Bioaccumulation of total and methyl mercury by arthropods

Three arthropods, Locusta migratoria manilensis, Acrida chinensis, and Paraten-odera sinensis were selected to study the bioaccumulation of total and methyl mercury. Concentrations of total mercury in the tissues of Locusta migratoria manilensis and Acrida chinensis were 0.013-0.154 and 0.009-0.138 mg/kg, respectively, while those of methylmercury were 0.001-0.012 and 0.001-0.006 mg/kg, respectively. Concentrations of total mercury in Locusta migratoria manilensis and Acrida chinensis, which are the primary consumers, are lower than those in their food, while mercury is accumulated more by the secondary consumer, Paraten-odera sinensis. For total mercury, the concentrations in Locusta migratoria manilensis are inversely proportional to body length and those in Acrida chinensis increase first and then decrease with increased body length. For methyl mercury, concentration is proportional to body length for both of these arthropods. Total mercury concentrations vary in different parts of the arthropod body, and increase in the order of: abdomen > thorax > head.

Importance of dissolved neutral mercury sulfides for methyl mercury production in contaminated sediments

Biotic transformation of inorganic mercury, Hg(II), to mono methyl mercury (MeHg) is proposed to be largely controlled by passive uptake of neutral Hg complexes by sulfate reducing bacteria (SRB). In this study, the chemical speciation of Hg(II) in seven locally contaminated sediments covering environments such as (i) brackish water, (ii) low-productivity freshwater, and, (iii) high-productivity freshwater was related to potential Hg methylation rates, determined by incubation at 23 degrees C for 48 h under N2(g), and to total MeHg concentrations in sediments. Pore water speciation was modeled considering Hg complexes with halides, organic thiols [Hg(SR)2(aq), associated to dissolved organic matter], monosulfides, and bisulfides. The sum of neutral mercury sulfides [Hg(SH)20(aq)] and [HgS0(aq)] was significantly, positively (p < 0.001, n = 20) correlated to the specific methylation rate constant (Km, day(-1)) at depths of 5-100 cm in two brackish water sediments. Total Hg, total mercury sulfides or Hg(SR)2(aq) in pore water gave no significant relationships with Km. In two subsets of freshwater sediments, neutral mercury sulfides were positively correlated to total Hg in pore water, and therefore, total Hg also gave significant relationships with Km. The sum of [Hg(SH)20(aq)] and [HgS0(aq)] was significantly, positively correlated to total sediment MeHg (microg kg-1) in brackish waters (p < 0.001, n = 23), in southern, high-productivity freshwaters (p < 0.001, n = 20), as well as in northern, low-productivity freshwater (p = 0.048, n = 6). The slopes (b, b') of the relationships Km (day-1) = a + b([Hg(SH)20(aq)] + [HgS0(aq)]) and MeHg (microg kg-1) = a' + b'([Hg(SH)20(aq)] + [HgS0(aq)]) showed an inverse relationship with the C/N ratio, supposedly reflecting differences in primary production and energy-rich organic matter availability among sites. We conclude that concentrations of neutral inorganic mercury sulfide species, together with the availability of energy-rich organic matter, largely control Hg methylation rates in contaminated sediments. Furthermore, Hg(SH)20(aq) is suggested to be the dominant species taken up by MeHg producing bacteria in organic-rich sediments without formation of HgS(s).

The toxicology of mercury and its chemical compounds

This review covers the toxicology of mercury and its compounds. Special attention is paid to those forms of mercury of current public health concern. Human exposure to the vapor of metallic mercury dates back to antiquity but continues today in occupational settings and from dental amalgam. Health risks from methylmercury in edible tissues of fish have been the subject of several large epidemiological investigations and continue to be the subject of intense debate. Ethylmercury in the form of a preservative, thimerosal, added to certain vaccines, is the most recent form of mercury that has become a public health concern. The review leads to general discussion of evolutionary aspects of mercury, protective and toxic mechanisms, and ends on a note that mercury is still an "element of mystery."

Biotransformation of Hg(II) by cyanobacteria

The biotransformation of Hg(II) by cyanobacteria was investigated under aerobic and pH-controlled culture conditions. Mercury was supplied as HgCl(2) in amounts emulating those found under heavily impacted environmental conditions where bioremediation would be appropriate. The analytical procedures used to measure mercury within the culture solution, including that in the cyanobacterial cells, used reduction under both acid and alkaline conditions in the presence of SnCl(2). Acid reduction detected free Hg(II) ions and its complexes, whereas alkaline reduction revealed that meta-cinnabar (beta-HgS) constituted the major biotransformed and cellularly associated mercury pool. This was true for all investigated species of cyanobacteria: Limnothrix planctonica (Lemm.), Synechococcus leopoldiensis (Racib.) Komarek, and Phormidium limnetica (Lemm.). From the outset of mercury exposure, there was rapid synthesis of beta-HgS and Hg(0); however, the production rate for the latter decreased quickly. Inhibitory studies using dimethylfumarate and iodoacetamide to modify intra- and extracellular thiols, respectively, revealed that the former thiol pool was required for the conversion of Hg(II) into beta-HgS. In addition, increasing the temperature enhanced the amount of beta-HgS produced, with a concomitant decrease in Hg(0) volatilization. These findings suggest that in the environment, cyanobacteria at the air-water interface could act to convert substantial amounts of Hg(II) into beta-HgS. Furthermore, the efficiency of conversion into beta-HgS by cyanobacteria may lead to the development of applications in the bioremediation of mercury.

Binding, distribution, and plant uptake of mercury in a soil from Oak Ridge, Tennessee, USA

A large amount of mercury has been discharged on the U.S. Department of Energy's Oak Ridge Site (Tennessee) as a part of the U.S. nuclear weapon program during the 1950s through the early 1960s. Increases in mercury concentration in fish and in lower East Fork Poplar Creek of Oak Ridge have been recently reported. This is an experimental study mimicking the initial stage of transformation and redistribution of mercury in soils, which are comparable to those of the Oak Ridge site. The objectives of this study were to investigate potential transformation, distribution, and plant uptake of mercury compounds in soils. Results show that the H(2)O(2)-oxidizable mercury fraction (organically bound mercury) was the major solid-phase fraction in soils freshly contaminated with soluble mercury compounds, while cinnabar fraction was the major solid phase fraction in soils contaminated with HgS. Langmuir relationships were found between mercury concentrations in plant shoots and in soil solid-phase components. Mercury in HgS-contaminated soils was to some extent phytoavailable to plants. Mercury transformation occurred from more labile fractions into more stable fractions, resulting in strong binding of mercury and decreasing its phytoavailability in soils. In addition, high mercury losses from soils contaminated with soluble mercury compounds were observed during a growing season through volatilization, accounting for 20-62% of the total initial mercury in soils.

Expression of mercuric reductase from Bacillus megaterium MB1 in eukaryotic microalga Chlorella sp. DT: an approach for mercury phytoremediation

A eukaryotic microalga, Chlorella sp. DT, was transformed with the Bacillus megaterium strain MB1 merA gene, encoding mercuric reductase (MerA), which mediates the reduction of Hg2+ to volatile elemental Hg0. The transformed Chlorella cells were selected first by hygromycin B and then by HgCl2. The existence of merA gene in the genomic DNA of transgenic strains was shown by polymerase chain reaction amplification, while the stable integration of merA into genomic DNA of transgenic strains was confirmed by Southern blot analysis. The ability to remove Hg2+ in merA transgenic strains was higher than that in the wild type. The merA transgenic strains showed higher growth rate and photosynthetic activity than the wild type did in the presence of a toxic concentration of Hg2+. Cultured with Hg2+, the expression level of superoxide dismutase in transgenic strains was lower than that in the wild type, suggesting that the transgenic strains faced a lower level of oxidative stress. All the results indicated that merA gene was successfully integrated into the genome of transgenic strains and functionally expressed to promote the removal of Hg2+.

Mercury analysis of acid- and alkaline-reduced biological samples: identification of meta-cinnabar as the major biotransformed compound in algae

The biotransformation of Hg(II) in pH-controlled and aerated algal cultures was investigated. Previous researchers have observed losses in Hg detection in vitro with the addition of cysteine under acid reduction conditions in the presence of SnCl2. They proposed that this was the effect of Hg-thiol complexing. The present study found that cysteine-Hg, protein and nonprotein thiol chelates, and nucleoside chelates of Hg were all fully detectable under acid reduction conditions without previous digestion. Furthermore, organic (R-Hg) mercury compounds could not be detected under either the acid or alkaline reduction conditions, and only beta-HgS was detected under alkaline and not under acid SnCl2 reduction conditions. The blue-green alga Limnothrix planctonica biotransformed the bulk of Hg(II) applied as HgCl2 into a form with the analytical properties of beta-HgS. Similar results were obtained for the eukaryotic alga Selenastrum minutum. No evidence for the synthesis of organomercurials such as CH3Hg+ was obtained from analysis of either airstream or biomass samples under the aerobic conditions of the study. An analytical procedure that involved both acid and alkaline reduction was developed. It provides the first selective method for the determination of beta-HgS in biological samples. Under aerobic conditions, Hg(II) is biotransformed mainly into beta-HgS (meta-cinnabar), and this occurs in both prokaryotic and eukaryotic algae. This has important implications with respect to identification of mercury species and cycling in aquatic habitats.

Molecular cloning and genetic analysis of functional merB gene from indian isolates of Escherichia coli

Studies were carried out to characterize organomercurial lyase genes from wild type mercury-resistant Escherichia coli isolates, previously collected from five geographically distinct regions of the Indian subcontinent. PCR amplification followed by DNA sequencing of amplified fragments showed three merB identical to the previously characterized mer B from E. coli pR831b that were thus considered as the same gene. The remaining two genes derived from E. coli isolates of an almost mercury-free site (Dal lake, Kashmir) and designated as pIAAD3 merB and pIAAD14 merB showed slight variation (2%) at base. However, this variation in pIAAD3 due to the absence of base "T" at 479 position results in complete frame shift and the predicted MerB-like polypeptide derived from it showed 21.53% divergent at its C terminal end from the previously characterized pR831b MerB. The expression profile of pIAAD3 merB in pQE30 and pUC18 vectors each demonstrated 22.2 kDa proteins. The induced DH5alpha E. coli cells possessing pIAAD3 merB cloned in pUC18 vector split phenyl mercuric acetate (PMA) into benzene and inorganic mercury efficiently, thus giving a clue that the expressed gene product is biologically active. The current study suggests that such genetic changes may take place in the continued absence of mercury pressure, and with such modifications, they finally break down to act as vestigial remnants. Further work is going on in our lab to exploit pIAAD3 merB for the bioremediation of mercury-polluted sites.

Investigation of metal-binding metallothioneins in the tissues of rats after oral intake of cinnabar

The multi-metal-binding MT fractions in rat tissues after oral intake of cinnabar were characterized by hyphenated size-exclusion chromatography (SEC) and inductively coupled plasma mass spectrometry (ICP-MS). With the increase of both the feeding time and dosage of cinnabar the amounts of Hg-binding MT fractions in rat kidney of groups fed cinnabar increased significantly compared with the control group. Meanwhile, more Cu-binding MT were synthesized in the rat kidney, which confirmed the manipulating effect of MT in the homeostasis of Cu for detoxification of nephritic mercury. Although the Hg-binding MT fractions in rat liver of all cinnabar groups were almost independent of cinnabar dosage (2.5-5.0 g kg(-1) bw) and feeding time (2-4 weeks), the amount was much higher than that of the control group. The amounts of Hg-MT in other rat tissues of the cinnabar groups and the control group were compared and their significance is discussed.

Increasing UV-B radiation at the earth's surface and potential effects on aqueous mercury cycling and toxicity

In the past two decades, a great deal of attention has been paid to the environmental fate of mercury (Hg), and this is exemplified by the growing number of international conferences devoted uniquely to Hg cycling and its impacts on ecosystem functions and life. This interest in the biogeochemistry of Hg has resulted in a significant improvement of our understanding of its impact on the environment and human health. However, both past and current research, have been primarily oriented toward the study of direct impact of anthropogenic activities on Hg cycling. Besides a few indirect effects such as the increase in Hg methylation observed in acid-rain impacted aquatic systems or the reported enhanced Hg bioaccumulation in newly flooded water reservoirs; changes in Hg transformations/fluxes that may be related to global change have received little attention. A case in point is the depletion of stratospheric ozone and the resulting increase in solar UV-radiation reaching the Earth. This review and critical discussion suggest that increasing UV-B radiation at earth's surface could have a significant and complex impact on Hg cycling including effects on Hg volatilization (photo-reduction), solubilization (photo-oxidation), methyl-Hg demethylation, and Hg methylation. Therefore, this paper is written to provoke discussions, and more importantly, to stimulate research on potential impacts of incoming solar UV-radiation on global Hg fluxes and any toxicity aspects of Hg that may become exacerbated by UV-radiation.

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.

Bacterial mercury resistance from atoms to ecosystems

Bacterial resistance to inorganic and organic mercury compounds (HgR) is one of the most widely observed phenotypes in eubacteria. Loci conferring HgR in Gram-positive or Gram-negative bacteria typically have at minimum a mercuric reductase enzyme (MerA) that reduces reactive ionic Hg(II) to volatile, relatively inert, monoatomic Hg(0) vapor and a membrane-bound protein (MerT) for uptake of Hg(II) arranged in an operon under control of MerR, a novel metal-responsive regulator. Many HgR loci encode an additional enzyme, MerB, that degrades organomercurials by protonolysis, and one or more additional proteins apparently involved in transport. Genes conferring HgR occur on chromosomes, plasmids, and transposons and their operon arrangements can be quite diverse, frequently involving duplications of the above noted structural genes, several of which are modular themselves. How this very mobile and plastic suite of proteins protects host cells from this pervasive toxic metal, what roles it has in the biogeochemical cycling of Hg, and how it has been employed in ameliorating environmental contamination are the subjects of this review.

Mercury transport and resistance

Resistance to mercuric ions in bacteria is conferred by mercuric reductase, which reduces Hg(II) to Hg(0) in the cytoplasmic compartment. Specific mercuric ion transport systems exist to take up Hg(II) salts and deliver them to the active site of the reductase. This short review discusses the role of transport proteins in resistance and the mechanism of transfer of Hg(II) between the mercury-resistance proteins.

Mercury interaction with the GABA(A) receptor modulates the benzodiazepine binding site in primary cultures of mouse cerebellar granule cells

Mercury compounds are neurotoxic compounds with a great specificity for cerebellar granule cells. The interaction of mercury compounds with proteins in the central nervous system may underlie some of their effects on neurotransmission. In this work we study the interaction of mercuric chloride (HgCl2) and methylmercury (MeHg) with the GABA(A) receptor in primary cultures of cerebellar granule cells. Both compounds increased, dose dependently, the binding of [3H]flunitrazepam to the benzodiazepine recognition site. EC50 values for this effect were 3.56 and 15.24 microM for HgCl2 and MeHg, respectively, after 30 min exposure of intact cultured cerebellar granule cells. The increase of [3H]flunitrazepam binding by mercury compounds was completely inhibited by the GABA(A) receptor antagonists bicuculline and picrotoxinin, and by the organochlorine pesticide alpha-endosulfan. It was also partially inhibited by the anion transporter blocker DIDS, however this effect could be due to a possible chelation of mercury by DIDS. Intracellular events, like intracellular calcium, kinase activation/inactivation or antioxidant conditions did not affect [3H]flunitrazepam binding or its increase induced by mercury compounds. The sulfhydryl alkylating agent N-ethylmaleimide mimicked the effect of mercury compounds on [3H]flunitrazepam binding suggesting a common mechanism. We conclude that mercury compounds interact with the GABA(A) receptor by the way of alkylation of SH groups of cysteinyl residues found in GABA(A) receptor subunit sequences.

Simulation of mercury capture by activated carbon injection in incinerator flue gas. 2. Fabric filter removal

Following a companion paper focused on the in-duct mercury capture in incinerator flue gas by powdered activated carbon injection, this paper is concerned with the additional mercury capture on the fabric filter cake, relevant to baghouse equipped facilities. A detailed model is presented for this process, based on material balances on mercury in both gaseous and adsorbed phases along the growing filter cake and inside the activated carbon particles,taking into account mass transfer resistances and adsorption kinetics. Several sorbents of practical interest have been considered, whose parameters have been evaluated from available literature data. The values and range of the operating variables have been chosen in order to simulate typical incinerators operating conditions. Results of simulations indicate that, contrary to the in-duct removal process, high mercury removal efficiencies can be obtained with moderate sorbent consumption, as a consequence of the effective gas/sorbent contacting on the filter. Satisfactory utilization of the sorbents is predicted, especially at long filtration times. The sorbent feed rate can be minimized by using a reactive sorbent and by lowering the filter temperature as much as possible. Minor benefits can be obtained also by decreasing the sorbent particle size and by increasing the cleaning cycle time of the baghouse compartments. Reverse-flow baghouses were more efficient than pulse-jet baghouses, while smoother operation can be obtained by increasing the number of baghouse compartments. Model results are compared with available relevant full scale data.

Simulation of mercury capture by activated carbon injection in incinerator flue gas. 1. In-duct removal

A detailed model for the in-duct mercury capture in incinerator flue gas by powdered activated carbon injection is presented. Material balances on mercury in both gaseous and adsorbed phases are carried out along the duct length and inside the activated carbon particles, taking into account mass transfer resistances and adsorption kinetics. The set of the coupled partial differential equations is transformed by means of an orthogonal collocation technique and integrated using a Runge-Kutta method with adaptive stepsize control. The model has been applied to several sorbents of practical interest, whose parameters have been evaluated from available literature data. The values and range of the operating variables have been chosen in order to simulate typical incinerators operating conditions. Results of simulations indicate that large sorbent loadings in the duct are needed to obtain high mercury removal efficiencies, due to the short residence times. As a consequence very low utilization of the sorbents is achieved in any case. To minimize the sorbent feed rate it is particularly advisable to use a reactive sorbent and to lower the operating temperature as much as possible. Improvements in the mercury capture performance can be obtained also by increasing the in-duct particles residence time and by decreasing the sorbent particles size. Model results are compared with available relevant full scale data.

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.

Unusually wide co-factor tolerance in a metalloenzyme; divalent metal ions modulate endo-exonuclease activity in T5 exonuclease

T5 5'-3' exonuclease is a member of a homologous group of 5' nucleases which require divalent metal co-factors. Structural and biochemical studies suggest that single-stranded DNA substrates thread through a helical arch or hole in the protein, thus bringing the phosphodiester backbone into close proximity with the active site metal co-factors. In addition to the expected use of Mg(2+), Mn(2+) and Co(2+) as co-factors, we found that divalent zinc, iron, nickel and copper ions also supported catalysis. Such a range of co-factor utilisation is unusual in a single enzyme. Some co-factors such as Mn(2+) stimulated the cleavage of double-stranded closed-circular plasmid DNA. Such endonucleolytic cleavage of circular double-stranded DNA cannot be readily explained by the threading model proposed for the cleavage of substrates with free 5'-ends as the hole observed in the crystal structure of T5 exonuclease is too small to permit the passage of double-stranded DNA. We suggest that such a substrate may gain access to the active site of the enzyme by a process which does not involve threading.

Abnormal auditory brainstem responses for mice treated with mercurial compounds: involvement of excessive nitric oxide

In this paper, we attempted to construct an animal (mouse) model for monitoring the oto-neurotoxicity of mercuric sulfide, comparing its toxicity with the well-known (organic) mercury compound methyl-mercury. Mice were treated with either mercuric sulfide (HgS, 0.1 and 1.0 g/kg per day) or methyl-mercury (MeHg, 0.2, 2.0 and 10 mg/kg per day) by gastric gavage for 7 consecutive days. Analysis of auditory brainstem response (ABR) indicated that significant elevation of the physiological hearing threshold as well as significant prolongation of interwave latency I-V was observed for MeHg -- (2.0 and 0.2 mg/kg per day) or HgS -- (1.0 g/kg per day, but not 0.1 g/kg per day) treated mice. Further, both MeHg- and HgS-treated animals demonstrated a significant prolongation of interwave latency I-V that increased with an increasing mean blood-Hg level. The oto-neurotoxicity of MeHg (2.0 mg/kg per day) persisted to at least 11 weeks subsequent to the cessation of its administration. The toxic effect of HgS, however, disappeared completely 5 weeks subsequent to the cessation of its administration. These results suggest a correlation between the Hg-elicited hearing dysfunction and the availability of mercury in brain tissue. Both inhibition of Na(+)/K(+)-ATPase activity and overproduction of nitric oxide in the brainstem are consistent with an analysis of the physiological hearing threshold and latencies of ABR waveform at all time points throughout the experimental process. Thus, it is proposed that high-dose HgS or MeHg intoxication is associated with a decrease in functional Na(+)/K(+)-ATPase activity in the brainstem of affected animals, this presumably arising via excessive nitric oxide production, and suggesting that brainstem damage may play a role in mercury-induced hearing loss.

Removal of mercury from mercury-contaminated sediments using a combined method of chemical leaching and volatilization of mercury by bacteria

A method for the removal of mercury sulfide from mercury-contaminated sediments was developed, which consists of chemical leaching and volatilization of mercury by bacteria. More than 85% of the mercury in sediment containing 0.11-37.4 mg/kg of mercury was efficiently extracted with 3 M HCl and 74 mM FeCl3. Subsequent volatilization by bacteria resulted in the removal of 62.9-75.1% of mercury from mercury-contaminated Minamata Bay sediments. Methylmercury was also eliminated from soil at a high efficiency. Thus, this combined method of chemical and microbial treatments could be used for efficient removal of both organic and inorganic mercurials from natural sediments.

Stability of inorganic mercury and methylmercury on yeast-silica gel microcolumns: field sampling capabilities

The stability of methylmercury and inorganic mercury retained on yeast-silica gel microcolumns was established and compared with the stability of these species in solution. Yeast-silica gel columns with the retained analytes were stored for two months at three different temperatures: -20 degrees C, 4 degrees C and room temperature. At regular time intervals, both mercury species were eluted and quantified by cold vapor atomic absorption spectrometry (CVAAS). Methylmercury was found stable in the columns over the two-month period at the three different temperatures tested while the concentration of inorganic mercury decreased after one week's storage even at -20 degrees C. These results are of great interest since the use of these microcolumns allows the preconcentration and storage of mercury species until analysis, thus saving laboratory space and avoiding the problems associated with maintaining species integrity in aqueous solution.

Rapid reduction of Hg(II) by mercuric ion reductase does not require the conserved C-terminal cysteine pair using HgBr2 as the substrate

Conditions are described under which the nonphysiological substrate mercuric bromide (HgBr2) is rapidly turned over, both by the wild type (CCCC) and by an active site double mutant (CCAA) of mercuric reductase in which the C-terminal cysteines 557' and 558' are replaced by alanine and only the redox-active pair Cys135 and Cys140 are available for catalysis. A maximum rate of turnover kcatapp of approximately 18 s-1 (at 3 degreesC) for both enzymes is observed, and at high [HgBr2]/[enzyme] ratios, inhibition is found. The UV-vis spectral changes during turnover are closely similar in both enzymes, indicating that catalysis follows the same enzymatic mechanism. Single-turnover analysis of the mutant enzyme shows that after binding of HgBr2, two further rapid events ensue, followed by reduction of the metal ion (kobs approximately 23.5 s-1). It is shown that under multiple-turnover conditions, completion of the catalytic cycle must occur via an ordered mechanism where rapid binding of a new molecule of HgBr2 to EH2.NADP+ precedes exchange of the pyridine nucleotide. Binding of HgBr2 to the active site triple mutant C135A/C557A/C558A (ACAA) is ca. 100-fold slower compared to that of the CCAA mutant and results in no detectable turnover. It is concluded that in the reducible enzyme.Hg(II) complex, the metal ion is coordinated to Cys135 and Cys140 and that for efficient catalysis both residues are required. Furthermore, the data imply that binding to EH2.NADPH occurs via initial rate-limiting attack of Cys135, followed by reaction with Cys140.

Genetically modified Escherichia coli for colorimetric detection of inorganic and organic Hg compounds

A sensitive colorimetric bacterial system was developed for the detection of Hg(II) and organomercury compounds. The bioactive species, a recombinant Escherichia coli, produces proportionally elevated levels of the enzyme beta-galactosidase with increasing amounts of Hg. This is due to a reporter plasmid which carriers a Hg(II)-inducible promoter (mer promoter) from the Hg resistance transposon Tn501 regulating the transcription of a promoterless lacZ gene. Additionally, a pMB1 origin of replication without the natural RNA polymerase start site is fused downstream of the mer promoter leading to a Hg(II)-inducible plasmid replication, which results in an improved signal-to-noise ratio. To enhance the sensitivity of this cellular biosensor, the transport proteins for Hg(II) uptake are constitutively produced by a helper plasmid. To enable the detection of organically bound Hg, the Streptomyces lividans organomercurical lyase, an enzyme which catalyses the cleavage of C-Hg-bonds of organomercurial compounds, is also provided by the helper plasmid. Hg(II) and phenylmercuric acetate (PMA) concentrations as low as 5 x 10(-10) M (0.1 ppb) may be detected within a few minutes.

Optical spectroscopic and reverse-phase HPLC analyses of Hg(II) binding to phytochelatins

Optical spectroscopy and reverse-phase HPLC were used to investigate the binding of Hg(II) to plant metal-binding peptides (phytochelatins) with the structure (gammaGlu-Cys)2Gly, (gammaGlu-Cys)3Gly and (gammaGlu-Cys)4Gly. Glutathione-mediated transfer of Hg(II) into phytochelatins and the transfer of the metal ion from one phytochelatin to another was also studied using reverse-phase HPLC. The saturation of Hg(II)-induced bands in the UV/visible and CD spectra of (gammaGlu-Cys)2Gly suggested the formation of a single Hg(II)-binding species of this peptide with a stoichiometry of one metal ion per peptide molecule. The separation of apo-(gammaGlu-Cys)2Gly from its Hg(II) derivative on a C18 reverse-phase column also indicated the same metal-binding stoichiometry. The UV/visible spectra of both (gammaGlu-Cys)3Gly and (gammaGlu-Cys)4Gly at pH 7.4 showed distinct shoulders in the ligand-to-metal charge-transfer region at 280-290 mm. Two distinct Hg(II)-binding species, occurring at metal-binding stoichiometries of around 1.25 and 2.0 Hg(II) ions per peptide molecule, were observed for (gammaGlu-Cys)3Gly. These species exhibited specific spectral features in the charge-transfer region and were separable by HPLC. Similarly, two main Hg(II)-binding species of (gammaGlu-Cys)4Gly were observed by UV/visible and CD spectroscopy at metal-binding stoichiometries of around 1.25 and 2.5 respectively. Only a single peak of Hg(II)-(gammaGlu-Cys)4Gly complexes was resolved under the conditions used for HPLC. The overall Hg(II)-binding stoichiometries of phytochelatins were similar at pH 2.0 and at pH 7.4, indicating that pH did not influence the final Hg(II)-binding capacity of these peptides. The reverse-phase HPLC assays indicated a rapid transfer of Hg(II) from glutathione to phytochelatins. These assays also demonstrated a facile transfer of the metal ion from shorter- to longer-chain phytochelatins. The strength of Hg(II) binding to glutathione and phytochelatins followed the order: gammaGlu-Cys-Gly<(gammaGlu-Cys)2Gly<(gammaGlu-Cy s)3Gly<(gamma Glu-Cys)4Gly.

Radial mass analysis of the flagellar filament of Salmonella: implications for the subunit folding

X-ray fiber diffraction patterns of the R-type straight flagellar filament of Salmonella typhimurium SJW1655 strain showed layer-lines with an axial spacing of 1/437 A-1, which could be resolved only due to very small disorientation angles (< 2 degrees) of the filaments in oriented sol specimens. Although the equatorial layer-line was situated between the relatively strong first layer-lines right above and below it, these small disorientation angles and a new method of two-dimensional angular deconvolution allowed us to determine the equatorial layer-line intensities quite accurately. The equatorial data were phased by using the amplitude difference between the native flagellar filament and its heavy atom derivatives. One of the heavy-atom derivatives was prepared by introducing a cysteine residue by site-directed mutagenesis and applying a mercury compound. From the equatorial structure factors, the radial density distribution of the filament was calculated at 11 A resolution. A prominent feature was two pairs of high density peaks at radii of around 25 and 45 A and a deep density trough between them, which corresponds to the concentric double tubular structure in the core region that has been found in the density map recently deduced by helical image reconstruction from electron micrographs of frozen hydrated filaments. The molecular masses were estimated for four radial segments that correspond to the morphological domains identified in the map of helical image reconstruction. Then the domains were assigned to sequence positions by correlating the estimated masses with those of proteolytic fragments of flagellin. The assignment is consistent with the distributions of secondary structures and in particular alpha-helical coiled-coils that were predicted from the sequence. It also helps to understand how the polymerization behaviour is affected by truncation of the disordered terminal regions of flagellin and why mutations in a specific region are responsible for changes in the polymorphic shape of the filament.

Form of mercury in stream fish exposed to high concentrations of dissolved inorganic mercury

The form of mercury predominating in mercury-contaminated fish from both pristine and industrialized waters in North America and Europe has almost universally been methylmercury. Sunfish (Lepomis auritus) living in a stream contaminated with 0.5-1 micrograms/L dissolved inorganic mercury accumulated greater concentrations of total mercury at headwater sites, where the dissolved mercury concentrations were greatest, than they did at downstream sites. However, despite evidence from laboratory studies that dissolved inorganic mercury is rapidly accumulated by fish without transformation to methylmercury, methylmercury constituted 85% or more of the total mercury concentration in fish at all sites.

Synthesis of polymer-bound 6-thiolatomercury and 6-mercuric sulfonate DOPA precursors and their halodemercuration reactivity

Fluorodemercuration has the greatest utility for the preparation of 6-[18F]DOPA, but requires separation from unreacted mercury precursor and other mercury-containing compounds. One approach is the development of a polymer-bound mercury precursor. In this study, polymer-bound 6-thiolatomercury and 6-mercuric sulfonate DOPA derivatives, and its monomeric analogs were synthesized. Fluorodemercuration of monomeric analog of mercuric sulfonate gave half the yield (14-15%) while iododemercuration gave the same yield (38%) compared with a 6-mercuric trifluoroacetate protected DOPA. The mercuric sulfonate undergoes halodemercuration, so polymer-bound halodemercuration precursors may be useful as precursors of 6-[18F]DOPA.

Inhibition of phosphomannose isomerase by mercury ions

Mercury ions can inhibit Candida albicans phosphomannose isomerase (PMI) by two different processes at sub-micromolar concentrations. Kinetic studies show that mercury ions are in rapid equilibrium with the enzyme and cause a clear partial noncompetitive inhibition when mannose 6-phosphate is used as the substrate. The inhibition constants at 37 degrees C in 50 mM Hepes buffer, pH 8.0, are 35 and 57 nM for Kii and Kis, respectively. In addition to this inhibition at rapid equilibrium, mercury ions also inactivate C. albicans PMI by a much slower process, involving an irreversible mechanism. This is shown to be a two-step process, proceeding via an intermediate complex with a dissociation constant of 5.6 microM, with a maximum rate of inactivation of 0.15 min-1. The rate of irreversible inactivation can be slowed by the addition of the substrate, mannose 6-phosphate. Incubation of the enzyme with [203Hg]Cl2 causes the formation of a stable adduct with one atom of mercury incorporated into each enzyme molecule during the inactivation. Since cysteine-150 is the only iodoacetate-modifiable cysteine in the protein, we propose that this is where the mercury ion reacts during the irreversible inactivation process. In the Escherichia coli enzyme this cysteine is replaced by an asparagine, and the enzyme cannot be irreversibly inactivated by mercury ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of mercurials to membrane suspensions and undenatured proteins

Binding capacities of membrane suspensions and dissolved compounds for mercurials were titrated by a new potentiometric method. Critical steps included a silver electrode of new design, the use of L-cysteine as a thiol buffer, a nitrogen atmosphere, and pretreatment of samples with equimolar mercurial and cysteine. Titrations had a sharp endpoint, accurate +/- 26 nmole methylmercury or +/- 8 nmole mercuric salt. Measurements of binding capacity of bovine serum albumin averaged 93% of the titer predicted for one SH group per molecule; those of human hemoglobin yielded 86-91% of the titer predicted for two SH groups per molecule. Yields dropped with exposure of protein solutions or membrane suspensions to atmospheric oxygen. Brain microsomes had significantly higher binding capacities (per milligram of protein) than red blood cell ghosts. The ratio of endpoint titers of CH3HgCl to HgCl2 averaged 2:1 in assays of cysteine, proteins, and membranes, showing that the assay was free of denaturation artifacts and protein-protein interference. Solutions of EDTA showed measurable binding of Hg2+ but not of CH3Hg+. Satisfactory titrations were also obtained with N-ethylmaleimide.