTT and CYS. Tetanic contractions were also studied. After treatment with DTT or CYS tetanic tension did not change. No significant reduction of tetanic tension was observed during treatment with 1 microM Hg2+ but its reduction was observed after 5 microM Hg2+. Myosin ATPase activity was also affect by Hg2+, being completely blocked by 1 microM Hg2+ and reduced by 50% with 0.15 microM Hg2+. Full activity was restored by using 500 nM DTT. These findings suggest that several but not all toxic effects of Hg2+ on the mechanical activity of the heart muscle are prevented by protectors of SH groups such as DTT and CYS. The enhancement of the Ca2+ release from the sarcoplasmic reticulum by Hg2+ during activation was not affected by prior treatment with DTT and CYS, suggesting that interactions with SH groups may not be important for the activation of the Ca2+ channel of the sarcoplasmic reticulum.

Captopril and glutathione inhibit the superoxide dismutase activity of Hg (II).

The purpose of this study was to determine if captopril, an angiotensin-converting enzyme inhibitor, and glutathione could interact with mercuric ions and so modify the catalytic dismutation of superoxide carried out by this metal. With an assay that generates superoxide anion radicals without the intervention of metal ions increasing concentrations of both reagents progressively inhibited the breakdown of superoxide brought about by mercury. Maximum inhibition was attained with a molar ratio of captopril (glutathione): Hg (II) = 1. These results may help to explain the protective and/or antioxidative effect of thiol compounds during mercury intoxication.

Thiol compounds inhibit mercury-induced immunological and immunopathological alterations in susceptible mice.

In vitro mercury induces a high proliferative response in splenic lymphocytes and in vivo it induces a systemic autoimmune disease in susceptible mouse strains. This disease is characterized by increased serum levels of IgE and IgG1 antibodies, by the production of anti-nucleolar antibodies and by the formation of renal immune complex deposits. We have previously found that the presence of 2-mercaptoethanol (2-ME) inhibited mercury-induced cell proliferation in vitro. In this study, we tested the effects of four other thiol compounds, namely dithiothreitol (DTT), L-cysteine, meso-2,3-dimercaptosuccinic acid (meso-DMSA) and 2,3-dimercapto-1-propanesulfonic acid, Na salt (DMPS) on mercury-induced immunological changes both in vitro and in vivo. We found that in vitro, the addition of all thiol compounds abrogated mercury-induced cell aggregation and proliferation. In vivo, injection of meso-DMSA and/or DMPS (s.c. or i.p.) immediately following exposure to mercury markedly decreased IgG1 synthesis in spleen cells and serum IgE levels in mercury-susceptible SJL mice. Treatment with DMPS also prevented mercury-induced IgG1 anti-nucleolar antibody synthesis and the development of mesangial IgG1 immune complex deposits in SJL mice.

The role of G proteins in the activity and mercury modulation of GABA-induced currents in rat neurons.

The role of G proteins in the functional modulation and potentiation by mercury chloride of the GABA(A) receptor-channel complex in rat dorsal root ganglion neurons was studied by using the whole-cell patch clamp technique. Stimulation of Gs proteins by application of GTP-gamma-S in the patch pipette or by incubation of neurons with cholera toxin reduced GABA-induced currents, suggesting modulation of GABA-induced currents via a Gs-protein-coupled pathway. GDP-beta-S in the pipette solution or pretreatment of dorsal root ganglion neurons with pertussis toxin suppressed GABA-induced currents, suggesting that basal Gi/Go-protein activity positively modulates the GABA(A) receptor-channel complex. Mercury chloride potentiation of GABA-activated currents was blocked by application of GTP-gamma-S in the patch pipette or by incubation of neurons with cholera toxin. Mercury chloride potentiation of GABA-activated currents was blocked by application of GDP-beta-S in the patch pipette or by incubation of neurons with pertussis toxin. G proteins, probably Gi/Go proteins, underlie the mercury chloride potentiation of GABA-induced currents.

Suppression of the thermosensitive replication phenotype of the derivative plasmid of pI9789::Tn552 in Staphylococcus aureus may involve integration of the plasmid into the host chromosome.

Plasmid-chromosome co-integration was found to be the mechanism of choice to overcome thermosensitivity of replication of the plasmid pS1 in PS80d and RN4220 strains of Staphylococcus aureus. The integration of the plasmid was sometimes accompanied by deletion of a specific section of the plasmid pS1 in PS80d. Growth of bacteriophage on strains containing the integrated plasmid and the subsequent use of the phage in transduction gave transductants containing plasmids that had regained their replication thermosensitivity. These plasmids had not acquired any detectable chromosomal DNA. The 16-kb EcoRI fragment of the PS80d chromosome that hybridizes to pS1 is the target for recombination in many cases, but apparently other sites are also used. This fragment contains sequence homologous to parts of the transposon Tn552 and it is probable that site-specific recombination is involved in the integration. The possible mechanisms for the integrations and the deletions are discussed.

Mercury uptake by primary cultures of rat renal cortical epithelial cells. I. Effects of cell density, temperature, and metabolic inhibitors.

Factors affecting the renal uptake of inorganic mercury were investigated using primary cultures of rat renal cortical epithelial (RCE) cells under protein- and amino acid-free conditions. The cells were isolated from kidneys of adult rats and cultured. Confluence of culture (cell density), monitored morphologically and by total protein content, was achieved on Day 5. The RCE cells were incubated with 1 microM Hg at 37 degrees C for 30 min, followed by washing with phosphate-buffered saline containing various chelating agents (i.e., EGTA, PEN, DMSA, and BAL) to remove the surface-bound, noninternalized Hg. A substantial portion of Hg was bound to the cell surface. The removal of Hg from these binding sites was dependent on the stability constants of the chelating agents for Hg and lipophilic BAL removed the most Hg. Hg accumulation by the cells was dependent on cell density and decreased as the cell culture became confluent, possibly due to the formation of tight junctions resulting in a majority of the Hg transport occurring through the apical membrane. As measured after BAL washing, metabolic inhibitors, NaF, DNP, and ouabain decreased Hg accumulation by 28% and low temperature (4 degrees C) decreased it by 62%. Dependence of Hg uptake on metabolic energy and temperature suggests that a part of Hg is transported via active transport system. The pronounced decrease of Hg uptake at 4 degrees C indicates that, in addition to active transport, Hg transport also involves simple diffusion, some of which is dependent on membrane fluidity. It is concluded that Hg transport in RCE cells through the apical membrane occurs mainly by diffusion, and to a smaller extent by active transport.

Mercury inhibition at the donor side of photosystem II is reversed by chloride.

Mercury is an environmental contaminant that strongly inhibits photosynthetic electron transport, photosystem II being the most sensitive target. We investigated in greater detail the effect of mercury using photosystem II submembrane fractions of higher plants. Oxygen evolution was strongly inhibited and variable chlorophyll fluorescence was severely quenched by mercury. Chloride, an inorganic cofactor known to be essential for the optimal function of photosystem II, significantly reversed the inhibitory effect of mercury. However, calcium, another essential cofactor, showed no reversal capacity. It is concluded that on the donor side of PSII, mercury exerts its action by perturbing chloride binding and/or function. Considering the exceptional affinity of mercury for sulfhydryl groups of proteins, the results suggest the implication of cystein residue(s) in maintaining structural and functional integrity of photosystem II.

In vitro interaction of heavy metals with ouabain receptors in rat brain microsomes.

This study investigates the influence of heavy metals on ouabain-binding in presence of thiol (sulfhydryl) compounds. The data on in vitro effects of mercury (Hg), lead (Pb) and cadmium (Cd) showed significant inhibition of 3H-ouabain binding to microsomal membrane in a concentration-dependent manner. Maximum inhibition of 3H-ouabain binding was observed at 1 microM for Hg and 100 microM each for Pb and Cd. Preincubation with monothiol (L-cysteine or glutathione) or dithiol (dithiothreitol) protected inhibition of 3H-ouabain binding to the membranes by Hg or Pb. Dithiol but not monothiols partially protected Cd-inhibition. The present data confirm that the heavy metals interact with ouabain receptors in a manner similar to SH-blocking agents and protection of metal-inhibited 3H-ouabain binding by thiol compounds is metal specific.

Change in toxicity effect of mercury at static concentration to Chlorella vulgaris with addition of organic carbon sources.

The effect of mercury on Chlorella vulgaris is influenced to a large extent by organic carbon sources like glucose, glutamate, and 2-oxoglutarate which is exhibited through changes in growth and metabolic processes of the alga in the presence of static dose (0.200 +/- 0.004, p = 0.05) of Hg2+. Addition of the three carbon sources to the Hg(2+)-amended Chu No 10+ medium resulted in an acceleration of pigment biomass, growth, and protein content of the green alga. Glucose at all concentrations was found to be more effective than the other two in reducing the mercury toxicity to C. vulgaris. However, the detoxifying effect is dependent on the type of carbon sources, their concentration and days of incubation. Glucose at 5 mg/l significantly reduced Hg2+ toxicity while glutamate and 2-oxoglutarate, at this concentration, did not show such capacity. Similarly the degree of significance of ameliorative effect increased with increase in incubation period which, however, varied with the carbon source. Availability of additional energy and formation of non-toxic or less toxic mercury are the probable causes of reduction in toxicity effects.

Studies on mercury-induced myotonia in the mouse diaphragm.

Muscle contracture and myotonia of mouse diaphragm, induced by HgCl2, were studied. HgCl2 induced myotonia of mouse diaphragm only on condition that the Hg2+ contracture was inhibited by 0.005-0.01 mM NaCN. A higher concentration of 0.05 mM NaCN abolished both the Hg2+ contracture and Hg2+ myotonia. This finding suggests that the Hg2+ contracture masked the Hg2+ myotonia which was less sensitive to the inhibitory action of NaCN. This differential inhibitory action of NaCN on Hg2+ contracture from Hg2+ myotonia implies a possibility that NaCN antagonized the actions of HgCl2 not only through a simple chemical interaction. Hg2+ myotonia was characterized by an increase in contractile amplitude and a prolongation of contractile duration which were associated with stimulus-bound repetitive action potentials and an increase in membrane input resistance. A low Cl- medium as well as a Cl- channel blocker (9-anthracene carboxylic acid) not only by themselves induced myotonia, but also antagonized Hg2+ myotonia. Thus, Hg2+ appeared to mimic the Cl- channel blocker in inducing myotonia through a blockade of the Cl- channel. K+ channel blockers (4-aminopyridine, uranyl nitrate and tetraethylammonium chloride), as well as low (0.25 mM) Ca2+ Krebs, augmented Hg2+ myotonia while ATP-sensitive K+ channel blockers (tolbutamide and glibenclamide) antagonized Hg2+ myotonia (in the presence of NaCN). Since glibenclamide did not affect myotonia induced by a Cl- channel blocker, it was suggested that glibenclamide inhibited the Hg2+ myotonia through an interaction either directly or indirectly with NaCN on the sarcolemma. All of these findings suggest that K+ channels (delayed rectifier and Ca(2+)-activated K+ channel) functionally cooperated with the Cl- channel of the sarcolemma in the regulation of the skeletal muscle contraction. In this study, K+ channel blockers synergistically cooperated with Cl- channel blockers in inducing myotonia of the mouse diaphragm, while an ATP-sensitive K+ channel blocker exerted only an opposite effect on NaCN. Ca2+ appeared to play an important role in regulating the ionic channel activities, especially the Cl- channel, since low Ca2+ markedly potentiated not only Hg2+ but also low Cl- in inducing myotonia.