Oxidative dimerization in metallothionein is a result of intermolecular disulphide bonds between cysteines in the alpha-domain

Metallothionein expression on oysters ( Crassostrea cuculata and Crassostrea glomerata) from the southern coastal region of East Java

Background: This study aimed to analyse levels of heavy metals (Pb, Hg and Cd) in the aquatic body, gills and stomach of the oysters Crassostrea cuculata and Crassostrea glomerata, the metallothionein (MT) level in the gills and stomach of both oysters, and relationships between heavy metals level (Pb, Hg and Cd) in the gills and stomach to MT level in both species of oysters. Methods: The research method utilized was a descriptive method. The oyster samples were taken from three stations: Sendang Biru, Popoh and Prigi beaches. MT values were assessed using confocal laser scanning microscopy. The heavy metal levels were assessed using atomic absorption spectrophotometry method. Results: Both oyster heavy metal content obtained in the southern coastal waters exceeded the safe limit set by the State Minister of Environment No. 51 of 2004. In general, the expression of MT was found to be higher in stomach tissue compared to gill tissue. Conclusions: The levels of the heavy metals Pb, Hg, and has a strong relationship with MT levels in the gills and stomach in both types of oysters.

Metallothionein expression on oysters ( Crassostrea cuculata and Crassostrea glomerata) from the southern coastal region of East Java

Background: This study aimed to analyse levels of heavy metals (Pb, Hg and Cd) in the aquatic body, gills and stomach of the oysters Crassostrea cuculata and Crassostrea glomerata, the metallothionein (MT) level in the gills and stomach of both oysters, and relationships between heavy metals level (Pb, Hg and Cd) in the gills and stomach to MT level in both types of oysters. Methods: The research method utilized was a descriptive method. The oyster samples were taken from three stations: Sendang Biru, Popoh and Prigi beaches. MT values were assessed using confocal laser scanning microscopy. The heavy metal levels were assessed using atomic absorption spectrophotometry method. Results: Both oyster heavy metal content obtained in the southern coastal waters exceeded the safe limit set by the State Minister of Environment No. 51 of 2004. In general, the expression of MT was found to be higher in gastric tissue compared to gill tissue. Conclusions: The relationship between levels of the heavy metals Pb, Hg, and Cd in oyster gills and stomach has a strong relationship with MT levels in the gills and stomach in both types of oysters.

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

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

Increased zinc and manganese in parallel with neurodegeneration, synaptic protein changes and activation of Akt/GSK3 signaling in ovine CLN6 neuronal ceroid lipofuscinosis

Mutations in the CLN6 gene cause a variant late infantile form of neuronal ceroid lipofuscinosis (NCL; Batten disease). CLN6 loss leads to disease clinically characterized by vision impairment, motor and cognitive dysfunction, and seizures. Accumulating evidence suggests that alterations in metal homeostasis and cellular signaling pathways are implicated in several neurodegenerative and developmental disorders, yet little is known about their role in the NCLs. To explore the disease mechanisms of CLN6 NCL, metal concentrations and expression of proteins implicated in cellular signaling pathways were assessed in brain tissue from South Hampshire and Merino CLN6 sheep. Analyses revealed increased zinc and manganese concentrations in affected sheep brain in those regions where neuroinflammation and neurodegeneration first occur. Synaptic proteins, the metal-binding protein metallothionein, and the Akt/GSK3 and ERK/MAPK cellular signaling pathways were also altered. These results demonstrate that altered metal concentrations, synaptic protein changes, and aberrant modulation of cellular signaling pathways are characteristic features in the CLN6 ovine form of NCL.

Use of (113)Cd NMR to probe the native metal binding sites in metalloproteins: an overview

Our laboratories have actively published in this area for several years and the objective of this chapter is to present as comprehensive an overview as possible. Following a brief review of the basic principles associated with (113)Cd NMR methods, we will present the results from a thorough literature search for (113)Cd chemical shifts from metalloproteins. The updated (113)Cd chemical shift figure in this chapter will further illustrate the excellent correlation of the (113)Cd chemical shift with the nature of the coordinating ligands (N, O, S) and coordination number/geometry, reaffirming how this method can be used not only to identify the nature of the protein ligands in uncharacterized cases but also the dynamics at the metal binding site. Specific examples will be drawn from studies on alkaline phosphatase, Ca(2+) binding proteins, and metallothioneins.In the case of Escherichia coli alkaline phosphatase, a dimeric zinc metalloenzyme where a total of six metal ions (three per monomer) are involved directly or indirectly in providing the enzyme with maximal catalytic activity and structural stability, (113)Cd NMR, in conjunction with (13)C and (31)P NMR methods, were instrumental in separating out the function of each class of metal binding sites. Perhaps most importantly, these studies revealed the chemical basis for negative cooperativity that had been reported for this enzyme under metal deficient conditions. Also noteworthy was the fact that these NMR studies preceded the availability of the X-ray crystal structure.In the case of the calcium binding proteins, we will focus on two proteins: calbindin D(9k) and calmodulin. For calbindin D(9k) and its mutants, (113)Cd NMR has been useful both to follow actual changes in the metal binding sites and the cooperativity in the metal binding. Ligand binding to calmodulin has been studied extensively with (113)Cd NMR showing that the metal binding sites are not directly involved in the ligand binding. The (113)Cd chemical shifts are, however, exquisitely sensitive to minute changes in the metal ion environment.In the case of metallothionein, we will reflect upon how (113)Cd substitution and the establishment of specific Cd to Cys residue connectivity by proton-detected heteronuclear (1)H-(113)Cd multiple-quantum coherence methods (HMQC) was essential for the initial establishment of the 3D structure of metallothioneins, a protein family deficient in the regular secondary structural elements of α-helix and β-sheet and the first native protein identified with bound Cd. The (113)Cd NMR studies also enabled the characterization of the affinity of the individual sites for (113)Cd and, in competition experiments, for other divalent metal ions: Zn, Cu, and Hg.

Role of metallothioneins in benign and malignant thyroid lesions

Recent findings in the past two decades have brought many insights into the biology of thyroid benign and malignant lesions, in particular the papillary and follicular thyroid cancers. Although, much progress have been made, thyroid cancers still pose diagnostic problems regarding differentiation of follicular lesions in relation to their aggressiveness and the treatment of advanced and undifferentiated thyroid cancers. Metallothioneins (MTs) were shown to induce cancer cells proliferation, mediate resistance to apoptosis, certain chemotherapeutics and radiotherapy. Therefore, MTs may be of utility in diagnosis and management of patients with benign and malignant lesions of the thyroid.

Sulfur mustard induces expression of metallothionein-1A in human airway epithelial cells

Background:

Sulfur mustard can cause several long-term complications in the organs of individuals exposed to this toxic gas, and among these, pulmonary sequelae are the most important. More than 25 years after the Iran–Iraq war, thousands of Iranians are suffering from the chronic respiratory complications of sulfur mustard. Currently, based on several clinical findings, bronchiolitis obliterans is confirmed as the major diagnosis in these patients. Numerous studies have revealed that this disorder is strongly associated with oxidative stress due to excessive production of harmful reactive substances and decreased levels of endogenous antioxidants. Metallothioneins (MTs) are a group of low molecular weight sulfhydryl-rich intra-cellular proteins, and several isoforms have been identified in humans. MT-1A is an inducible and important MT isoform, which is transcriptionally activated by a variety of stress stimuli, such as free radicals.

Methods:

MT-1 mRNA expression and protein levels in endobronchial biopsy samples from 24 sulfur mustard-exposed patients and 15 unexposed control cases were evaluated by semi-quantitative reverse transcriptase polymerase chain reaction, real-time reverse transcriptase polymerase chain reaction, and immunohistochemistry.

Results:

mRNA- MT-1A expression levels in sulfur mustard-exposed patients were upregulated compared with normal samples. Protein expression was also markedly higher in controls than in sulfur mustard-exposed patients.

Conclusion:

Upregulation of MT-1A mRNA in patients who have been exposed to sulfur mustard seems to be due to oxidative stress, which is induced in an attempt to ameliorate this harmful situation by reestablishment of homeostasis, but depletion of its protein might be due to secondary consequences of sulfur mustard toxicity, which are as yet not understood.

Expression of MT1E during the formation of hepatocellular carcinoma and its function in hepatocarcinoma cells

AIM: To investigate the expression of MT1E mRNA at different stages of liver cancer development and its biological functions in hepatocarcinoma cells.

METHODS: Rat hepatocellular carcinoma (HCC) models were induced by diethylnitrosamine (DEN), and the dynamic histological changes of liver tissue and the differential expression of MT1E gene were observed after the 4^th, 8^th, 16^th and 20^th weeks. Two siRNA targets toward MT1E gene were designed, recombinant plasmid was transfected into HCC cell line SMMC-7721, the gene expression of MT1E was determined using real-time quantitative PCR, and the cell viability was determined by MTT assay.

RESULTS: At the 4^th week and 8^th week the major pathological changes presented inflammatory changes in liver tissue, after 16^th week presented typical proliferation changes, until 20^th week developed into HCC. According to the gene chip results, the expression of MT1E was increased significantly after 16^th week, with significant difference compared with control group (gene chip reading: 11524 vs 5462). An effective siRNA target sequence in MT1E was got, the gene expression was decreased greatly at the 72 h after transfection compared with blank control group and negative control group (0.38 vs 1.00, 0.93, both P < 0.01). MTT assay result displayed, the cell growth and proliferation were depressed obviously when interference target was effectively transfected 144 h (0.1700 ± 0.0313 vs 0.5748 ± 0.0480, P < 0.01).

CONCLUSION: DEN-induced rat HCC models were established successfully, the expression of MT1E is increased obviously at the later stage of development of HCC, perhaps related to the malignant proliferation of tumor cells.

Metal binding and antioxidant properties of chimeric tri- and tetra-domained metallothioneins

An unusual tri-domained (alpha-beta-beta) natural oyster metallothionein (MT) is known, and non-oxidative MT dimers occur in vivo in mollusk species and in mammals. To assess the respective role of the MT domains, two chimeric MTs were constructed: a tetra-domained oyster MT corresponding to the alpha-beta-alpha-beta structure, in order to mimic the natural non-oxidative dimeric form, and a tri-domained alpha-beta-alpha oyster MT. Metal binding and putative antioxidant properties of these two chimeric MTs were investigated using expression of the related genes in the bacteria Escherichia coli. In a wild-type strain these MTs could efficiently bind Cd. In a superoxide dismutase (sodA sodB) null mutant, the tri-domained MT was found to exacerbate Cd toxicity whereas the tetra-domained MT efficiently protected bacteria from Cd. The paradoxical toxicity displayed by the tri-domained MT upon Cd contamination was linked to the generation of superoxide radicals generated by a mechanism which most probably involves a copper-redox cycling reaction, since a Cd-contaminated sodA sodB strain expressing this MT produced 4 times more O2(-) than the control bacteria, and MT toxicity disappeared in the presence of bathocuproine disulfonic acid, a copper chelator. In contrast, the tetra-domained form did not. Interestingly, in bacteria producing superoxide dismutase but hypersensitive to oxidative stress due to either mutations in thioredoxin and glutathione reductase pathways (WM104 mutant) or to a lack of gamma-glutamylcysteine synthetase (gshA mutant), both chimeric MTs were protecting against Cd toxicity. However, an unexpected lack of antioxidant function was observed for both chimeric MTs, which were found to enhance the toxicity of hydrogen peroxide in WM104, or that of menadione in QC1726. Altogether, our results suggest that superoxide dismutase activity counteracts the potential prooxidative effect of the tri-domained MT mediated by Cu ions and that the tetra-domained form is a very efficient protector against metal toxicity in vivo.

Studies of metal ion binding by apo-metallothioneins attached onto preformed self-assembled monolayers using a highly sensitive surface plasmon resonance spectrometer

The use of a flow-injection surface plasmon resonance (FI-SPR) spectrometer equipped with a bicell detector or a position-sensitive device for determining coordination of heavy metal ions (Cd(2+) and Hg(2+)) by surface-confined apo-metallothionein (apo-MT) molecules is described. To facilitate the formation of a compact MT adsorbate layer with a uniform surface orientation, MT molecules were attached onto a preformed alkanethiol self-assembled monolayer. The method resorts to the generation of apo-MT at the surface by treating the MT-covered sensor chip with glycine-HCl and the measurement of the apo-MT conformation changes upon metal ion incorporation. Domain-specific metal ion binding processes by the apo-MT molecules were observed. Competitive replacement of one metal ion by another can be monitored in real time by FI-SPR. The tandem use of an immobilization scheme for forming a sub-monolayer of MT molecules at the sensor surface and the highly sensitive FI-SPR instrument affords a low concentration detection level. The detection level for Cd(2+) (0.1 μM or 15 ppb) compares favorably with similar studies and the methodology complements to other well-established sensitive analytical techniques. The extent of metal incorporation by apo-MT molecules was also determined.

Different redox states of metallothionein/thionein in biological tissue

Mammalian metallothioneins are redox-active metalloproteins. In the case of zinc metallothioneins, the redox activity resides in the cysteine sulfur ligands of zinc. Oxidation releases zinc, whereas reduction re-generates zinc-binding capacity. Attempts to demonstrate the presence of the apoprotein (thionein) and the oxidized protein (thionin) in tissues posed tremendous analytical challenges. One emerging strategy is differential chemical modification of cysteine residues in the protein. Chemical modification distinguishes three states of the cysteine ligands (reduced, oxidized and metal-bound) based on (i) quenched reactivity of the thiolates when bound to metal ions and restoration of thiol reactivity in the presence of metal-ion-chelating agents, and (ii) modification of free thiols with alkylating agents and subsequent reduction of disulfides to yield reactive thiols. Under normal physiological conditions, metallothionein exists in three states in rat liver and in cell lines. Ras-mediated oncogenic transformation of normal HOSE (human ovarian surface epithelial) cells induces oxidative stress and increases the amount of thionin and the availability of cellular zinc. These experiments support the notion that metallothionein is a dynamic protein in terms of its redox state and metal content and functions at a juncture of redox and zinc metabolism. Thus redox control of zinc availability from this protein establishes multiple methods of zinc-dependent cellular regulation, while the presence of both oxidized and reduced states of the apoprotein suggest that they serve as a redox couple, the generation of which is controlled by metal ion release from metallothionein.

Metal-binding proteins and peptides in the aquatic fungi Fontanospora fusiramosa and Flagellospora curta exposed to severe metal stress

The production of thiol-containing proteins/peptides and its role in metal-binding was examined in the aquatic hyphomycetes Fontanospora fusiramosa and Flagellospora curta exposed to Cu, Cd, or Zn at concentrations inhibiting the biomass production in 80%. Heat-treated cell-free extracts were separated by size-exclusion chromatography and the thiol and metal content in the fractions was determined. F. curta, the species tolerant to metals, showed higher absolute levels of thiol compounds, which bound higher amounts of Cu and Cd than F. fusiramosa. Peptides with very low molecular weight (<9 kDa), most likely glutathione and phytochelatins, were the major Cu- and Zn-binding components in both species of aquatic hyphomycetes. In most cases, proteins with high molecular weight (>26 kDa) were induced by metal ions and they were the major Cd-binding component in both species. Proteins with characteristics of metallothioneins were also induced by exposure to metals in both species, but they showed a minor role in metal-binding, suggesting they might have other functions in fungal cells.

Expression, purification and preliminary characterization of mussel (Mytilus galloprovincialis) metallothionein MT20

Metallothioneins are rather ubiquitous metal-binding proteins induced by stressing or physiological stimuli. Two major metallothionein isoforms have been identified in mussel: MT10 and MT20. Nevertheless the high sequence homology, the two isoforms exhibit different expression and inducibility in vivo. We cloned and produced in Escherichia coli the MT20 isoform from Mytilus galloprovincilis. cDNA was subcloned into pGEX-6P.1 vector, in frame with a sequence encoding a glutathione-S-transferase (GST) tail. Recombinant protein was purified to electrophoretic homogeneity by affinity chromatography. After enzymatic cleavage of the GST tail the MT moiety was recovered with a final yield of about 5 mg of protein per litre of bacterial culture. The metal-binding ability of MT20 was assessed by absorption spectroscopy upon addition of cadmium equivalents and the metal release was checked as a function of the environment pH. Moreover the protein was analysed for the propensity to polymerization, typical of this class of protein, before and after exposure to reducing and alkylating agents.

Zinc coordination environments in proteins as redox sensors and signal transducers

Zinc/cysteine coordination environments in proteins are redox-active. Oxidation of the sulfur ligands mobilizes zinc, while reduction of the oxidized ligands enhances zinc binding, providing redox control over the availability of zinc ions. Some zinc proteins are redox sensors, in which zinc release is coupled to conformational changes that control varied functions such as enzymatic activity, binding interactions, and molecular chaperone activity. Whereas the released zinc ion in redox sensors has no known function, the redox signal is transduced to specific and sensitive zinc signals in redox transducers. Released zinc can bind to sites on other proteins and modulate signal transduction, generation of metabolic energy, mitochondrial function, and gene expression. The paradigm of such redox transducers is the zinc protein metallothionein, which, together with its apoprotein, thionein, functions at a central node in cellular signaling by redistributing cellular zinc, presiding over the availability of zinc, and interconverting redox and zinc signals. In this regard, the transduction of nitric oxide (NO) signals into zinc signals by metallothionein has received particular attention. It appears that redox-inert zinc has been chosen to control some aspects of cellular thiol/disulfide redox metabolism. Tight control of zinc is essential for redox homeostasis because both increases and decreases of cellular zinc elicit oxidative stress. Depending on its availability, zinc can be cytoprotective as a pro-antioxidant or cytotoxic as a pro-oxidant. Any condition with acute or chronic oxidative stress is expected to perturb zinc homeostasis.

Induction of hepatic and renal metallothionein synthesis by ferric nitrilotriacetate in mice: the role of MT as an antioxidant

Metallothionein (MT) demonstrates strong antioxidant properties, yet the physiological relevance of its antioxidant action is not clear. Injection of mice with ferric nitrilotriacetate (Fe-NTA) caused a dose-dependent increase in hepatic and renal MT. Fe-NTA caused a greater increase in hepatic and renal MT concentration (2.5- and 4-fold) compared with FeCl(3) at the same dose of ferric ion. MT mRNA levels were markedly elevated in both of tissues. Thiobarbituric acid (TBA) values in both tissues reached a maximum after 2-4 h. The MT concentrations were significantly increased after 2-4 h in liver and after 8-16 h in kidneys. Plasma concentrations of cytokines such as IL-6 and TNFalpha were elevated by 4 h; IL-6 levels were 24 times higher after Fe-NTA than that after injection of FeCl(3). Pretreatment of mice with ZnSO(4) attenuated nephrotoxicity induced by Fe-NTA after 2 h, but was not effective 4 h after injection. After a Fe-NTA injection, a loss of Cd-binding properties of preinduced MT was observed only in kidneys of Zn-pretreated mice but not in liver. Treatment with BSO, glutathione (GSH) depletor, intensified a loss of its Cd-binding properties after a Fe-NTA injection. These results indicate that induction of MT synthesis may result from reactive oxygen species (ROS) generated by Fe-NTA, and MT may act in vivo as a complementary antioxidant.

Expression, purification, and characterization of metallothionein-A from rainbow trout

Recombinant metallothionein A (MT-A) from rainbow trout has been successfully produced in milligram quantities in Escherichia coli. cDNA has been subcloned into pGEX-6P.1 vector, in-frame with a sequence encoding an N-terminal glutathione-S-transferase (GST) tail. Purification to electrophoretic homogeneity has been obtained by affinity chromatography using GSH-Sepharose. After enzymatic cleavage of GST tail, the MT-A moiety shows a molecular weight, corresponding to the expected one (6630 Da). The final yield of the entire expression and purification process was about 5 mg of pure metallothionein per liter of bacterial culture. The effects of different reducing and alkylating agents have been evaluated at the level of the formation of higher molecular weight aggregates. To investigate the metal-binding ability of the recombinant MT-A, we carried out a spectrophotometrical titration with cadmium ions. Finally, we checked the metal dissociation by recording the UV absorbance of the protein as a function of the environmental pH.

Dynamics of interdomain and intermolecular interactions in mammalian metallothioneins

The structures of mammalian metallothioneins (MTs), as solved by X-ray crystallography and NMR spectroscopy, all show seven divalent metals bound in two separate domains. The marked differences in metal-mobilities found for the two domains has led to the proposal for a dual role for the two MT metal domains. The tight metal binding in the C-terminal alpha-domain supposedly constitutes the basis for the detoxification of excess heavy metals, while the more labile metals in the N-terminal beta-domain function in the homeostasis of the essential elements zinc and copper. In this overview, we compare the two types of dimers found for MTs and their influence on metal-mobilities. In the presence of excess metal, the N-terminal domain is responsible for the formation of metal-bridged dimers while under aerobic conditions, a specific intermolecular disulfide is formed between the C-terminal domains. Both forms of dimers not only involve different domains for their intermolecular protein interactions, they also exhibit radical differences in the reactive properties of their respective cluster bound metal ions. Since the metal exchange within each domain is also influenced by interdomain interactions, the relative orientation of the domains is also most likely important for MT functions. Thus far, the relative orientation of the two domains could only be obtained from the crystal structure. Here, we present evidence for increased mobility in the linker region as the reason for the lack of interdomain constraints in the solution NMR studies of mammalian MTs.