Mercury binding to proteins disclosed by ESI MS experiments: The case of three organomercurials

Solution interactions of three organomercury compounds, i.e., methylmercury chloride, thimerosal and phenylmercury acetate, with a group of biochemically relevant proteins, namely cytochrome c (Cyt c), ribonuclease A (RNase A), carbonic anhydrase I (hCA I), superoxide dismutase (SOD), and serum albumin (HSA), were investigated using an established ESI MS approach. Temporal analysis of sample aliquots provided insight into the binding kinetics, while comparative analysis of the obtained mass spectra disclosed adduct formation of each mercurial with the tested proteins and the relative abundance of the species. The three organomercurials bind, exclusively and tightly, to free cysteine residues as no binding was observed in the case of proteins lacking such groups. hCA I, SOD and HSA formed distinct mercury adducts, preserving the Hg bound alkyl/aryl ligands; yet, the three organomercurials displayed significant differences in reactivity in relation to their chemical structure. The investigation was then extended to analyze the reactions with the C-terminal dodecapeptide of the enzyme human thioredoxin reductase, which contains a characteristic selenol-thiol moiety: tight Hg binding was observed. Notably, this peptide was able to remove effectively and completely the alkyl/aryl ligands of the three tested organomercurials; this behavior may be relevant to the detoxification mechanism of organomercurials in mammals. Finally, a competition experiment was carried out to establish whether protein bound mercury centers may be displaced by other competing metals. Interestingly, and quite unexpectedly, we observed that a protein bound mercury fragment may be partially displaced from its coordination site in hCA I by the medicinal gold compound auranofin.

Covalent inhibition of P. falciparum ferredoxin-NADP+ reductase: Exploring alternative strategies for the development of new antimalarial drugs

The protozoan Plasmodium falciparum is the main aetiological agent of tropical malaria. Characteristic of the phylum is the presence of a plastid-like organelle which hosts several homologs of plant proteins, including a ferredoxin (PfFd) and its NADPH-dependent reductase (PfFNR). The PfFNR/PfFd redox system is essential for the parasite, while mammals share no homologous proteins, making the enzyme an attractive target for novel and much needed antimalarial drugs. Based on previous findings, three chemically reactive residues important for PfFNR activity were identified: namely, the active-site Cys99, responsible for hydride transfer; Cys284, whose oxidation leads to an inactive dimeric form of the protein; and His286, which is involved in NADPH binding. These amino acid residues were probed by several residue-specific reagents and the two cysteines were shown to be promising targets for covalent inhibition. The quantitative and qualitative description of the reactivity of few compounds, including a repurposed drug, set the bases for the development of more potent and specific antimalarial leads.

Assessment of mobility and bioavailability of mercury compounds in sewage sludge and composts

Content of heavy metals, including mercury, determines the method of management and disposal of sewage sludge. Excessive concentration of mercury in composts used as organic fertilizer may lead to accumulation of this element in soil and plant material. Fractionation of mercury in sewage sludge and composts provides a better understanding of the extent of mobility and bioavailability of the different mercury species and helps in more informed decision making on the application of sludge for agricultural purposes. The experimental setup comprises the composing process of the sewage sludge containing 13.1mgkg^-1 of the total mercury, performed in static reactors with forced aeration. In order to evaluate the bioavailability of mercury, its fractionation was performed in sewage sludge and composts during the process. An analytical procedure based on four-stage sequential extraction was applied to determine the mercury content in the ion exchange (water soluble and exchangeable Hg), base soluble (Hg bound to humic and fulvic acid), acid soluble (Hg bound to Fe/Mn oxides and carbonates) and oxidizable (Hg bound to organic matter and sulphide) fractions. The results showed that from 50.09% to 64.55% of the total mercury was strongly bound to organo-sulphur and inorganic sulphide; that during composting, increase of concentrations of mercury compounds strongly bound with organic matter and sulphides; and that mercury content in the base soluble and oxidizable fractions was strongly correlated with concentration of dissolved organic carbon in those fractions.

An Acute Mercuric Mercury Poisoning: Chemical Speciation of Hair Mercury Shows a Peak of Inorganic Mercury Value

A woman ingested a dose of sublimate (approximately 0.9 g) in an attempted suicide. She survived and recovered in response to a combination of therapies including chelate (BAL) therapy, plasma exchange, haemodialysis and peritoneal dialysis.

Serum inorganic mercury concentration, urinary inorganic mercury excretion and hair inorganic and organic mercury and selenium concentrations, along the length from the scalp to the distal part, were measured. Longitudinal analysis of hair, revealed a peak in inorganic mercury corresponding to the time of mercury ingestion. Organic mercury and selenium in the hair had different patterns of longitudinal variation from that of inorganic mercury. The biological half-life (23.5 d) of serum inorganic mercury levels was in good agreement with values previously reported in the literature.

Total and organic mercury concentrations in the muscles of Pacific albacore (Thunnus alalunga) and bigeye tuna (Thunnus obesus)

Muscles of 115 North Pacific albacore (ALB, Thunnus alalunga) and 75 Pacific bigeye tuna (BET, Thunnus obesus), collected from 2001 to 2006, were analyzed. No ALB, but 13 large BET had organic mercury (OH g) concentrations exceeding 1 μg g(-1) wet weight. For both ALB and BET, total mercury (THg) and OH g concentrations were significantly and positively correlated with fork length (FL) and body weight. The muscle Hg bioaccumulation rates of BET were higher than those of ALB, particularly in the adult fish. Moreover, the lines had crossover points among the two species that imply the young BET (FL<110 cm) contains lower muscle Hg concentrations than ALB of the same size. The suggested weekly dietary intake of ALB and small-BET meats is 340 g, and of BET meat it is 150 g for a 60-kg person based on the provisional tolerable weekly intake (PTWI) of methylmercury set by the WHO.

Risks associated with the transfer of toxic organo-metallic mercury from soils into the terrestrial feed chain

Although the transfer of organo-metallic mercury (OrgHg) in aquatic food webs has long been studied, it has only been recently recognized that there is also accumulation in terrestrial systems. There is still however little information about the exposure of grazing animals to OrgHg from soils and feed as well as on risks of exposure to animal and humans. In this study we collected 78 soil samples and 40 plant samples (Lolium perenne and Brassica juncea) from agricultural fields near a contaminated industrial area and evaluated the soil-to-plant transfer of Hg as well as subsequent trophic transfer. Inorganic Hg (IHg) concentrations ranged from 0.080 to 210mgkg(-1) d.w. in soils, from 0.010 to 84mgkg(-1) d.w. in roots and from 0.020 to 6.9mgkg(-1) d.w. in shoots. OrgHg concentrations in soils varied between 0.20 and 130μgkg(-1) d.w. representing on average 0.13% of the total Hg (THg). In root and shoot samples OrgHg comprised on average 0.58% (roots) and 0.66% (shoots) of THg. Average bioaccumulation factors (BAFs) for OrgHg in relation to soil concentrations were 3.3 (for roots) and 1.5 (for shoots). The daily intake (DI) of THg in 33 sampling sites exceeded the acceptable daily intake (ADI) of THg of both cows (ADI=1.4mgd(-1)) and sheep (ADI=0.28mgd(-1)), in view of food safety associated with THg in animal kidneys. Estimated DI of OrgHg for grazing animals were up to 220μgd(-1) (for cows) and up to 33μgd(-1) (for sheep). This study suggested that solely monitoring the levels of THg in soils and feed may not allow to adequately taking into account accumulation of OrgHg in feed crops and properly address risks associated with OrgHg exposure for animals and humans. Hence, the inclusion of limits for OrgHg in feed quality and food safety legislation is advised.

Mercury in human brain, blood, muscle and toenails in relation to exposure: an autopsy study

BACKGROUND

The main forms of mercury (Hg) exposure in the general population are methylmercury (MeHg) from seafood, inorganic mercury (I-Hg) from food, and mercury vapor (Hg0) from dental amalgam restorations. While the distribution of MeHg in the body is described by a one compartment model, the distribution of I-Hg after exposure to elemental mercury is more complex, and there is no biomarker for I-Hg in the brain. The aim of this study was to elucidate the relationships between on the one hand MeHg and I-Hg in human brain and other tissues, including blood, and on the other Hg exposure via dental amalgam in a fish-eating population. In addition, the use of blood and toenails as biological indicator media for inorganic and organic mercury (MeHg) in the tissues was evaluated.

METHODS

Samples of blood, brain (occipital lobe cortex), pituitary, thyroid, abdominal muscle and toenails were collected at autopsy of 30 deceased individuals, age from 47 to 91 years of age. Concentrations of total-Hg and I-Hg in blood and brain cortex were determined by cold vapor atomic fluorescence spectrometry and total-Hg in other tissues by sector field inductively coupled plasma-mass spectrometry (ICP-SFMS).

RESULTS

The median concentrations of MeHg (total-Hg minus I-Hg) and I-Hg in blood were 2.2 and 1.0 microg/L, and in occipital lobe cortex 4 and 5 microg/kg, respectively. There was a significant correlation between MeHg in blood and occipital cortex. Also, total-Hg in toenails correlated with MeHg in both blood and occipital lobe. I-Hg in both blood and occipital cortex, as well as total-Hg in pituitary and thyroid were strongly associated with the number of dental amalgam surfaces at the time of death.

CONCLUSION

In a fish-eating population, intake of MeHg via the diet has a marked impact on the MeHg concentration in the brain, while exposure to dental amalgam restorations increases the I-Hg concentrations in the brain. Discrimination between mercury species is necessary to evaluate the impact on Hg in the brain of various sources of exposure, in particular, dental amalgam exposure.

Cleaving Mercury-Alkyl Bonds: A Functional Model for Mercury Detoxification by MerB

The extreme toxicity of organomercury compounds that are found in the environment has focused attention on the mechanisms of action of bacterial remediating enzymes. We describe facile room-temperature protolytic cleavage by a thiol of the Hg-C bond in mercury-alkyl compounds that emulate the structure and function of the organomercurial lyase MerB. Specifically, the tris(2-mercapto-1-t-butylimidazolyl)hydroborato ligand [Tm(Bu(t))], which features three sulfur donors, has been used to synthesize [Tm(Bu(t))]HgR alkyl compounds (R = methyl or ethyl) that react with phenylthiol (PhSH) to yield [Tm(Bu(t))]HgSPh and RH. Although [Tm(Bu(t))]HgR compounds exist as linear two-coordinate complexes in the solid state, 1H nuclear magnetic resonance spectroscopy indicates that the complexes exist in rapid equilibrium with their higher-coordinate [kappa2-Tm(Bu(t))]HgR and [kappa3-Tm(Bu(t))]HgR isomers in solution. Facile access to a higher-coordinate species is proposed to account for the exceptional reactivity of [Tm(Bu(t))]HgR relative to that of other two-coordinate mercury-alkyl compounds.

Coupling two mercury resistance genes in Eastern cottonwood enhances the processing of organomercury

Eastern cottonwood (Populus deltoides Bartr. ex Marsh.) trees were engineered to express merA (mercuric ion reductase) and merB (organomercury lyase) transgenes in order to be used for the phytoremediation of mercury-contaminated soils. Earlier studies with Arabidopsis thaliana and Nicotiana tabacum showed that this gene combination resulted in more efficient detoxification of organomercurial compounds than did merB alone, but neither species is optimal for long-term field applications. Leaf discs from in vitro-grown merA, nptII (neomycin phosphotransferase) transgenic cottonwood plantlets were inoculated with Agrobacterium tumefaciens strain C58 carrying the merB and hygromycin resistance (hptII) genes. Polymerase chain reaction of shoots regenerated from the leaf discs under selection indicated an overall transformation frequency of 20%. Western blotting of leaves showed that MerA and MerB proteins were produced. In vitro-grown merA/merB plants were highly resistant to phenylmercuric acetate, and detoxified organic mercury compounds two to three times more rapidly than did controls, as shown by mercury volatilization assay. This indicates that these cottonwood trees are reasonable candidates for the remediation of organomercury-contaminated sites.

Volatilization and recovery of mercury from mercury-polluted soils and wastewaters using mercury-resistant Acidithiobacillus ferrooxidans strains SUG 2-2 and MON-1

Iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, is one of the most important bacteria for the bioleaching of copper and gold ores. In order to use the mercury reducing activity of A. ferrooxidans for the bioremediation of mercury, mercury-resistant A. ferrooxidans strains SUG 2-2 and MON-1 were screened among 150 strains of iron-oxidizing bacteria isolated from natural environments. It was found that strains SUG 2-2 and MON-1 have a novel ferrous iron-dependent mercury volatilization activity as well as an NADPH-dependent mercury reductase activity. Strain MON-1 has an organomercurial lyase-like activity and grew most rapidly in an iron medium with 0.1 microM p-chloromercuribenzoic acid among 11 A. ferrooxidans strains tested. Nearly 100% of the total mercury in mercury-polluted soil or mercury wastewater was volatilized and recovered by incubating SUG 2-2 or MON-1 cells in 20 ml of an acidified water (pH 2.5) with ferrous iron, suggesting that these mercury-resistant strains can be used for the bioremediation of inorganic and organic mercurial compounds. We show for the first time that MON-1 cells immobilized in polyvinyl alcohol (PVA) resins could efficiently volatilize mercury from 2 L of a synthetic mercury-polluted wastewater (pH 2.5) containing 40 microM Hg(2+) and ferrous iron. The MON-1-immobilized PVA resins were used repeatedly.

Inter-individual variations of human mercury exposure biomarkers: a cross-sectional assessment

BACKGROUND

Biomarkers for mercury (Hg) exposure have frequently been used to assess exposure and risk in various groups of the general population. We have evaluated the most frequently used biomarkers and the physiology on which they are based, to explore the inter-individual variations and their suitability for exposure assessment.

METHODS

Concentrations of total Hg (THg), inorganic Hg (IHg) and organic Hg (OHg, assumed to be methylmercury; MeHg) were determined in whole blood, red blood cells, plasma, hair and urine from Swedish men and women. An automated multiple injection cold vapour atomic fluorescence spectrophotometry analytical system for Hg analysis was developed, which provided high sensitivity, accuracy, and precision. The distribution of the various mercury forms in the different biological media was explored.

RESULTS

About 90% of the mercury found in the red blood cells was in the form of MeHg with small inter-individual variations, and part of the IHg found in the red blood cells could be attributed to demethylated MeHg. THg in plasma was associated with both IHg and MeHg, with large inter-individual variations in the distribution between red blood cells and plasma. THg in hair reflects MeHg exposure at all exposure levels, and not IHg exposure. The small fraction of IHg in hair is most probably emanating from demethylated MeHg. The inter-individual variation in the blood to hair ratio was very large. The variability seemed to decrease with increasing OHg in blood, most probably due to more frequent fish consumption and thereby blood concentrations approaching steady state. THg in urine reflected IHg exposure, also at very low IHg exposure levels.

CONCLUSION

The use of THg concentration in whole blood as a proxy for MeHg exposure will give rise to an overestimation of the MeHg exposure depending on the degree of IHg exposure, why speciation of mercury forms is needed. THg in RBC and hair are suitable proxies for MeHg exposure. Using THg concentration in plasma as a measure of IHg exposure can lead to significant exposure misclassification. THg in urine is a suitable proxy for IHg exposure.

Existence of an iron-oxidizing bacterium Acidithiobacillus ferrooxidans resistant to organomercurial compounds

Acidithiobacillus ferrooxidans MON-1 which is highly resistant to Hg2+ could grow in a ferrous sulfate medium (pH 2.5) with 0.1 microM p-chloromercuribenzoic acid (PCMB) with a lag time of 2 d. In contrast, A. ferrooxidans AP19-3 which is sensitive to Hg2+ did not grow in the medium. Nine strains of A. ferrooxidans, including seven strains of the American Type Culture Collection grew in the medium with a lag time ranging from 5 to 12 d. The resting cells of MON-1, which has NADPH-dependent mercuric reductase activity, could volatilize Hg0 when incubated in acidic water (pH 3.0) containing 0.1 microM PCMB. However, the resting cells of AP19-3, which has a similar level of NADPH-dependent mercuric reductase activity compared with MON-1, did not volatilize Hg0 from the reaction mixture with 0.1 microM PCMB. The activity level of the 11 strains of A. ferrooxidans to volatilize Hg0 from PCMB corresponded well with the level of growth inhibition by PCMB observed in the growth experiments. The resting cells of MON-1 volatilized Hg0 from phenylmercury acetate (PMA) and methylmercury chloride (MMC) as well as PCMB. The cytosol prepared from MON-1 could volatilize Hg0 from PCMB (0.015 nmol mg(-1) h(-1)), PMA (0.33 nmol mg(-1) h(-1)) and MMC (0.005 nmol mg(-1) h(-1)) in the presence of NADPH and beta-mercaptoethanol.

Removal of inorganic and organic mercurials by immobilized bacteria having mer-ppk fusion plasmids

Feasibility of biological mercury removal from wastewater was examined by using alginate-immobilized cells of Escherichia coli carrying mer-ppk fusion plasmid pMKB18. Immobilized cells engineered to express mercury-transport system, organomercurial lyase and polyphosphate efficiently removed organic and inorganic mercury from contaminated wastewater over a wide concentration range of mercurials, probably via intracellular accumulation mediated by ppk-specified polyphosphate. Bioaccumulation of mercury was selective compared to other metals such as Cd(2+), Pb(2+) and Cr(6+). The immobilized cells could be used repeatedly (at least three times) without large loss of mercury removal activity. From these results, it is concluded that the mer-ppk fusion plasmid and the immobilized cells are useful for simultaneous removal of organic and inorganic mercury from contaminated wastewater.

Phytoremediation of organomercurial compounds via chloroplast genetic engineering

Mercury (Hg), especially in organic form, is a highly toxic pollutant affecting plants, animals, and man. In plants, the primary target of Hg damage is the chloroplast; Hg inhibits electron transport and photosynthesis. In the present study, chloroplast genetic engineering is used for the first time to our knowledge to enhance the capacity of plants for phytoremediation. This was achieved by integrating a native operon containing the merA and merB genes (without any codon modification), which code for mercuric ion reductase (merA) and organomercurial lyase (merB), respectively, into the chloroplast genome in a single transformation event. Stable integration of the merAB operon into the chloroplast genome resulted in high levels of tolerance to the organomercurial compound, phenylmercuric acetate (PMA) when grown in soil containing up to 400 micro M PMA; plant dry weights of the chloroplast transformed lines were significantly higher than those of wild type at 100, 200, and 400 micro M PMA. That the merAB operon was stably integrated into the chloroplast genome was confirmed by polymerase chain reaction and Southern-blot analyses. Northern-blot analyses revealed stable transcripts that were independent of the presence or absence of a 3'-untranslated region downstream of the coding sequence. The merAB dicistron was the more abundant transcript, but less abundant monocistrons were also observed, showing that specific processing occurs between transgenes. The use of chloroplast transformation to enhance Hg phytoremediation is particularly beneficial because it prevents the escape of transgenes via pollen to related weeds or crops and there is no need for codon optimization to improve transgene expression. Chloroplast transformation may also have application to other metals that affect chloroplast function.

Amino acid transporters involved in luminal transport of mercuric conjugates of cysteine in rabbit proximal tubule

The primary aim of the present study was to test the hypothesis that amino acid transport systems are involved in absorptive transport of dicysteinylmercury (cysteine-Hg-cysteine). Luminal disappearance flux [JD, fmol x min(-1) (mm tubular length)(-1)] of inorganic mercury (Hg2+), in the form of dicysteinylmercury, was measured in isolated perfused S2 segments with various amino acids or amino acid analogs in the luminal compartment under one of two conditions, in the presence or absence of Na+. The control perfusion fluid contained 20 microM dicysteinylmercury. Replacing Na+ in both the bathing and perfusing solutions with N-methyl-D-glucamine reduced the JD of Hg2+ by about 40%. Nine amino acids and two amino acid analogs were coperfused individually (at millimolar concentrations) with dicysteinylmercury. The amino acids and amino acid analogs that had the greatest effect on the JD of Hg2+ were L-cystine, L-serine, L-histidine, L-tryptophan, and 2-(-)-endoamino-bicycloheptane-2-carboxylic acid. The greatest reduction (76%) in the total JD of Hg2+ occurred when L-cystine was coperfused with dicysteinylmercury in the presence of Na+. Overall, the current findings indicate that Hg2+ is transported from the lumen into proximal tubular epithelial cells via amino acid transporters that recognize dicysteinylmercury. In addition, the data indicate that multiple amino acid transporters are involved in the luminal uptake of dicysteinylmercury, including the Na+-dependent low-affinity L-cystine, B(0), and ASC systems and the Na+-independent L-system. Furthermore, the transport data obtained when L-cystine was added to the luminal fluid indicate strongly that dicysteinylmercury is likely transported as a molecular homolog of L-cystine.

Technical report: mercury in the environment: implications for pediatricians

Mercury is a ubiquitous environmental toxin that causes a wide range of adverse health effects in humans. Three forms of mercury (elemental, inorganic, and organic) exist, and each has its own profile of toxicity. Exposure to mercury typically occurs by inhalation or ingestion. Readily absorbed after its inhalation, mercury can be an indoor air pollutant, for example, after spills of elemental mercury in the home; however, industry emissions with resulting ambient air pollution remain the most important source of inhaled mercury. Because fresh-water and ocean fish may contain large amounts of mercury, children and pregnant women can have significant exposure if they consume excessive amounts of fish. The developing fetus and young children are thought to be disproportionately affected by mercury exposure, because many aspects of development, particularly brain maturation, can be disturbed by the presence of mercury. Minimizing mercury exposure is, therefore, essential to optimal child health. This review provides pediatricians with current information on mercury, including environmental sources, toxicity, and treatment and prevention of mercury exposure.

Bacterial detoxification of Hg(II) and organomercurials

The most common bacterial mechanism for resistance to mercuric-ion species involves intracellular reduction of Hg(II) to Hg(0). Key proteins of the pathway typically include: MerR, which regulates pathway expression; MerP, which protects the external environment; MerT or MerC, which transport Hg(II) species across the inner membrane; MerA, which catalyses reduction of Hg(II); and sometimes MerB, which catalyses cleavage of C-Hg bonds in organomercurials. Cysteine residues of varying number are arranged in each of the key proteins to optimize their unique roles in sensing (high affinity), transporting (exchangeability), and reducing (redox accessibility) Hg(II). Nature's regulator of this pathway, MerR, is an exquisitely sensitive, Hg(II)-binding, DNA-binding protein that holds the system primed for immediate transcription at the slightest influx of Hg(II).

Modulation of the calmodulin-induced inhibition of sarcoplasmic reticulum calcium release channel (ryanodine receptor) by sulfhydryl oxidation in single channel current recordings and [(3)H]ryanodine binding

The modulation of the calmodulin-induced inhibition of the calcium release channel (ryanodine receptor) by two sulfhydryl oxidizing compounds, 4-(chloromercuri)phenyl-sulfonic acid (4-CMPS) and 4, 4'-dithiodipyridine (4,4'-DTDP) was determined by single channel current recordings with the purified and reconstituted calcium release channel from rabbit skeletal muscle sarcoplasmic reticulum (HSR) and [(3)H]ryanodine binding to HSR vesicles. 0.1 microm CaM reduced the open probability (P(o)) of the calcium release channel at maximally activating calcium concentrations (50-100 microm) from 0.502 +/- 0.02 to 0.137 +/- 0.022 (n = 28), with no effect on unitary conductance. 4-CMPS (10-40 microm) and 4,4'-DTDP (0.1-0.3 mm) induced a concentration dependent increase in P(o) (> 0.9) and caused the appearance of longer open states. CaM shifted the activation of the calcium release channel by 4-CMPS or 4,4'-DTDP to higher concentrations in single channel recordings and [(3)H]ryanodine binding. 40 microm 4-CMPS induced a near maximal (P(o) > 0.9) and 0.3 mm 4,4'-DTDP a submaximal (P(o) = 0.74) channel opening in the presence of CaM, which was reversed by the specific sulfhydryl reducing agent DTT. Neither 4-CMPS nor 4,4'-DTDP affected Ca-[(125)I]calmodulin binding to HSR. 1 mm MgCl(2) reduced P(o) from 0.53 to 0.075 and 20-40 microm 4-CMPS induced a near maximal channel activation (P(o) > 0.9). These results demonstrate that the inhibitory effect of CaM or magnesium in a physiological concentration is diminished or abolished at high concentrations of 4-CMPS or 4,4'-DTDP through oxidation of activating sulfhydryls on cysteine residues of the calcium release channel.

Phytodetoxification of hazardous organomercurials by genetically engineered plants

Methylmercury is a highly toxic, organic derivative found in mercury-polluted wetlands and coastal sediments worldwide. Though commonly present at low concentrations in the substrate, methylmercury can biomagnify to concentrations that poison predatory animals and humans. In the interest of developing an in situ detoxification strategy, a model plant system was transformed with bacterial genes (merA for mercuric reductase and merB for organomercurial lyase) for an organic mercury detoxification pathway. Arabidopsis thaliana plants expressing both genes grow on 50-fold higher methylmercury concentrations than wild-type plants and up to 10-fold higher concentrations than plants that express merB alone. An in vivo assay demonstrated that both transgenes are required for plants to detoxify organic mercury by converting it to volatile and much less toxic elemental mercury.

Specific fluorescent labeling of two functional domains in RNA polymerase alpha subunit

A monomercury derivative of fluoresceine acetate (FMMA) was previously suggested as a specific reagent reacting with only one of four cysteine (Cys) residues in the alpha. subunit of Escherichia coli RNA polymerase. Here, we analyzed the reactivity against FMMA of both isolated alpha subunit and alpha subunit assembled in the holoenzyme. In both cases, the highest reactivity was identified for Cys-269 positioned in the regulatory helix of C-terminal domain (CTD) which includes the contact sites for both class-I transcription factors and DNA UP elements. Substitution of Ala for both Cys-269 and Cys-176 completely eliminates the reactivity of alpha subunit against the fluorescent dye, supporting the prediction that another reactive amino acid under native conformation is Cys-176, which is positioned within or near the region important for alpha dimerization and its binding of beta' subunit. In the isolated alpha subunit, the reactivity against FMMA is different between these two Cys residues and the order is from Cys-269 to Cys-176. Mutant alpha-subunits, bearing only one Cys residue at either 269 or 176, could be reconstituted into locally modified and active enzymes. This FMMA modification system may provide a tool suitable for studies of intra- and intermolecular interactions of this subunit.

Studies on mercury-detoxicating enzymes from a broad-spectrum mercury-resistant strain of Flavobacterium rigense

Flavobacterium rigense strain PR2, a broad-spectrum mercury-resistant bacterium abundantly present in soil exhibited multiple metal resistance properties. Mercury resistance was due to the sequential action of two mercury-detoxicating enzymes, organomercurial lyase and mercuric reductase. The levels of these enzyme activities were determined using different mercury compounds as inducers and substrates. Mercuric reductase was partially purified from the bacterium and the physicochemical properties of the enzyme were studied. The effect of several enzyme inhibitors and heavy metal ions on the enzyme activity was also studied.

Structure of HIV-1 reverse transcriptase in a complex with the non-nucleoside inhibitor alpha-APA R 95845 at 2.8 A resolution

BACKGROUND

HIV-1 reverse transcriptase (RT) is a multifunctional enzyme that copies the RNA genome of HIV-1 into DNA. It is a heterodimer composed of a 66 kDa (p66) and a 51 kDa (p51) subunit. HIV-1 RT is a crucial target for structure-based drug design, and potent inhibitors have been identified, whose efficacy, however, is limited by drug resistance.

RESULTS

The crystal structure of HIV-1 RT in complex with the non-nucleoside inhibitor alpha-anilinophenyl-acetamide (alpha-APA) R95845 has been determined at 2.8 A resolution. The inhibitor binds in a hydrophobic pocket near the polymerase active site. The pocket contains five aromatic amino acid residues and the interactions of the side chains of these residues with the aromatic rings of non-nucleoside inhibitors appear to be important for inhibitor binding. Most of the amino acid residues where mutations have been correlated with high levels of resistance to non-nucleoside inhibitors of HIV-1 RT are located close to alpha-APA. The overall fold of HIV-1 RT in complex with alpha-APA is similar to that found when in complex with nevirapine, another non-nucleoside inhibitor, but there are significant conformational changes relative to an HIV-1 RT/DNA/Fab complex.

CONCLUSIONS

The non-nucleoside inhibitor-binding pocket has a flexible structure whose mobility may be required for effective polymerization, and may be part of a hinge that permits relative movements of two subdomains of the p66 subunit denoted the 'palm' and 'thumb'. An understanding of the structure of the inhibitor-binding pocket, of the interactions between HIV-1 RT and alpha-APA, and of the locations of mutations that confer resistance to inhibitors provides a basis for structure-based design of chemotherapeutic agents for the treatment of AIDS.

Molecular analysis of mercury-resistant Bacillus isolates from sediment of Minamata Bay, Japan

Bacillus isolates from Minamata Bay sediment were selected for the ability to volatilize mercury from a range of organomercurials (including methylmercury). Chromosomal DNA from 74 of 78 such strains hybridized with the mer mercury resistance operon DNA from marine Bacillus sp. strain RC607 (Y. Wang, M. Moore, H. S. Levinson, S. Silver, C. Walsh, and I. Mahler, J. Bacteriol. 171:83-92, 1989). The most frequent classes with regard to restriction nuclease site maps of the mer operon for the new isolates were identical to or closely related to the mer determinant of strain RC607. PCR amplification analysis with primers designed from the strain RC607 mer operon gave products of precisely the predicted size with the 74 Minamata Bay isolates.

Engineering cysteine mutants to obtain crystallographic phases with a cutinase from Fusarium solani pisi

Cutinases are extracellular enzymes involved in the disruption of cutine, an insoluble polyester which covers the surface of plants. They belong to a class of serine esterases that are able to hydrolyse fatty acid esters and emulsified triglycerides as efficiently as lipases, but without displaying interfacial activation. Classical crystallographic methods for obtaining heavy-atom derivatives failed, so the cutinase structure has been solved exclusively by the multiple isomorphous replacement method using four Hg derivatives obtained from mutants S4C, S92C, S120C and S129C. Two of these derivatives behaved as expected: (i) the cys mutant of the catalytic Ser S120C, located at the surface of the active site pocket, leads to a good derivative; and (ii) the Hg atom of the derivative obtained with the S92C mutant is completely accessible to the solvent and occupies two alternative positions--consequently a poor derivative results. In contrast, two mutants show an unexpected behaviour: (i) the Hg atom in the S129C mutant was completely buried 10 A below the protein surface and yielded the best derivative; and (ii) a poor quality derivative was obtained with the S4C mutant. Cys 4 belongs to the disordered propeptide 1-16. The Cys 4 bound Hg atom is located in front of the Asp58 side chain, but neither Cys4 nor parts of the propeptide are clearly visible in the electron density maps of the derivative structure.

A new mercury-penicillin V derivative as a probe for ultrastructural localization of penicillin-binding proteins in Escherichia coli

The precise ultrastructural localization of penicillin-binding protein (PBP)-antibiotic complexes in Escherichia coli JM101, JM101 (pBS96), and JM101(pPH116) was investigated by high-resolution electron microscopy. We used mercury-penicillin V (Hg-pen V) as a heavy-metal-labeled, electron-dense probe for accurately localizing PBPs in situ in single bacterial cells grown to exponential growth phase. Biochemical data derived from susceptibility tests and bacteriolysis experiments revealed no significant differences between Hg-pen V and the parent compound, penicillin V, or between strains. Both antibiotics revealed differences in the binding affinities for PBPs of all strains. Deacylation rates for PBPs were slow despite the relatively low binding affinities of antibiotics. Cells bound most of the Hg-pen V added to cultures, and the antibiotic-PBP complex could readily be seen by electron microscopy of unstained whole mounts as distinct, randomly situated electron-dense particles. Fifty to 60% of the antibiotic was retained by cells during processing for conventional embedding so that thin sections could also be examined. These revealed similar electron-dense particles located predominantly on the plasma membrane and less frequently in the cytoplasm. Particles positioned on the plasma membranes were occasionally shown to protrude into the periplasmic space, thereby reflecting the high resolution of the Hg-pen V probe. Moreover, some particles were observed free in the periplasm, suggesting, for the first time, that a proportion of PBPs may not be restricted to the plasma membrane but may be tightly associated with the peptidoglycan for higher efficiency of peptidoglycan assembly. All controls were devoid of the electron-dense particles. The presence of electron-dense particles in cells of the wild-type JM101, demonstrated that our probe could identify PBPs in naturally occurring strains without inducing PBP overproduction.

Disproportionation involving an organomercurial and a sulfhydryl-containing protein. Reaction between chloromercuryferrocene and bakers'-yeast cytochrome c

Reaction between iso-1 cytochrome c from bakers' yeast and chloromercuryferrocene, FcHgCl, does not result in simple replacement of the sulfhydryl hydrogen atom in Cys 102 with the ferrocenylmercury group, FcHg. Instead, this reaction yields the protein monomer modified at Cys 102 with an HgCl+ group and the protein dimer in which the thiolate groups of Cys 102 are bridged by a mercury(II) atom. These proteins and other organometallic products are identified by chromatographic, spectroscopic, and electrochemical methods. Organomercurials of the type RHgX and biological thiols can undergo not only substitution reactions, but disproportionation reactions as well.

Synthesis and characterization of a heterobifunctional mercurial cross-linking agent: incorporation into cobratoxin and interaction with the nicotinic acetylcholine receptor

The heterobifunctional organomercurial reagents 3-(acetoxymercurio)- and 3-(chloromercurio)-5-nitrosalicylaldehyde were prepared, characterized in model studies, and used to probe the interaction between cobratoxin, purified from the venom of the Thailand cobra (Naja naja siamensis), and the affinity-purified nicotinic acetylcholine receptor (AcChR) from Torpedo california electroplax. These reagents may also be useful in introducing chemically well-defined heavy metal atoms into proteins containing no reactive thiols. Model reagent adducts were prepared in situ by reductive amination with N-butylamine and N alpha-acetyllysine-N-methylamide. The nitrophenolic pKaS of the amine adducts were similar to those of the aldehyde reagents through reduced by 1.3-1.5 units when compared with the hydroxylmethyl reduction product. Reaction of either mercuriosalicylaldehyde with cobratoxin led to a single major modification product incorporating 1 mol of the reagent into cobratoxin at Lys 23. The Lys 23 modified toxin had a reduced binding affinity for the AcChR over that of the native toxin (Kd 2.75 nM cf. 0.3 nM). Reduction of the purified AcChR with 1 mM dithiothreitol (DTT) followed by removal of excess thiol led to cross-linking reactions with the Lys 23 modified cobratoxin to both the alpha and beta subunits of the AcChR complex. Reaction of DTT-treated AcChR with N-ethylmaleimide (NEM) blocked cross-linking, while treatment of the initially cross-linked toxin-AcChR complex with mercaptoethanol leads to reversal of cross-linking. These observations strongly support cross-linking mediated by the formation of a mercury-sulfur bond and further lend support the identity of the respective interacting sites in AcChR and toxin.

Transformations of inorganic mercury by Candida albicans and Saccharomyces cerevisiae

Saccharomyces cerevisiae and Candida albicans were incubated with 0.25, 0.5, or 0.75 micrograms of Hg (as HgCl2) per ml of Nelson's medium in the presence of trace amounts of oxygen at 28 degrees C for 12 days. Two control media were used, one without added Hg and one without yeast inoculum. Yeast cell growth was estimated after 1, 2, 3, and 8 days of incubation. The contents of organomercury in the system and of elemental mercury released from the media and collected in traps were determined at the end of the experiments. The results were as follows. (i) C. albicans was the more mercury-resistant species, but both yeast species failed to grow in the media containing 0.75 micrograms of Hg per ml. (ii) The amounts of organomercury produced by the two species were proportional to the amount of HgCl2 added to the medium. In all cases C. albicans produced considerably larger amounts of methylmercury than S. cerevisiae. (iii) The amounts of elemental Hg produced were inversely proportional to the HgCl2 level added in the case of S. cerevisiae but were all similar in the case of C. albicans. (iv) Neither organomercury nor elemental Hg was produced in any of the control media.

Biotoxicity of mercury as influenced by mercury(II) speciation

Integration of physicochemical procedures for studying mercury(II) speciation with microbiological procedures for studying the effects of mercury on bacterial growth allows evaluation of ionic factors (e.g., pH and ligand species and concentration) which affect biotoxicity. A Pseudomonas fluorescens strain capable of methylating inorganic Hg(II) was isolated from sediment samples collected at Buffalo Pound Lake in Saskatchewan, Canada. The effect of pH and ligand species on the toxic response (i.e., 50% inhibitory concentration [IC50]) of the P. fluorescens isolated to mercury were determined and related to the aqueous speciation of Hg(II). It was determined that the toxicities of different mercury salts were influenced by the nature of the co-ion. At a given pH level, mercuric acetate and mercuric nitrate yielded essentially the same IC50s; mercuric chloride, on the other hand, always produced lower IC50s. For each Hg salt, toxicity was greatest at pH 6.0 and decreased significantly (P = 0.05) at pH 7.0. Increasing the pH to 8.0 had no effect on the toxicity of mercuric acetate or mercuric nitrate but significantly (P = 0.05) reduced the toxicity of mercuric chloride. The aqueous speciation of Hg(II) in the synthetic growth medium M-IIY (a minimal salts medium amended to contain 0.1% yeast extract and 0.1% glycerol) was calculated by using the computer program GEOCHEM-PC with a modified data base. Results of the speciation calculations indicated that complexes of Hg(II) with histidine [Hg(H-HIS)HIS+ and Hg(H-HIS)2(2+)], chloride (HgCl+, HgCl2(0), HgClOH0, and HgCl3-), phosphate (HgHPO4(0), ammonia (HgNH3(2+), glycine [Hg(GLY)+], alanine [Hg(ALA)+], and hydroxyl ion (HgOH+) were the Hg species primarily responsible for toxicity in the M-IIY medium. The toxicity of mercuric nitrate at pH 8.0 was unaffected by the addition of citrate, enhanced by the addition of chloride, and reduced by the addition of cysteine. In the chloride-amended system, HgCl+, HgCl2(0), and HgClOH0 were the species primarily responsible for observed increases in toxicity. In the cysteine-amended system, formation of Hg(CYS)2(2-) was responsible for detoxification effects that were observed. The formation of Hg-citrate complexes was insignificant and had no effect on Hg toxicity.

[Fertility of workers exposed to herbicides and pesticides]

Twenty-one workmen exposed to herbicide as organic mercury compounds and to pesticide as halogenic hydrocarbon has been investigated concerning their fertility. If the concentrations of organic mercury compounds increase in the air of the place of employment then the levels of this heavy metal are also higher in the urine and in the ejaculate, and the fertility is reduced. It could also be established that a correlation exists between the concentration of pesticide in the place of employment and the male fertility, especially in cases with an excess of maximum permitted concentrations.

Affinity chromatography of mammalian and yeast nucleosomes. Two modes of binding of transcriptionally active mammalian nucleosomes to organomercurial-agarose columns, and contrasting behavior of the active nucleosomes of yeast

The reasons for the selective binding of nucleosomes from transcriptionally active genes to the organomercurial-agarose columns have been investigated. At least two modes of binding are identified by a new two-stage elution procedure that discriminates between nucleosomes which are retained by the Hg-column because of their salt-labile associations with SH-reactive non-histone proteins, and nucleosomes in which a conformational change has made the thiol groups of histone H3 accessible to SH-reagents. The first class is released from the column in 0.5 M NaCl; the second class is eluted in 10 mM dithiothreitol which displaces the bound H3-thiols. In mammalian cells, both classes of Hg-bound nucleosomes are enriched in the DNA sequences being transcribed at the time, and their histones H3 and H4 are hyperacetylated. In yeast cells, in which histone H3 lacks cysteinyl residues, only a small fraction of nucleosomes binds to the mercury column, and it has no enrichment of DNA sequences derived from the actively transcribed GAL, HIS4, and ACT1 genes. Since few nucleosomes remain on the column after elution in 0.5 M NaCl, the bound nucleosomes of yeast are retained primarily because of salt-labile associations with thiol-reactive nonhistone proteins. Thus, the presence of histone H3-thiol groups appears to be essential for the mercury binding of the second class of nucleosomes which, in mammalian cells, is derived from the transcriptionally active genes. The results support models of reversible nucleosome unfolding during transcription in mammalian cells to reveal previously inaccessible H3-SH groups, and they also indicate that other thiol-containing proteins, including high mobility group 1 and 2, become closely but transiently associated with the chromatin subunits during their transcription.

Organomercurial-volatilizing bacteria in the mercury-polluted sediment of Minamata Bay, Japan

A total of 4,604 bacterial strains isolated from the sediments of Minamata Bay and nearby low-level-mercury stations (control stations) were screened for the ability to volatilize mercury from inorganic and organic mercurial compounds. The strains that volatilize mercury from several kinds of organomercurials were found only in the sediments of Minamata Bay.

[Study of the binding of diuretics by serum proteins according to changes in tryptophan fluorescence]

The effect of diuretics on tryptophan fluorescence of blood serum proteins divided by electrophoresis was studied. It was shown that ethacrynic acid produces the most significant extinction of tryptophan fluorescence in albumin fraction and novurit in globulin fraction. Furosemide possessing a high affinity for all three obtained fractions of protein does not exhibit a preferential binding to one or another of these fractions. It was also found that furosemide and ethacrynic acid by binding to human serum albumin molecule produce its conformational alterations. Mercury diuretic does not possess such effect.

Fluorescein mercuric acetate specifically displaces zinc from cytochrome oxidase

Treatment of beef heart cytochrome oxidase with fluorescein mercuric acetate (FMA) was found to specifically displace zinc from the enzyme and inhibit the steady-state activity in a parallel fashion. The native cytochrome oxidase preparation contained 2.3 Cu: 2.0 Fe: 1.1 Zn: 0.9 Mg. Addition of 2 equivalents of FMA inhibited the activity by 50% and displaced 60% of the zinc from the enzyme, but did not affect the copper, iron or magnesium content. The pre-steady-state reduction of cytochrome oxidase by ferrocytochrome c was not affected by the FMA treatment, in contrast to the inhibition of steady state activity. These results suggest a possible structural or functional role for zinc in cytochrome oxidase.

Bacterial organomercurial lyase: overproduction, isolation, and characterization

Organomercurial lyase mediates the first of two steps in the microbial detoxification of organomercurial salts. This enzyme encoded on the plasmid R831 obtained from Escherichia coli J53-1 has been overproduced to the level of 3% of the soluble cell protein in E. coli by a construction using the T7 promoter. The enzyme has been purified to homogeneity in quantity in three steps. It is a monomer of Mr 22,400 with no detectable cofactors or metal ions. It catalyzes the protonolysis of the C-Hg bond in a wide range of organomercurial salts (primary, secondary, tertiary, alkyl, vinyl, allyl, and aryl) to the hydrocarbon and mercuric ion with turnover rates in the range of 1-240 min-1.

Mechanistic studies of a protonolytic organomercurial cleaving enzyme: bacterial organomercurial lyase

Mechanistic studies of the protonolytic carbon-mercury bond cleavage by organomercurial lyase from Escherichia coli (R831) suggest that the reaction proceeds via an SE2 pathway. Studies with stereochemically defined substrates cis-2-butenyl-2-mercuric chloride (1) and endo-norbornyl-2-mercuric bromide (2) reveal that a high degree of configurational retention occurs during the bond cleavage, while studies with exo-3-acetoxynortricyclyl-5-mercuric bromide (3) and cis-exo-2-acetoxy-bicyclo[2.2.1]hept-5-enyl-3-mercuric bromide (4) show that the protonolysis proceeds without accompanying skeletal rearrangement. Kinetic data for the enzymatic reactions of cis-2-butenyl-2-mercuric chloride (1) and trans-1-propenyl-1-mercuric chloride (6) indicate that these substrates show enhanced reaction rates of ca. 10-200-fold over alkylvinylmercurials and unsubstituted vinylmercurials, suggesting that the olefinic methyl substituent may stabilize an intermediate bearing some positive charge. Enzymatic reaction of 2-butenyl-1-mercuric bromide (5) yields a 72/23/5 mixture of 1-butene/trans-2-butene/cis-2-butene, indicative of intervening SE2' cleavage. The observation of significant solvent deuterium isotope effects at pH 7.4 of Vmax (H2O)/Vmax(D2O) = 2.1 for cis-2-butenyl-2-mercuric chloride (1) turnover and Vmax(H2O)/Vmax(D2O) = 4.9 for ethylmercuric chloride turnover provides additional support for a kinetically important proton delivery. Finally, the stoichiometric formation of butene and Hg(II) from 1 and methane and Hg(II) from methylmercuric chloride eliminates the possibility of an SN1 solvolytic mechanism. As the first well-characterized enzymatic reaction of an organometallic substrate and the first example of an enzyme-mediated SE2 reaction the organomercurial lyase catalyzed carbon-mercury bond cleavage provides an arena for investigating novel enzyme structure-function relationships.

Hybromet: a ligand for purifying opioid receptors

Condensation of the Grignard reagent derive from 2-[4-(allyloxy)phenyl]ethyl bromide (4b) with 7 alpha-acetyl-6,14-endo-ethenotetrahydrothebaine (5) furnished the (R) tertiary carbinol, 7, which upon methoxymercuration followed by treatment with the KBr gave the bromomercurio compound 10 (Hybromet). The corresponding N-cyclopropylmethyl analogue, 11, was prepared also. The bromomercurio compound, 1, and the mercaptobenzothiazole derivative, 3, gave allyl phenyl ether when treated with BAL at room temperature. Similar treatment of 10 with BAL gave 7 in high yield. Binding studies using rat brain homogenates indicated that 7, 13, and 14 have moderately high affinities for mu rather than delta binding sites. Although much weaker, 10 showed preferential mu binding also. These results along with the fact that 10 reacted smoothly with sulfhydryl groups suggest that Hybromet would be a suitable ligand for use in affinity chromatography.

[Diuretic binding with biological membranes studied by means of a fluorescent probe]

The binding of furosemide, novurit and ethacrynic acid with human serum albumin, phospholipid liposomes, mitochondria and plasmatic membranes of rat kidneys was studied by the fluorescent probe 1-anilino naphthalene-8-sulfonate (1,8-ANS) method. It is shown that ethacrynic acid has high associative constants with serum albumin molecule and plasmatic membranes. Novurit has the greatest affinity with rat kidney mitochondria. Furosemide does not bind with plasmatic membranes and has very low constants of association with mitochondria and phospholipid liposomes.

Specific labeling and partial inactivation of cytochrome oxidase by fluorescein mercuric acetate

Addition of 1 eq of fluorescein mercuric acetate (FMA) to beef heart cytochrome oxidase was found to inhibit the steady-state electron transfer activity by 50%, but further additions up to 10 eq had no additional effect on activity. The partial inhibition caused by FMA is thus similar to that observed with other mercury compounds (Mann, A. J., and Auer, H. E. (1980) J. Biol. Chem. 255, 454-458). The fluorescence of FMA was quenched by a factor of 10 upon binding to cytochrome oxidase, consistent with the involvement of a sulfhydryl group. However, addition of mercuric chloride to FMA-cytochrome oxidase resulted in an increase in fluorescence, suggesting that FMA was displaced from the high affinity binding site. Cytochrome c binding to FMA-cytochrome oxidase resulted in a 10% decrease in the fluorescence, possibly caused by Forster energy transfer from FMA to the cytochrome c heme. The binding site for FMA in cytochrome oxidase was investigated by carrying out sodium dodecyl sulfate gel electrophoresis under progressively milder dissociation conditions. When FMA-cytochrome oxidase was dissociated with 3% sodium dodecyl sulfate and 6 M urea, FMA was predominantly bound to subunit II following electrophoresis. However, when the dissociation was carried out at 4 degrees C in the absence of urea with progressively smaller amounts of lithium dodecyl sulfate, the labeling of subunit II decreased and that of subunit I increased. These experiments demonstrate that mercury compounds bind to a high affinity site on cytochrome oxidase, possibly located in subunit I, but then migrate to subunit II under the normal sodium dodecyl sulfate gel electrophoresis conditions. A definitive assignment of the high affinity binding site in the native enzyme cannot be made, however, because it is possible that mercury compounds can migrate from one sulfhydryl to another under even the mildest electrophoresis conditions.

Determination of mercury and organomercurial resistance in obligate anaerobic bacteria

A methodology for determining the minimum inhibitory concentration of inorganic and organomercurial compounds for obligate anaerobic bacteria is described. A wide variation in the susceptibility of anaerobic clinical and sewage isolates was observed. Isolates of Bacteroides ruminicola and Clostridium perfringens resistant to mercury were examined for their plasmid content and ability to demonstrate inducible resistance. None of the resistant anaerobes contained any plasmids, while resistant facultative isolates from the same source contained several plasmids. In 24 h, resistant strains of clostridia and Bacteroides volatilized 20 and 43% of the 203Hg2+ added to cultures, while Escherichia coli R100 and a sewage isolate of Enterobacter cloacae volatilized 63 and 27%, respectively, of the added 203Hg2+. Attempts to induce mercury resistance in the aerobic isolates were successful, but no induction was seen in the anaerobes. Thus, mercury resistance in these anaerobic isolates was neither inducible nor plasmid mediated.

Octanol/water partition coefficients as a model system for assessing antidotes for methylmercury(II) poisoning, and for studying mercurials with medicinal applications

1-Octanol/water partition coefficients, [HgII]octanol/[HgII]water, provide a simple but limited model system for aspects of the biological behavior of methylmercury(II) and commonly used organomercury(II) medicinal compounds. In an octanol/water system some widely studied antidotes for mercury poisoning at least partly displace the biological thiols L-cysteine and glutathione from binding to MeHgII at pH 6.9. Addition of the antidote meso-dimercaptosuccinic acid to MeHgII in the presence of glutathione results in formation of metallic mercury. For RHgII derivatives of L-cysteine and glutathione, octanol/water partition coefficients follow the order Ph greater than Et greater than Me. An exceptionally high value for diphenylmercury, compared with PhHgII derivatives of L-cysteine and glutathione, is consistent with reported results of the distribution of mercury compounds in rats. Ethylmercury(II) is partly displaced from thimerosal by L-cysteine and glutathione in the octanol/water system, indicating that the active form of thimerosal in vivo may involve binding of EtHgII to biological ligands.

Enzymatic polymerization of 5-mercuriuridine-5'-diphosphate with polynucleotide phosphorylase from E. coli

We report the polymerization of 5-mercuriuridine-5'-diphosphate (ppUHgX), in the presence of an excess of beta-mercaptoethanol, with polynucleotide phosphorylase from E. coli. A degradation of mercurated nucleotides with mercaptans was observed and about 30% of incorporation of mercury in the polymer was obtained after treatment with pancreatic RNase. The influence of ppUHgX and pUHgX with or without beta-mercaptoethanol was also studied on the polymerization of UDP. The ppUHgX did not polymerize in the absence of beta-mercaptoethanol.

Close range interactions between nucleotide bases and tryptophan residues in an Escherichia coli single-stranded DNA binding protein-mercurated poly(uridylic acid) complex. A study by optically detected magnetic resonance spectroscopy

Optically detected triplet state magnetic resonance spectra are reported for the complex formed between mercurated poly(Urd) and Escherichia coli single-stranded DNA binding protein. Upon forming a complex, the triplet state properties of Trp residue(s) in the protein are perturbed by the heavy mercury atom and are characterized by a shortened triplet state lifetime and the appearance of a strong D + E slow passage optically detected magnetic resonance signal. These features, which signal an external heavy atom effect, provide direct evidence for a close range interaction between mercurated nucleotide bases and Trp residues owing to the requirement of a van der Waals contact between the perturbed molecule and the heavy atom perturber. The amplitude-modulated phosphorescence microwave double resonance technique selectively displays the phosphorescence spectrum of the heavy atom-perturbed Trp triplet states. A van der Waals contact manifested through a stacked structure of the mercurated uridine base and the indole moiety of Trp is strongly suggested as the most plausible mode of interaction from steric considerations, since other approaches of the mercury atom are blocked by the covalent attachment of 2-mercaptoethanol to mercury. The magnitude of the heavy atom perturbation also is consistent with Hg approach to the pi-system from above or below the indole aromatic plane, and is at least an order of magnitude larger than effects expected from an edge-on approach.

Perspectives on the central nervous system toxicity of methylmercury

Methylmercury is a widespread and highly toxic environmental pollutant. The source of the substance in the environment is industrial and agricultural use. Chronic methylmercury poisoning is characterized by peripheral and central nervous system damage. The rate of absorption and distribution of this organomercurial into neural tissue determines the rate of development and the severity of the neural lesion. Furthermore, the rate of metabolism and excretion of an organomercurial will greatly influence its neural toxicity. There are differences in the accumulation of methylmercury in different regions of the brain, as well as by the different cell types in these regions. The significance of this variable accumulation of methylmercury is not known. Methylmercury influences a large number of neurocellular functions ranging from inhibition of membrane integrity to alteration in the synthesis and release of transmitter substances.