Properties of the reaction of chromate with metallothionein

Microalgal Metallothioneins and Phytochelatins and Their Potential Use in Bioremediation

The persistence of heavy metals (HMs) in the environment causes adverse effects to all living organisms; HMs accumulate along the food chain affecting different levels of biological organizations, from cells to tissues. HMs enter cells through transporter proteins and can bind to enzymes and nucleic acids interfering with their functioning. Strategies used by microalgae to minimize HM toxicity include the biosynthesis of metal-binding peptides that chelate metal cations inhibiting their activity. Metal-binding peptides include genetically encoded metallothioneins (MTs) and enzymatically produced phytochelatins (PCs). A number of techniques, including genetic engineering, focus on increasing the biosynthesis of MTs and PCs in microalgae. The present review reports the current knowledge on microalgal MTs and PCs and describes the state of art of their use for HM bioremediation and other putative biotechnological applications, also emphasizing on techniques aimed at increasing the cellular concentrations of MTs and PCs. In spite of the broad metabolic and chemical diversity of microalgae that are currently receiving increasing attention by biotechnological research, knowledge on MTs and PCs from these organisms is still limited to date.

Immunolocalization of metallothionein in hepatocytes of guppy fish (Poecilia reticulata) exposed to tannery effluent: A biomarker study

Composite tannery effluent (TE) contaminated with a load of metals, especially chromium, is used in East Calcutta Wetland Ecosystem (ECW; a Ramsar site of West Bengal; No 1208) after natural stabilization for pisciculture and agriculture. Quantity and synergistic and antagonistic activities of both essential and nonessential metals in the effluents can induce a variety of complex changes in animal physiology. High fish yield and no apparent adverse influence on the fish biosystems thriving under pollutant stress at ECW allowed the practice to thrive sustainably over the last nearly 100 years. Thus, the present study was conducted to investigate the role of metallothionein (MT) as a potential biomarker in a fish biosystem following acute and chronic exposure to sublethal concentrations of tannery wastewater. MT localization in the liver tissue of guppy fish (Poecilia reticulata) was studied by the immuno gold-labeled electron microscopic technique. The data demonstrated increased metal content in fish body upon exposure to TE. Electron micrographs showed significant induction of hepatic MT in exposed P. reticulata over control specimens. Liver, being the detoxification site of an organism, plays a crucial role in the synthesis of MT and arrest of excess metal. The method employed in the present study for demonstrating the induction and localization of MT is innovative and could be used as a biomarker in fish exposed to metal stress. Our study also indicated that the metalloprotein complexes in fish tissue sequestered excess load of metals and thereby arrested unwanted interferences of excess metal loads in metabolic processes.

Apo-Metallothionein Emerging as a Major Player in the Cellular Activities of Metallothionein

Observations of apo-metallothlonein (apo-MT) have been made under a variety of physiologic circumstances, including zinc deficiency in cell culture and in rodents, cellular induction of MT by dexamethasone with concurrent Zn deficiency, a variety of tumors under normal Zn conditions, MT induction by Zn and Bi citrate, induction of hepatic MT after tumor cell Injection into nude mice, and overexpression of cardiac MT in MT transgenic mice. Experiments demonstrating both the lability of Zn and Cu bound to MT and the cellular stability of apo-MT are described to help rationalize the widespread presence of this metal-depleted species. Finally, comparative in vitro and cellular experiments examined the relative reactivity of Zn- and apo-MT with nitric oxide species, showing that apo-MT is much more reactive chemically and that in cells it may be a principal reactive species within the MT pool.

The ins and outs of metal homeostasis by the root nodule actinobacterium Frankia

BACKGROUND

Frankia are actinobacteria that form a symbiotic nitrogen-fixing association with actinorhizal plants, and play a significant role in actinorhizal plant colonization of metal contaminated areas. Many Frankia strains are known to be resistant to several toxic metals and metalloids including Pb(2+), Al(+3), SeO2, Cu(2+), AsO4, and Zn(2+). With the availability of eight Frankia genome databases, comparative genomics approaches employing phylogeny, amino acid composition analysis, and synteny were used to identify metal homeostasis mechanisms in eight Frankia strains. Characterized genes from the literature and a meta-analysis of 18 heavy metal gene microarray studies were used for comparison.

RESULTS

Unlike most bacteria, Frankia utilize all of the essential trace elements (Ni, Co, Cu, Se, Mo, B, Zn, Fe, and Mn) and have a comparatively high percentage of metalloproteins, particularly in the more metal resistant strains. Cation diffusion facilitators, being one of the few known metal resistance mechanisms found in the Frankia genomes, were strong candidates for general divalent metal resistance in all of the Frankia strains. Gene duplication and amino acid substitutions that enhanced the metal affinity of CopA and CopCD proteins may be responsible for the copper resistance found in some Frankia strains. CopA and a new potential metal transporter, DUF347, may be involved in the particularly high lead tolerance in Frankia. Selenite resistance involved an alternate sulfur importer (CysPUWA) that prevents sulfur starvation, and reductases to produce elemental selenium. The pattern of arsenate, but not arsenite, resistance was achieved by Frankia using the novel arsenite exporter (AqpS) previously identified in the nitrogen-fixing plant symbiont Sinorhizobium meliloti. Based on the presence of multiple tellurite resistance factors, a new metal resistance (tellurite) was identified and confirmed in Frankia.

CONCLUSIONS

Each strain had a unique combination of metal import, binding, modification, and export genes that explain differences in patterns of metal resistance between strains. Frankia has achieved similar levels of metal and metalloid resistance as bacteria from highly metal-contaminated sites. From a bioremediation standpoint, it is important to understand mechanisms that allow the endosymbiont to survive and infect actinorhizal plants in metal contaminated soils.

Chromium

Chromium is ubiquitous in the environment as Cr(III) and Cr(VI) oxidation states, which interconvert under environmentally and biologically relevant conditions (although Cr(III) usually predominates). While Cr(VI) is an established human carcinogen and a major occupational and environmental hazard, Cr(III) has long been regarded as an essential human micronutrient, although recent literature has cast serious doubts on the validity of this postulate. Despite five decades of research, no functional Cr-containing enzymes or cofactors have been characterized conclusively, and several hypotheses on their possible structures have been refuted. Gastrointestinal absorption pathways for both Cr(III) and Cr(VI) are apparent and whole-blood speciation can involve Cr(VI) uptake and reduction by red blood cells, as well as Cr(III) binding to both proteins and low-molecular-mass ligands in the plasma. DNA-damaging effects of Cr(VI) and anti-diabetic activities of Cr(III) are likely to arise from common mechanistic pathways that involve reactive Cr(VI/V/IV) intermediates and kinetically inert Cr(III)-protein and Cr(III)-DNA adducts. Both Cr(III) and Cr(VI) are toxic to plants and microorganisms, particularly Cr(VI) due to its higher bioavailability and redox chemistry. Some bacteria reduce Cr(VI) to Cr(III) without the formation of toxic Cr(V) intermediates and these bacteria are being considered for use in the bioremediation of Cr(VI)-polluted environments.

Detoxification of Cr(VI) in Salvinia minima is related to seasonal-induced changes of thiols, phenolics and antioxidative enzymes

In this study, protein- and non-protein-thiol-containing compounds (THCC), soluble phenolics (SP), proline (Pro), proteins and malondialdehyde (MDA) contents, and antioxidative enzyme activities were analyzed in floating and submerged leaves of Salvinia minima to establish their role against Cr-induced oxidative stress. We analyzed relationships among biochemical responses to different Cr(VI) concentrations to explore underlying mechanisms of Cr detoxification in plants growing under field conditions during summer and winter seasons. Significant increases in THCC were observed in submerged leaves from both seasons, while in floating leaves THCC increased only in summer being decreased in winter. Contrarily SP increased in floating leaves and decreased in submerged ones. MDA increased significantly in winter-leaves, but in summer-leaves remained unchanged. Antioxidative enzymes, i.e. guaiacol peroxidase (G-POD), superoxide dismutase (SOD) and catalase (CAT) showed different activity patterns. G-POD significantly increased in Cr-treated leaves from both seasons, while SOD increased in submerged leaves only, remaining practically unchanged in floating ones. CAT activity increased in floating leaves from both seasons, whereas in submerged ones was decreased or increased. Proteins increased in both leaf types during summer whereas decreased or remained unchanged in winter. Pro increased in winter-submerged leaves only. Results show that seasonal-induced changes occur in all measured parameters.

Mammalian metallothionein in toxicology, cancer, and cancer chemotherapy

The present paper centers on mammalian metallothionein 1 and 2 in relationship to cell and tissue injury beginning with its reaction with Cd²⁺ and then considering its role in the toxicology and chemotherapy of both metals and non-metal electrophiles and oxidants. Intertwined is a consideration of MTs role in tumor cell Zn²⁺ metabolism. The paper updates and expands on our recent review by Petering et al. (Met Ions Life Sci 5:353-398, 2009).

Metal-binding mechanisms in metallothioneins

Metallothionein are small, cysteine-rich, metal-binding proteins that are found ubiquitously in nature. Most metallothioneins bind multiple metals in two well-defined metal-thiolate clusters. This perspective discusses the use of optical spectroscopy to study the metalation of metallothioneins and the emergence of electrospray ionization mass spectrometry as a means of studying the mechanism of metalation for metallothioneins. A brief history of past kinetic studies of cadmium metallothioneins and recent kinetic study advances for the arsenic metalation of metallothionein will be presented. Lastly, a possible functional role for the two-domain structure of metallothionein will be briefly discussed.

Kinetic analysis of arsenic-metalation of human metallothionein: significance of the two-domain structure

Metallothionein (MT) is ubiquitous in Nature, underlying MT's importance in the cellular chemistry of metals. Mammalian MT consists of two metal-binding domains while microorganisms like cyanobacteria consist of a single metal-binding domain MT. The evolution of a two-domain protein has been speculated on for some time; however, no conclusive evidence explaining the evolutionary necessity of the two-domain structure has been reported. The results presented in this report provide the complete kinetic analysis and subsequent mechanism of the As(3+)-metalation of the two-domain beta alpha hMT and the isolated single domain fragments using time- and temperature-resolved electrospray ionization mass spectrometry. The mechanism for beta alpha hMT binding As(3+) is noncooperative and involves six sequential bimolecular reactions in which the alpha domain binds As(3+) first followed by the beta domain. At room temperature (295 K) and pH 3.5, the sequential individual rate constants, k(n) (n = 1-6) for the As(3+)-metalation of beta alpha hMT starting at k(1beta alpha) are 25, 24, 19, 14, 8.7, and 3.7 M(-1)s(-1). The six rate constants follow an almost linear trend directly dependent on the number of unoccupied sites for the incoming metal. Analysis of the temperature-dependent kinetic electrospray ionization mass spectra data allowed determination of the activation energy for the formation of As(1)-H(17)-beta alpha hMT (14 kJ mol(-1)) and As(2-6)-beta alpha hMT (22 kJ mol(-1)). On the basis of the increased rate of metalation for the two-domain protein when compared with the isolated single-domain, we propose that there is an evolutionary advantage for the two-domain MT structures in higher organism, which allows MT to bind metals faster and, therefore, be a more efficient metal scavenger.

Metallothionein Toxicology: Metal Ion Trafficking and Cellular Protection

The literature is replete with reports about the involvement of metallothionein in host defense against injurious chemical, biological, and physical agents. Yet, metallothionein's functional roles are still being debated. This review addresses the issues that have left the physiological significance of metallothionein in doubt and moves on to assess the MT's importance in cell toxicology. It is evident that the protein is broadly involved in protecting cells from injury due to toxic metal ions, oxidants, and electrophiles. Attention is focused on MT's structural and chemical properties that confer this widespread role in cell protection. Particular emphasis is placed on the implications of finding that metal ion unsaturated metallothionein is commonly present in many cells and tissues and the question, how does selectivity of reaction with metallothionein take place in the cellular environment that includes large numbers of competing metal binding sites and high concentrations of protein and glutathione sulfhydryl groups?

Zinc binding ligands and cellular zinc trafficking: apo-metallothionein, glutathione, TPEN, proteomic zinc, and Zn-Sp1

Many cell types contain metal-ion unsaturated metallothionein (MT). Considering the Zn(2+) binding affinity of metallothionein, the existence of this species in the intracellular environment constitutes a substantial "thermodynamic sink". Indeed, the mM concentration of glutathione may be thought of in the same way. In order to understand how apo-MT and the rest of the Zn-proteome manage to co-exist, experiments examined the in vitro reactivity of Zn-proteome with apo-MT, glutathione (GSH), and a series of common Zn(2+) chelating agents including N,N,N',N'-(2-pyridylethyl)ethylenediammine (TPEN), EDTA, and [(2,2'-oxyproplylene-dinitrilo]tetraacetic acid (EGTA). Less than 10% of Zn-proteome from U87mg cells reacted with apo-MT or GSH. In contrast, each of the synthetic chelators was 2-3 times more reactive. TPEN, a cell permeant reagent, also reacted rapidly with both Zn-proteome and Zn-MT in LLC-PK(1) cells. Taking a specific zinc finger protein for further study, apo-MT, GSH, and TPEN inhibited the binding of Zn(3)-Sp1 with its cognate DNA site (GC-1) in the sodium-glucose co-transporter promoter of mouse kidney. In contrast, preformation of Zn(3)-Sp1-(GC-1) prevented reaction with apo-MT and GSH; TPEN remained active but at a higher concentration. Whereas, Zn(3)-Sp1 is active in cells containing apo-MT and GSH, exposure of LLC-PK(1) cells to TPEN for 24h largely inactivated its DNA binding activity. The results help to rationalize the steady state presence of cellular apo-MT in the midst of the many, diverse members of the Zn-proteome. They also show that TPEN is a robust intracellular chelator of proteomic Zn(2+).

Arsenic binding to human metallothionein

The number of reported cases of chronic arsenic poisoning is on the rise throughout the world, making the study of the long-term effects of arsenic critical. As(3+) binds readily to biological thiols, including mammalian metallothionein (MT), which is an ubiquitous sulfur-rich metalloprotein known to coordinate a wide range of metals. The two-domain mammalian protein binds divalent metals (M) into two metal-thiolate clusters with stoichiometries of M(3)S(cys9) (beta) and M(4)S(cys11) (alpha). We report that As(3+) binds with stoichiometries of As(3)S(cys9) (beta) and As(3)S(cys11) (alpha) to the recombinant human metallothionein (rhMT) isoform 1a protein. Further, we report the complete kinetic analysis of the saturation reactions of the separate alpha and beta domains of rhMT with As(3+). Speciation in the metalation reactions was determined using time- and temperature-resolved electrospray ionization mass spectrometry. The binding reaction of As(3+) to the alpha and beta MT domains is shown to be noncooperative and involves three sequential, bimolecular metalation steps. The analyses allow for the first time the complete simulation of the experimental data for the stepwise metalation reaction of MT showing the relative concentrations of the metal-free, apo MT and each of the As-MT intermediate species as a function of time and temperature. At room temperature (298 K) and pH 3.5, the individual rate constants for the first, second, and third As(3+) binding to apo-alphaMT are 5.5, 6.3, and 3.9 M(-)(1) s(-)(1) and for apo-betaMT the constants are 3.6, 2.0, and 0.6 M(-)(1) s(-)(1). The activation energy for formation of As(1)-H(6)-betaMT is 32 kJ mol(-)(1), for As(2)-H(3)-betaMT it is 35 kJ mol(-)(1), for As(3)-betaMT it is 29 kJ mol(-)(1), for As(1)-H(8)-alphaMT it is 33 kJ mol(-)(1), for As(2)-H(5)-alphaMT it is 29 kJ mol(-)(1), and for As(3)-H(2)-alphaMT it is 23 kJ mol(-)(1).

Heavy metal accumulation and metallothionein concentration in the frog Rana ridibunda after exposure to chromium or a mixture of chromium and cadmium

The accumulation of two heavy metals (chromium (Cr) and cadmium (Cd)) in the liver, kidney and gut of Rana ridibunda exposed to Cr or to a mixture of Cr and Cd was investigated. The concentration of metallothioneins (MTs) in the same tissues was also studied. Both metals accumulated mainly in the kidney. Cr accumulation in the liver and gut was not affected by the presence of Cd. Furthermore, Cr concentration in the kidney was doubled when Cd was present. MT concentration did not increase after Cr treatment but it increased two- to six-fold over control values in mixture-exposed frogs, the highest value being observed in the gut. MTs in the gut could act as a barrier preventing ingested heavy metals from entering the blood stream. MT concentration correlated positively with Cd concentrations in both the liver and the gut of mixture-exposed animals.