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

Detoxifying the heavy metals: a multipronged study of tolerance strategies against heavy metals toxicity in plants

Heavy metal concentrations exceeding permissible limits threaten human life, plant life, and all other life forms. Different natural and anthropogenic activities emit toxic heavy metals in the soil, air, and water. Plants consume toxic heavy metals from their roots and foliar part inside the plant. Heavy metals may interfere with various aspects of the plants, such as biochemistry, bio-molecules, and physiological processes, which usually translate into morphological and anatomical changes. They use various strategies to deal with the toxic effects of heavy metal contamination. Some of these strategies include restricting heavy metals to the cell wall, vascular sequestration, and synthesis of various biochemical compounds, such as phyto-chelators and organic acids, to bind the free moving heavy metal ions so that the toxic effects are minimized. This review focuses on several aspects of genetics, molecular, and cell signaling levels, which integrate to produce a coordinated response to heavy metal toxicity and interpret the exact strategies behind the tolerance of heavy metals stress. It is suggested that various aspects of some model plant species must be thoroughly studied to comprehend the approaches of heavy metal tolerance to put that knowledge into practical use.

Divalent Mercury in Dissolved Organic Matter Is Bioavailable to Fish and Accumulates as Dithiolate and Tetrathiolate Complexes

The freshwater cyprinid Tanichthys albonubes was used to assess the bioavailability of divalent mercury (Hg(II)) complexed in dissolved organic matter (DOM) to fish. The fish acquired 0.3 to 2.2 μg Hg/g dry weight after 8 weeks in aquaria containing DOM from a Carex peat with complexed mercury at initial concentrations of 14 nM to 724 nM. Changes in the relative proportions of dithiolate Hg(SR)₂ and nanoparticulate β-HgS in the DOM, as quantified by high energy-resolution XANES (HR-XANES) spectroscopy, indicate that Hg(SR)₂ complexes either produced by microbially induced dissolution of nanoparticulate β-HgS in the DOM or present in the original DOM were the forms of mercury that entered the fish. In the fish with 2.2 μg Hg/g, 84 ± 8% of Hg(II) was bonded to two axial thiolate ligands and one or two equatorial N/O electron donors (Hg[(SR)₂+(N/O)_1-2] coordination), and 16% had a Hg(SR)₄ coordination, as determined by HR-XANES. For comparison, fish exposed to Hg²⁺ from 40 nM HgCl₂ contained 10.4 μg Hg/g in the forms of dithiolate (20 ± 10%) and tetrathiolate (23 ± 10%) complexes, and also Hg _xS _y clusters (57 ± 15%) having a β-HgS-type local structure and a dimension that exceeded the size of metallothionein clusters. There was no evidence of methylmercury in the fish or DOM within the 10% uncertainty of the HR-XANES. Together, the results indicate that inorganic Hg(II) bound to DOM is a source of mercury to biota with dithiolate Hg(SR)₂ complexes as the immediate species bioavailable to fish, and that these complexes transform in response to cellular processes.

Interactions between mercury and phytoplankton: speciation, bioavailability, and internal handling

The present review describes and discusses key interactions between mercury (Hg) and phytoplankton to highlight the role of phytoplankton in the biogeochemical cycle of Hg and to understand direct or indirect Hg effects on phytoplankton. Phytoplankton are exposed to various Hg species in surface waters. Through Hg uptake, phytoplankton affect the concentration, speciation, and fate of Hg in aquatic systems. The mechanisms by which phytoplankton take up Hg are still not well known, but several studies have suggested that both facilitated transport and passive diffusion could be involved. Once internalized, Hg will impact several physiological processes, including photosynthesis. To counteract these negative effects, phytoplankton have developed several detoxification strategies, such as the reduction of Hg to elemental Hg or its sequestration by intracellular ligands. Based on the toxicological studies performed so far in the laboratory, Hg is unlikely to be toxic to phytoplankton when they are exposed to environmentally relevant Hg concentrations. However, this statement should be taken with caution because questions remain as to which Hg species control Hg bioavailability and about Hg uptake mechanisms. Finally, phytoplankton are primary producers, and accumulated Hg will be transferred to higher consumers. Phytoplankton are a key component in aquatic systems, and their interactions with Hg need to be further studied to fully comprehend the biogeochemical cycle of Hg and the impact of this ubiquitous metal on ecosystems.

Intracellular speciation and transformation of inorganic mercury in marine phytoplankton

Metal speciation is closely related to toxicity in aquatic organisms, but quantitative study of mercury transformation has rarely been reported. In this study, the ability of three marine phytoplankton species, including a green alga Chlorella autotrophica, a flagellate Isochrysis galbana and a diatom Thalassiosira weissflogii, to convert inorganic mercury were examined. We found that all algae tested were able to transform Hg(II) into dissolved gaseous mercury (DGM), phytochelatin (PC) complexes and metacinnabar (β-HgS). The most tolerant species, T. weissflogii, generally produced the highest level of PCs and β-HgS. Attributed to the highest DGM production ability, C. autotrophica accumulated the least Hg, but was the most sensitive due to low PC induction and β-HgS formation. Of the added Hg(II), less than 5% was reduced to DGM per day in all species. Of the intracellular Hg, <20% and 20-90% were chelated by PCs and transformed into β-HgS, respectively. These results suggest that intracellular biotransformation might be more important than bioavailability regulation in Hg(II) detoxification in marine phytoplankton.

Differential acclimation of a marine diatom to inorganic mercury and methylmercury exposure

Aquatic organisms originating from metal polluted water may exhibit differences in their sensitivity to metals, but the underlying physiological mechanisms resulting in such responses have not been well reported. In the present study, a marine diatom Thalassiosira weissflogii was chronically exposed to different inorganic mercury (Hg(II), 0.5 and 5 μg Hg/L) or methylmercury (MeHg, 0.02 and 0.4 μg Hg/L) concentrations for over 18 generations. We then quantified the changes in the Hg(II) or MeHg sensitivity, Hg accumulation, subcellular distribution, as well as thiol compound induction in the diatoms. We found an unchanged tolerance to Hg(II) but an enhanced tolerance to MeHg in the preconditioned T. weissflogii. The underlying mechanisms may be related to the changes in cellular mercury accumulation and the detoxification ability of the cells. Specifically, exposure to high-Hg(II) led to increased metal distribution in cellular debris fraction, as well as the induction of a variety of non-protein thiol compounds, but the uptake kinetics was not significantly modified by Hg(II) exposure. Instead, exposure to high-MeHg decreased the mercury uptake rate along with the synthesis of glutathione (GSH) and (γ-EC)₂-Gly (PC₂). All these responses contributed to the different tolerance developments between Hg(II) and MeHg. This study suggests that moderation of Hg bioavailability was probably more important than internal detoxification in the development of Hg acclimation in marine diatoms.

Thiol compounds induction kinetics in marine phytoplankton during and after mercury exposure

We investigated the kinetics of Hg(II) and MeHg accumulation and the synthesis of phytochelatins (PCs), cysteine (Cys), glutathione (GSH), and γ-glutamylcysteine (γ-EC) in a marine diatom Thalassiosira weissflogii during a 3-h (short-term) and a 96-h (long-term) exposure period, and during a subsequent 96-h recovery period. MeHg induced the synthesis of a significant level of GSH, but it was Hg(II) that gave rise to significant levels of other non-protein thiol compounds. The thiol compounds Cys, γ-EC, and PC(2-3) were induced in T. weissflogii within the first 30 min of exposure, followed by PC(4), but the concentrations of all six compounds returned to the control levels after the 96-h recovery period. The kinetics of these non-protein thiol compounds pointed to a rapid cellular response to environmental mercury pollution. After a first decrease, the molar ratio of PC-SH (sulfhydryl in PCs) to intracellular Hg increased slightly which demonstrated the role of PCs in Hg(II) detoxification. However, PC-SH was bound with Hg(II) at a stoichiometric ratio of 0.1-0.3, indicating the involvement of other detoxification mechanisms. Elucidating the effects of mercury on intracellular non-protein thiol pools may help us better understand the metal detoxification in phytoplankton.

The phytochelatin transporters AtABCC1 and AtABCC2 mediate tolerance to cadmium and mercury

Heavy metals such as cadmium (Cd) and mercury (Hg) are toxic pollutants that are detrimental to living organisms. Plants employ a two-step mechanism to detoxify toxic ions. First, phytochelatins bind to the toxic ion, and then the metal-phytochelatin complex is sequestered in the vacuole. Two ABCC-type transporters, AtABCC1 and AtABCC2, that play a key role in arsenic detoxification, have recently been identified in Arabidopsis thaliana. However, it is unclear whether these transporters are also implicated in phytochelatin-dependent detoxification of other heavy metals such as Cd(II) and Hg(II). Here, we show that atabcc1 single or atabcc1 atabcc2 double knockout mutants exhibit a hypersensitive phenotype in the presence of Cd(II) and Hg(II). Microscopic analysis using a Cd-sensitive probe revealed that Cd is mostly located in the cytosol of protoplasts of the double mutant, whereas it occurs mainly in the vacuole of wild-type cells. This suggests that the two ABCC transporters are important for vacuolar sequestration of Cd. Heterologous expression of the transporters in Saccharomyces cerevisiae confirmed their role in heavy metal tolerance. Over-expression of AtABCC1 in Arabidopsis resulted in enhanced Cd(II) tolerance and accumulation. Together, these results demonstrate that AtABCC1 and AtABCC2 are important vacuolar transporters that confer tolerance to cadmium and mercury, in addition to their role in arsenic detoxification. These transporters provide useful tools for genetic engineering of plants with enhanced metal tolerance and accumulation, which are desirable characteristics for phytoremediation.

Metallothionein-like multinuclear clusters of mercury(II) and sulfur in peat

Strong mercury(II)-sulfur (Hg-SR) bonds in natural organic matter, which influence mercury bioavailability, are difficult to characterize. We report evidence for two new Hg-SR structures using X-ray absorption spectroscopy in peats from the Florida Everglades with added Hg. The first, observed at a mole ratio of organic reduced S to Hg (S(red)/Hg) between 220 and 1140, is a Hg(4)S(x) type of cluster with each Hg atom bonded to two S atoms at 2.34 Å and one S at 2.53 Å, and all Hg atoms 4.12 Å apart. This model structure matches those of metal-thiolate clusters in metallothioneins, but not those of HgS minerals. The second, with one S atom at 2.34 Å and about six C atoms at 2.97 to 3.28 Å, occurred at S(red)/Hg between 0.80 and 4.3 and suggests Hg binding to a thiolated aromatic unit. The multinuclear Hg cluster indicates a strong binding environment to cysteinyl sulfur that might impede methylation. Along with a linear Hg(SR)(2) unit with Hg-S bond lengths of 2.34 Å at S(red)/Hg of about 10 to 20, the new structures support a continuum in Hg-SR binding strength in natural organic matter.

Complexation of Hg with phytochelatins is important for plant Hg tolerance

Three-week-old alfalfa (Medicago sativa), barley (Hordeum vulgare) and maize (Zea mays) were exposed for 7 d to 30 µm of mercury (HgCl(2) ) to characterize the Hg speciation in root, with no symptoms of being poisoned. The largest pool (99%) was associated with the particulate fraction, whereas the soluble fraction (SF) accounted for a minor proportion (<1%). Liquid chromatography coupled with electro-spray/time of flight mass spectrometry showed that Hg was bound to an array of phytochelatins (PCs) in root SF, which was particularly varied in alfalfa (eight ligands and five stoichiometries), a species that also accumulated homophytochelatins. Spatial localization of Hg in alfalfa roots by microprobe synchrotron X-ray fluorescence spectroscopy showed that most of the Hg co-localized with sulphur in the vascular cylinder. Extended X-ray Absorption Fine Structure (EXAFS) fingerprint fitting revealed that Hg was bound in vivo to organic-S compounds, i.e. biomolecules containing cysteine. Albeit a minor proportion of total Hg, Hg-PCs complexes in the SF might be important for tolerance to Hg, as was found with Arabidopsis thaliana mutants cad2-1 (with low glutathione content) and cad1-3 (unable to synthesize PCs) in comparison with wild type plants. Interestingly, high-performance liquid chromatography-electrospray ionization-time of flight analysis showed that none of these mutants accumulated Hg-biothiol complexes.

Electrochemical survey of the chain length influence in phytochelatins competitive binding by cadmium

Multivariate curve resolution with alternating least squares (MCR-ALS) was applied to voltammetric data obtained in the analysis of the competitive binding of glutathione (GSH) and phytochelatins [(gammaGlu-Cys)(n)-Gly, PC(n), n=2-5] by Cd(2+). The displacements between ligands and chain length influence on the competitive binding of PC(n) toward Cd(2+) were investigated. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggests that ligands containing more thiol groups are able to displace the shortest chain ligands from their metal complexes, whereas the opposite does not happen. However, when the length of the chain surpasses that of PC(3), the binding capacity of the molecule still increases (i.e., it can bind more metal ions), but the position and shape of the voltammetric signals practically rest unchanged. This suggests that at this level, the stability of metal binding could depend more on the nature of the binding sites separately than on the quantity of the sites (i.e., the chain length).

Changes in the non-protein thiol pool and production of dissolved gaseous mercury in the marine diatom Thalassiosira weissflogii under mercury exposure

Two detoxification mechanisms working in the marine diatom Thalassiosira weissflogii to cope with mercury toxicity were investigated. Initially, the effect of mercury on the intracellular pool of non-protein thiols was studied in exponentially growing cultures exposed to sub-toxic HgCl(2) concentrations. T. weissflogii cells responded by synthesizing metal-binding peptides, named phytochelatins (PCs), besides increasing the intracellular pool of glutathione and gamma-glutamylcysteine (gamma-EC). Intracellular Hg and PC concentrations increased with the Hg concentration in the culture medium, exhibiting a distinct dose-response relationship. However, considerations of the PCs-SH:Hg molar ratio suggest that glutathione could also be involved in the intracellular mercury sequestration. The time course of the non-protein thiol pool and Hg intracellular concentration shows that PCs, glutathione and gamma-EC represent a rapid cellular response to mercury, although their role in Hg detoxification seems to lose importance at longer incubation times. The occurrence of a process of reduction of Hg(II) to Hg degrees and subsequent production of dissolved gaseous mercury (DGM) was also investigated at lower Hg concentrations, at which the PC synthesis doesn't seem to be involved. The significant (P<0.01) correlation between the cellular density in solution and the production of DGM suggests that this diatom is capable of directly producing DGM, both in light and dark conditions. This finding has been confirmed by the absence of DGM production in the culture media containing formaldehyde-killed cells. Finally, the relationship between these two different pathways of Hg detoxification is discussed.

Binding of Hg2+ with phytochelatins: study by differential pulse voltammetry on rotating Au-disk electrode, electrospray ionization mass-spectrometry, and isothermal titration calorimetry

The binding of Hg2+ with synthetic phytochelatins ((gamma-Glu-Cys)n-Gly, PCn, n = 2, 3, 4) was investigated by a recently proposed electroanalytical method, using differential pulse voltammetry on the rotating Au-disk electrode, Electrospray ionization mass-spectrometry (ESI-MS) and isothermaltitration calorimetry (ITC). ESI-MS experiments provided the exact stoichiometries of the complexes formed at different PCn/Hg2+ ratios. Voltammetry provided more detailed information on the complexation processes through the use of multivariate curve resolution by alternating least squares of the data matrix obtained from titrations withfine increments of metal or ligand. The system Hg2+-GSH-PC2 was investigated by voltammetry in order to obtain an estimation of the Hg2+ behavior in the presence of two related ligands. The additional assessment of the stability of Hg2+-PCn complexes was achieved through ITC by using the therapeutic chelator sodium 2,3-dimercaptopropanesulfate (DMPS) over Hg2+-PCn systems. The stability of various Hg2+-PCn complexes and the ability of DMPS to replace PCn from these complexes were examined.

Detection and quantification of unbound phytochelatin 2 in plant extracts of Brassica napus grown with different levels of mercury

The mercury (Hg) accumulation mechanism was studied in rape (Brassica napus) plants grown under a Hg concentration gradient (0 microm-1,000 microm). Hg mainly accumulated in roots. Therefore, the presence of phytochelatins (PCs) was studied in the roots of the plants. The high stability of the PC-Hg multicomplexes (mPC-nHg) seems to be the main reason for the lack of previous Hg-PC characterization studies. We propose a modification of the method to detect and quantify unbound PC of Hg in plant extracts via high-performance liquid chromatography coupled to electrospray tandem mass spectrometry and inductively coupled plasma mass spectrometry in parallel. We separated the PC from the Hg by adding the chelating agent sodium 2,3-dimercaptopropanesulfonate monohydrate. We only detected the presence of PC after the addition of the chelating agent. Some multicomplexes mPC-nHg could be formed but, due to their large sizes, could not be detected. In this study, only PC(2) was observed in plant samples. Hg accumulation was correlated with PC(2) concentration (r(2) = 0.98).

Cellular damage induced by cadmium and mercury in Medicago sativa

Alfalfa (Medicago sativa) plantlets were exposed to Cd or Hg to study the kinetics of diverse stress indexes. In the so-called beaker-size hydroponic system, plantlets were grown in 30 microM of Cd or Hg for 7 d. Oxidative stress took place and increased over time, a linear response being observed with Cd but not with Hg. To improve the sensitivity of the stress assays used, a micro-assay system, in which seedlings were exposed for 24 h, was developed. Phytotoxicity of metals, quantified as growth inhibition, was observed well before there was any change in the non-protein thiol tissue concentration. When measured with conventional techniques, oxidative stress indexes did not show significant variation. To trace early and small plant responses to Cd and Hg, a microscopic analysis with novel fluorescent dyes, which had not yet been exploited to any significant extent for use in plants, was conducted. These fluorescent probes, which allowed minute cellular responses to 0, 3, 10, and 30 microM of both metals to be visualized in the roots of the alfalfa seedlings, were: (i) 2',7'-dichlorofluorescin diacetate that labels peroxides; (ii) monochlorobimane that stains reduced glutathione/homoglutathione (GSH/hGSH); and (iii) propidium iodide that marks nuclei of dead cells. Oxidative stress and cell death increased after exposure for 6-24 h to Cd and Hg, but labelling of GSH/hGSH decreased acutely. This diminution might be the result of direct interaction of GSH/hGSH with both Cd and Hg, as inferred from an in vitro conjugation assay. Therefore, both Cd and Hg not only compromised severely the cellular redox homeostasis, but also caused cell necrosis. In plants treated with 1 mM L-buthionine sulphoximine, a potent inhibitor of GSH/hGSH synthesis, only the oxidative stress symptoms appeared, indicating that the depletion of the GSH/hGSH pool was not sufficient to promote cell death, and that other phytotoxic mechanisms might be involved.

Clonal differences in mercury tolerance, accumulation, and distribution in willow

This study was performed to investigate mercury (Hg) tolerance, accumulation, and translocation within the genus Salix for the potential use of this plant to remediate Hg-contaminated sites. Six clones of willow (Salix spp.) were tested on tolerance to Hg by treating plants grown in solution culture with 0 to 15 microM HgCl(2). Results showed that willow had a large variation in its sensitivity to Hg. However, the accumulation and translocation of Hg to shoots was similar in the eight tested willow clones as shown by cold vapor atomic absorption spectrometry analysis when plants were treated with 0.5 microM HgCl(2) in a nutrient solution. The majority of total Hg accumulated was localized to the roots, whereas only 0.45 to 0.62% of the total Hg accumulated via roots was translocated to the shoots. Thus, the root system is the main tissue of willow that accumulates Hg and the majority of the Hg in the root system (80%) was bound in the cell wall.

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

A series of 24 compounds of general formula R(1)S-Hg-SR(2), R(1) and R(2) being biologically relevant thiol-containing amino acids and peptides (cysteine, homo-cysteine, penicillamine, N-acetyl-cysteine, N-acetyl-penicillamine, cysteinyl-glycine, gamma-glutamyl-cysteine and glutathione) were prepared by direct reaction of mercury(II) ions and thiols in water at millimolar concentration. The obtained products were characterized by electrospray ionization and triple quadrupole tandem mass spectrometry. The source spectra of equimolar mixtures of two different thiols reacting with a stoichiometric amount of mercury(II) show the peak clusters of the three theoretically expected bis-thiolato-mercury(II) complexes with relative intensities close to the theoretically expected 1:2:1 ratio, thus pointing at lack of substantial discrimination between the different thiols, the only observed exception being homo-cysteine, which is less reactive than cysteine and penicillamine. The fragment spectra are structure-specific for the different ligands bound to the metal ion and allow a stand-alone discrimination of some constitutional isomer pairs. Among the peculiar fragmentation processes observed, loss of neutral ammonia from protonated symmetrical and unsymmetrical mercury(II)-bis-thiolates with free, protonizable amino groups leads to the formation of thiirane-carboxylic bound species; this process is suppressed when the protonated amino group is in the gamma-position with respect to the sulfur atom, as in the case of compounds with homo-cysteine. This unusual behavior may hint at unforeseen mechanisms for the interaction of mercury(II) with biological structures, ultimately leading to cellular and organ toxicity. Compounds with N-acetylated amino acids show distinctive fragment ions to which the connectivity of a protonated 2-methyl-oxazoline-5-carboxylic acid may be proposed on the basis of the loss of water and of the elements of formic acid. Finally, the adducts of mercury(II) with glutathione and gamma-glutamyl-cysteine feature a distinctive decomposition channel by loss of a pyroglutamic unit, much the same as protonated glutathione, glutathione disulfide, the S-glutathionyl adducts of biologically occurring electrophiles and other (pseudo)-peptides with gamma-glutamyl bonds.

The nature of arsenic-phytochelatin complexes in Holcus lanatus and Pteris cretica

We have developed a method to extract and separate phytochelatins (PCs)-metal(loid) complexes using parallel metal(loid)-specific (inductively coupled plasma-mass spectrometry) and organic-specific (electrospray ionization-mass spectrometry) detection systems-and use it here to ascertain the nature of arsenic (As)-PC complexes in plant extracts. This study is the first unequivocal report, to our knowledge, of PC complex coordination chemistry in plant extracts for any metal or metalloid ion. The As-tolerant grass Holcus lanatus and the As hyperaccumulator Pteris cretica were used as model plants. In an in vitro experiment using a mixture of reduced glutathione (GS), PC(2), and PC(3), As preferred the formation of the arsenite [As((III))]-PC(3) complex over GS-As((III))-PC(2), As((III))-(GS)(3), As((III))-PC(2), or As((III))-(PC(2))(2) (GS: glutathione bound to arsenic via sulphur of cysteine). In H. lanatus, the As((III))-PC(3) complex was the dominant complex, although reduced glutathione, PC(2), and PC(3) were found in the extract. P. cretica only synthesizes PC(2) and forms dominantly the GS-As((III))-PC(2) complex. This is the first evidence, to our knowledge, for the existence of mixed glutathione-PC-metal(loid) complexes in plant tissues or in vitro. In both plant species, As is dominantly in non-bound inorganic forms, with 13% being present in PC complexes for H. lanatus and 1% in P. cretica.

Purification and Characterization of Thiols in an Arsenic Hyperaccumulator under Arsenic Exposure

Pteris vittata (Chinese brake fern) is the first reported arsenic hyperaccumulator. To investigate the arsenic tolerance mechanism in this plant, reversed-phase HPLC with postcolumn derivatization was used to analyze the thiols induced under arsenic exposure. A major thiol in the plant leaflets was found to be responsive to arsenic exposure. The arsenic-induced compound was purified on a large scale by combining covalent chromatography and preparative reversed-phase HPLC. About 2 mg of this compound was isolated from 1 kg of fresh leaflets. The purified arsenic-induced compound was characterized using electrospray ionization mass spectrometry. A molecular ion (M + 1) of 540 and fragments were obtained, which indicated that the arsenic-induced thiol was a phytochelatin with two subunits (PC(2)). Compared to the classical methods for purification of phytochelatins, this new method is more specific, simple, and rapid and is suitable for purification of PCs in a large scale as well as sample preparation for mass spectrometry analysis.

Genetic engineering of Escherichia coli for enhanced uptake and bioaccumulation of mercury

Synthetic phytochelatins (ECs) are a new class of metal-binding peptides with a repetitive metal-binding motif, (Glu-Cys)(n)Gly, which were shown to bind heavy metals more effectively than metallothioneins. However, the limited uptake across the cell membrane is often the rate-limiting factor for the intracellular bioaccumulation of heavy metals by genetically engineered organisms expressing these metal-binding peptides. In this paper, two potential solutions were investigated to overcome this uptake limitation either by coexpressing an Hg(2+) transport system with (Glu-Cys)(20)Gly (EC20) or by directly expressing EC20 on the cell surface. Both approaches were equally effective in increasing the bioaccumulation of Hg(2+). Since the available transport systems are presently limited to only a few heavy metals, our results suggest that bioaccumulation by bacterial sorbents with surface-expressed metal-binding peptides may be useful as a universal strategy for the cleanup of heavy metal contamination.

Accumulation of metal-binding peptides in fission yeast requires hmt2+

The fission yeast Schizosaccharomyces pombe detoxifies cadmium by synthesizing phytochelatins, peptides of the structure (gamma-GluCys)nGly, which bind cadmium and mediate its sequestration into the vacuole. The fission yeast protein HMT2, a mitochondrial enzyme that can oxidize sulphide, appears to be essential for tolerance to multiple forms of stress, including exposure to cadmium. We found that the hmt2- mutant is unable to accumulate normal levels of phytochelatins in response to cadmium, although the cells possess a phytochelatin synthase that is active in vitro. Radioactive pulse-chase experiments demonstrated that the defect lies in two steps: the synthesis of phytochelations and the upregulation of glutathione production. Phytochelatins, once formed, are stable. hmt2- cells accumulate high levels of sulphide and, when exposed to cadmium, display bright fluorescent bodies consistent with cadmium sulphide. We propose that the precipitation of free cadmium blocks phytochelatin synthesis in vivo, by preventing upregulation of glutathione production and formation of the cadmium-glutathione thiolate required as a substrate by phytochelatin synthase. Thus, although sulphide is required for phytochelatin-mediated metal tolerance, aberrantly high sulphide levels can inhibit this pathway. Precise regulation of sulphur metabolism, mediated in part by HMT2, is essential for metal tolerance in fission yeast.

Enhanced bioaccumulation of heavy metals by bacterial cells displaying synthetic phytochelatins

A novel strategy using synthetic phytochelatins is described for the purpose of developing microbial agents for enhanced bioaccumulation of toxic metals. Synthetic genes encoding for several metal-chelating phytochelatin analogs (Glu-Cys)(n)Gly (EC8 (n = 8), EC11 (n = 11), and EC20 (n = 20)) were synthesized, linked to a lpp-ompA fusion gene, and displayed on the surface of E. coli. For comparison, EC20 was also expressed periplasmically as a fusion with the maltose-binding protein (MBP-EC20). Purified MBP-EC20 was shown to accumulate more Cd(2+) per peptide than typical mammalian metallothioneins with a stoichiometry of 10 Cd(2+)/peptide. Cells displaying synthetic phytochelatins exhibited chain-length dependent increase in metal accumulation. For example, 18 nmoles of Cd(2+)/mg dry cells were accumulated by cells displaying EC8, whereas cells exhibiting EC20 accumulated a maximum of 60 nmoles of Cd(2+)/mg dry cells. Moreover, cells with surface-expressed EC20 accumulated twice the amount of Cd(2+) as cells expressing EC20 periplasmically. The ability to genetically engineer ECs with precisely defined chain length could provide an attractive strategy for developing high-affinity bioadsorbents suitable for heavy metal removal.

Derivatization of phytochelatins from Silene vulgaris, induced upon exposure to arsenate and cadmium: comparison of derivatization with Ellman's reagent and monobromobimane

Phytochelatins (PCs) are a family of thiol-rich peptides, with the general structure (gamma-Glu-Cys)(n)()-Gly, with n = 2-11, induced in plants upon exposure to excessive amounts of heavy metals and some metalloids, such as arsenic. Two types of PC analyses are currently used, i.e., acid extraction and separation on HPLC with either precolumn derivatization (pH 8.2) with monobromobimane (mBBr) or postcolumn derivatization (pH 7.8) with Ellman's reagent [5, 5'-dithiobis(2-nitrobenzoic acid), DTNB]. Although both methods were satisfactory for analysis of Cd-induced PCs, formation of (RS)(3)-As complexes during extraction of As-induced PCs rendered the DTNB method useless. This paper shows that precolumn derivatization with mBBr, during which the (RS)(3)-As complexes are disrupted, provides a qualitative and quantitative analysis of both Cd- and As-induced PCs. In addition, derivatization efficiencies of both methods for the oligomers with n = 2-4 (PC(2)(-)(4)) are compared. Derivatization efficiency decreased from 71.8% and 81.4% for mBBr and DTNB derivatization, respectively, for PC(2) to 27.4% and 50.2% for PC(4). This decrease is most likely due to steric hindrance. Correction of measured thiol concentration is therefore advised for better quantification of PC concentrations in plant material.

Detoxification of arsenic by phytochelatins in plants

As is a ubiquitous element present in the atmosphere as well as in the aquatic and terrestrial environments. Arsenite and arsenate are the major forms of As intoxication, and these anions are readily taken up by plants. Both anions efficiently induce the biosynthesis of phytochelatins (PCs) ([gamma-glutamate-cysteine](n)-glycine) in vivo and in vitro. The rapid induction of the metal-binding PCs has been observed in cell suspension cultures of Rauvolfia serpentina, in seedlings of Arabidopsis, and in enzyme preparations of Silene vulgaris upon challenge to arsenicals. The rate of PC formation in enzyme preparations was lower compared with Cd-induced biosynthesis, but was accompanied by a prolonged induction phase that resulted finally in higher peptide levels. An approximately 3:1 ratio of the sulfhydryl groups from PCs to As is compatible with reported As-glutathione complexes. The identity of the As-induced PCs and of reconstituted metal-peptide complexes has unequivocally been demonstrated by electrospray ionization mass spectroscopy. Gel filtration experiments and inhibitor studies also indicate a complexation and detoxification of As by the induced PCs.

Structure and function of metal chelators produced by plants: the case for organic acids, amino acids, phytin, and metallothioneins

Plants produce a range of ligands for cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn). Cd- and Zn-citrate complexes are prevalent in leaves, even though malate is more abundant. In the xylem sap moving from roots to leaves, citrate and histidine are the principal ligands for Cu, Ni, and Zn. Phosphorus-rich globular bodies in young roots are probably Zn-phytate. Metallothioneins (MTs) are cysteine (Cys)-rich ligands. Plants produce class II MTs (MT-IIs) which differ from the archetypal mammalian MT-I in the location and number of Cys. The Ec protein from wheat embryos has Cys in three domains, binds Zn, and disappears with seedling development. The first 59 amino acids have been sequenced for the protein. Fifty-eight genes for MT-IIs, from a range of plants and tissues, predict proteins with Cys in two domains. Most of the predicted proteins have not been isolated, and their metal binding is poorly documented. Three protein bands, corresponding to six MT genes, have been isolated from Arabidopsis, and the amino acids sequenced for nine fragments. The MT-IIIs are atypical, nontranslationally synthesized polypeptides with variously repeating gamma-glutamylcysteine units. Of the five families known, those with carboxy-terminal glycine are the most widespread among plants, algae, and certain yeasts. A heterogeneous grouping of these molecules form Cd-binding complexes with tetrahedral coordination and a Cd-sulfur interatomic distance of 2.52 A. One complex is cytosolic, the dominant one is vacuolar. Together, they can bind a large proportion of cellular Cd; other ligands may also function. Little is known about the counterpart situation for Cu and Zn.

A role for HEM2 in cadmium tolerance

A Candida glabrata cadmium-sensitive mutant partially defective in glutathione production and exhibiting a complete absence of phytochelatins was used to clone a gene required for Cd tolerance. Transformation of the Cd-sensitive mutant with a genomic library from the wild-type C. glabrata led to the cloning of a gene that restored Cd tolerance and formation of Cd-glutathione and Cd-phytochelatin complexes. The cloned gene showed high levels of nucleic acid and protein sequence homology to the HEM2 genes, encoding porphobilinogen synthases, from several sources. It was shown that the C, glabrata Cd-sensitive mutant indeed exhibited a significant reduction in porphobilinogen synthase levels. The cloned C. glabrata gene complemented a hem2 mutant of Saccharomyces cerevisiae and restored porphobilinogen synthase activity in the mutant. The Cd sensitive mutant predictably showed decreased levels of sulfite reductase that requires siroheme, a metabolite produced in the heme biosynthetic pathway. The addition of cysteine, but not methionine, increased glutathione levels and Cd tolerance of both the wild-type and the mutant strain. However, addition of hemin chloride and methionine together restored Cd tolerance indicating that heme was required for transsulfuration of homocysteine to cysteine.

Recent applications of high-performance liquid chromatography to the analysis of metal complexes

Interest in metal complexes in modern inorganic chemistry has resulted in increasing demands for the analysis of these compounds. This paper reviews the most recent applications of high-performance liquid chromatography (HPLC) to the analysis of metal complexes. The review centres on the use of the technique in metal complex syntheses, reactions, characterizations and complexations and retention behaviour of these compounds, as reported in the literature since 1994.

Characteristics of glutathione-capped ZnS nanocrystallites

The titration of increasing equivalents of inorganic sulfide into preformed Zn-glutathione led to the appearance of UV/VIS spectral features attributable to ZnS nanocrystallites. Glutathione-ZnS complexes upon irradiation caused reduction of methylviologen confirming their semiconductor properties. Size-fractionation of glutathione-ZnS samples on a gel filtration column showed the formation of a range of complexes whose spectral properties were correlated with the sulfide content. The stoichiometry of Zn:glutathione increased from 1:2 to a maximum of about 7:1 as the sulfide/Zn ratios increase from 0 to approximately 1.0 in Zn-glutathione complex indicating up to 14-fold increase in the Zn-binding capacity of glutathione upon sulfide incorporation. Spectral characteristics of GSH-capped ZnS nanocrystallites were significantly influenced by pH and by the stoichiometry of Zn, sulfide and glutathione in the complex. Samples containing least glutathione and highest sulfide showed maximal luminescence at pH 6, whereas those with higher glutathione and lower sulfide content showed maximal luminescence at pH 11.