Domain specificity in metal binding to metallothionein. A circular dichroism and magnetic circular dichroism study of cadmium and zinc binding at temperature extremes

Human apo-metallothionein 1a is not a random coil: Evidence from guanidinium chloride, high temperature, and acidic pH unfolding studies

The structures of apo-metallothioneins (apo-MTs) have been relatively elusive due to their fluxional, disordered state which has been difficult to characterize. However, intrinsically disordered protein (IDP) structures are rather diverse, which raises questions about where the structure of apo-MTs fit into the protein structural spectrum. In this paper, the unfolding transitions of apo-MT1a are discussed with respect to the effect of the chemical denaturant GdmCl, temperature conditions, and pH environment. Cysteine modification in combination with electrospray ionization mass spectrometry was used to probe the unfolding transition of apo-MT1a in terms of cysteine exposure. Circular dichroism spectroscopy was also used to monitor the change in secondary structure as a function of GdmCl concentration. For both of these techniques, cooperative unfolding was observed, suggesting that apo-MT1a is not a random coil. More GdmCl was required to unfold the protein backbone than to expose the cysteines, indicating that cysteine exposure is likely an early step in the unfolding of apo-MT1a. MD simulations complement the experimental results, suggesting that apo-MT1a adopts a more compact structure than expected for a random coil. Overall, these results provide further insight into the intrinsically disordered structure of apo-MT.

Homo- and Heteronuclear Group 12 Metallothionein Type B Cluster Analogs: Synthesis, Structure, 1H NMR and ESI-MS

Metallothioneins (MTs) are a ubiquitous class of small cysteine-rich metal-binding proteins involved in metal homeostasis and detoxification with highly versatile metal binding properties. Despite the long-standing association of MT with M₃S₃ and M₄S₅ metal clusters, synthetic complexes with these core architectures are exceptionally rare. Here, we demonstrate an approach to synthesizing and characterizing aggregates of group 12 metal ions with monocyclic M₃S₃ cores in acetonitrile solution without the protection of a protein. Multidentate monothiol ligand N,N-bis(2-pyridylmethyl)-2-aminoethanethiol (L1H) provided [Cd₃(L1)₃](ClO₄)₃ (1), the first structurally characterized nonproteinaceous aggregate with a metallothionein-like monocyclic Cd₃S₃ core. In addition, [Zn₃(L1)₃](ClO₄)₃·4CH₃CN (2·4CH₃CN) was characterized by X-ray crystallography. The complex cations of 1 and 2 had comparable structures despite being nonisomorphic. Variable temperature and concentration ¹H NMR were used to investigate aggregation equilibria of 1, 2, and a precipitate with composition "Hg(L1)(ClO₄)" (3). Cryogenic ¹H NMR studies of 3 revealed a J(¹⁹⁹Hg¹H) coupling constant pattern consistent with an aggregate possessing a cyclic core. ESI-MS was used for gas-phase characterization of 1-3, as well as mixed-metal [M₂M'(L1)₃(ClO₄)₂]⁺ ions prepared in situ by pairwise acetonitrile solution combinations of the group 12 complexes of L1. Access to synthetic variants of metallothionein-like group 12 aggregates provides an additional approach to understanding their behavior.

Oleic acid protects against cadmium induced cardiac and hepatic tissue injury in male Wistar rats: A mechanistic study

AIMS

The aim of the present study was to evaluate the possible antioxidant role of oleic acid (OA) against Cd-induced injuries in the heart and liver tissues of male Wistar rats.

MAIN METHODS

Rats were treated with either vehicle (control), or OA (10 mg/kg b.w., fed orally), or Cd (0.44 mg/kg b.w., s.c.), or both (OA + Cd) for 15 days. Following completion of the treatment period, biomarkers of organ damage and oxidative stress including ROS, activities of antioxidant enzymes and their level, activities of Krebs cycle enzymes and respiratory chain enzymes were measured. Levels of interleukins (IL-1β, IL-6, IL-10), tumor necrosis factor (TNF-α) and nuclear factor kappa B (NFκB) were estimated to evaluate the state of inflammation. In addition, changes in mitochondrial membrane potential and status of cytochrome c (Cyt c) were also studied.

KEY FINDINGS

Pre-treatment of rats with OA significantly protected against Cd-induced detrimental changes possibly by decreasing endogenous ROS through regulation of antioxidant defense system, inflammatory responses and activities of metabolic enzymes. Moreover, OA was also found to restore mitochondrial membrane potential possibly by regulating Cyt c leakage thereby increasing mitochondrial viability.

SIGNIFICANCE

Our results for the first time demonstrated systematically that OA provided protection against Cd-induced oxidative stress mediated injuries in rat heart and liver tissues through its antioxidant mechanism. The results raise the possibility of using OA singly or in combination with other antioxidants or diet in the treatment of situations arising due to oxidative stress and may have future therapeutic relevance.

Putting the pieces into place: Properties of intact zinc metallothionein 1A determined from interaction of its isolated domains with carbonic anhydrase

Competitive metallation reactions between the isolated domain fragments and apo-carbonic anhydrase [CA; metal-free CA (apo-CA)] provided the binding affinities for each of the eight sites and showed that CA competed more efficiently for added zinc with the β-domain fragment. The combined effects of the number of sites, chain length and cysteine accessibility modulate the zinc-binding properties of mammalian metallothionein (MT).

Hepatoprotective activity of Moringa oleifera against cadmium toxicity in rats

Aim:

The present investigation has been conducted to evaluate the hepatoprotective activity of Moringa oleifera against cadmium-induced toxicity in rats.

Materials and Methods:

For this study, 18 Wistar albino rats were taken. Control group, Group I rats were given cadmium chloride @ 200 ppm per kg and Group II rats were treated with M. oleifera extract @ 500 mg/kg along with cadmium chloride @ 200 ppm per kg (daily oral for 28 days). On 29^th day, animals were slaughtered and various parameters were determined. Serum biomarkers, oxidative stress parameters, histomorphological examination were carried out with estimation of cadmium concentration in liver tissues.

Results:

Oral administration of cadmium chloride @ 200 ppm/kg for 28 days resulted in a significant increase in aspartate aminotransferase (AST), alanine transaminase (ALT), alkaline phosphatase (ALP), significant (p≤0.01) increase of lipid peroxidation (LPO) and decrease in superoxide dismutase (SOD), and increase in cadmium accumulation in liver. Treatment with M. oleifera @ 500 mg/kg significantly (p<0.01) decreased the elevated ALP, AST, ALT, LPO levels and increase in SOD levels, and as compared to cadmium chloride treated group. However, there was no significant difference in cadmium concentration in liver when compared with cadmium chloride treated group.

Conclusion:

The study conclude that supplementation of M. oleifera (500 mg/kg), daily oral for 28 days has shown protection against cadmium-induced hepatotoxicity.

Challenging conventional wisdom: single domain metallothioneins

Metallation studies of human metallothioneins support the role of single metal-binding-domains as commonplace with the typical two-domain-cluster structure as exceptional.

Chemistry and biology of mammalian metallothioneins

Metallothioneins (MTs) are a class of ubiquitously occurring low molecular mass, cysteine- and metal-rich proteins containing sulfur-based metal clusters formed with Zn(II), Cd(II), and Cu(I) ions. In mammals, four distinct MT isoforms designated MT-1 through MT-4 exist. The first discovered MT-1/MT-2 are widely expressed isoforms, whose biosynthesis is inducible by a wide range of stimuli, including metals, drugs, and inflammatory mediators. In contrast, MT-3 and MT-4 are noninducible proteins, with their expression primarily confined to the central nervous system and certain squamous epithelia, respectively. MT-1 through MT-3 have been reported to be secreted, suggesting that they may play different biological roles in the intracellular and extracellular space. Recent reports established that these isoforms play an important protective role in brain injury and metal-linked neurodegenerative diseases. In the postgenomic era, it is becoming increasingly clear that MTs fulfill multiple functions, including the involvement in zinc and copper homeostasis, protection against heavy metal toxicity, and oxidative damage. All mammalian MTs are monomeric proteins, containing two metal-thiolate clusters. In this review, after a brief summary of the historical milestones of the MT-1/MT-2 research, the recent advances in the structure, chemistry, and biological function of MT-3 and MT-4 are discussed.

Protective effects of selenium, zinc, or their combination on cadmium-induced oxidative stress in rat kidney

The present study was conducted to investigate whether the combined treatment with Se and Zn offers more beneficial effects than that provided by either of them alone in reversing Cd-induced oxidative stress in the kidney of rat. For this purpose, 30 adult male Wistar albino rats, equally divided into control and four treated groups, received either 200 ppm Cd (as CdCl(2)), 200 ppm Cd + 500 ppm Zn (as ZnCl(2)), 200 ppm Cd + 0.1 ppm Se (as Na(2)SeO(3)), or 200 ppm Cd + 500 ppm Zn + 0.1 ppm Se in their drinking water for 35 days. The results showed that Cd treatment decreased significantly the catalase (CAT) and glutathione peroxidase (GSH-Px) activities, whereas the superoxide dismutase (SOD) activity and the renal levels of lipid peroxidation (as malondialdehyde, MDA) were increased compared to control rats. The treatment of Cd-exposed rats with Se alone had no significant effect on the Cd-induced increase in the MDA concentrations but increased significantly the CAT activities and reversed Cd-induced increase in SOD activity. It also partially prevented Cd-induced decrease in GSH-Px activity. The treatment of Cd-exposed animals with Zn alone increased significantly the CAT activity and partially protected against Cd-induced increase in the MDA concentrations, whereas it had no significant effect on the Cd-induced increase in SOD activity and decrease in GSH-Px activity. The combined treatment of Cd-exposed animals with Se and Zn was more effective than that with either of them alone in reversing Cd-induced decrease in CAT and GSH-Px activities and Cd-induced increase in MDA concentrations. Results demonstrated beneficial effects of combined Se and Zn treatment in Cd-induced oxidative stress in kidney and suggest that Se and Zn can have a synergistic role against Cd toxicity.

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.

Protective role of selenium against renal toxicity induced by cadmium in rats

Cadmium is an environmental toxic metal implicated in human diseases. The mechanism of its toxicity is not fully understood. Therefore, the role of cadmium in renal toxicity, and the protective role of selenium against this toxicity were investigated. Forty-five male rats were used through out the study and divided into three groups of 15. The first group received saline solution daily for 10 days. The second group, received cadmium chloride (CdCl2) (2 mg/kg body weight) intraperitoneally daily for a period of 10 days. The third group, received sodium selenite (1 mg/kg body weight, twice in 10 days) and cadmium chloride (CdCl2) once a day [corrected] The results showed that cadmium treatment increased renal lipid peroxidation (measured as malondialdehyde, MDA) which was associated with a significant decrease in the antioxidant systems such as reduced glutathione levels and the activities of glutathione peroxidase (GPx) and thioredoxin reductase (TrxR). On the other hand, pretreatment of rats with selenium and cadmium led to a significant decrease in MDA concentration, and increased levels of GSH and the activities of GPx and TrxR when compared with those of cadmium-treated group. The total levels of phospholipid, triglyceride, and cholesterolester classes were decreased, while free fatty acids levels were markedly increased after cadmium treatment. In addition, the total levels of both mono- and poly-unsaturated fatty acids of different lipid classes were significantly decreased, while the total saturated fatty acids was significantly increased by cadmium treatment. Pretreatment of rats with selenium, was found to protect kidney tissues of rats against the biochemical changes resulting from cadmium administration. These results suggest that cadmium causes renal toxicity by inducing lipid peroxidation, decreasing antioxidant systems, and also by altering lipid metabolism. In addition, selenium treatment could protect the kidney tissues against the toxicity of cadmium since it reduced MDA levels and increased the activities of antioxidant enzymes in these tissues. These results could be important for the further understanding of the complex mechanisms of cadmium toxicity in kidney tissues and in the development of better treatments for people and/or animals exposed to the heavy metal.

Determination of the Cd/S cluster stoichiometry in Fucus vesiculosus metallothionein

The seaweed Fucus vesiculosus is unusual when compared with other algal species, in that it can survive in toxic-metal-contaminated aquatic environments. The metallothionein gene has been identified in F. vesiculosus by Kille and co-workers (Morris, C. A., Nicolaus, B., Sampson, V., Harwood, J. L., and Kille, P. (1999) Biochem. J. 338, 553), which suggests a possible protective mechanism against toxic metals for this species. We report the first detailed study of the metal binding properties of F. vesiculosus metallothionein using UV absorption, circular dichroism (CD), and electrospray mass spectral techniques. The overall metal-to-sulfur ratios of this novel algal protein when bound to divalent cadmium and zinc were determined to be Cd(6)S(16) and Zn(6)S(16), respectively. Mixed Cd/Zn species were also formed when Cd(2+) was added to the Zn-containing Fucus metallothionein. Only one conformation was identified at low pH for the native protein. Analysis of the UV absorption, CD, and ESI-MS spectral data recorded during stepwise, acid-induced demetalation supports a two-domain structure for the protein, with two 3-metal binding sites. The data suggest that one of the domains is significantly less stable than the other, and we tentatively propose from the arrangement of cysteines in the sequence that the two domains are M(3)S(7) and M(3)S(9) (where M = Cd(2+) or Zn(2+)). While the M(3)S(9) cluster is known in the beta clusters of crab, lobster, and mammalian metallothioneins, the M(3)S(7) is a hitherto unknown cluster structure. Metallothionein in F. vesiculosus is thought to act as a protective mechanism against incoming toxic metals. The metal binding studies reported are a putative model for metal binding in vivo.

Cu+ distribution in metallothionein fragments

The differential distribution of Cu+ between separate alpha and beta domains of metallothionein (the isolated peptide fragments) and the rate of transfer of Cu+ between the two domains using copper-thiolate specific emission spectroscopy are reported. Kinetic data show the rate of transfer of Cu+ from the Cu6alpha to the Cd3beta domain is 2 x 10(-1) s(-1) while the transfer from Cu6beta to the Cd4alpha domain is much slower at 8 x 10(-3) s(-1), indicating the greater binding affinity of Cu+ for the MT beta domain. We report that the emission intensity of Cu6beta is 0.45 the emission intensity of Cu6alpha-MT. Lambda(max) is shown to be a probe of the environment of the Cu+. A series of copper-containing domain intermediates to the formation of the filled Cu6S9-beta and Cu6S11-alpha-clusters are identified. A mechanism is proposed for the formation of Cu12(betaalpha)-MT that involves metal exchange reactions of Cu+ ions from the alpha to the beta domain with initial formation of a Cu4beta-cluster.

Stability and conformational dynamics of metallothioneins from the antarctic fish Notothenia coriiceps and mouse

The structural properties and the conformational dynamics of antarctic fish Notothenia coriiceps and mouse metallothioneins were studied by Fourier-transform infrared and fluorescence spectroscopy. Infrared data revealed that the secondary structure of the two metallothioneins is similar to that of other metallothioneins, most of which lack periodical secondary structure elements such as alpha-helices and beta-sheets. However, the infrared spectra of the N. coriiceps metallothionein indicated the presence of a band, which for its typical position in the spectrum and for its sensitivity to temperature was assigned to alpha-helices whose content resulted in 5% of the total secondary structure of the protein. The short alpha-helix found in N. coriiceps metallothionein showed an onset of denaturation at 30 degrees C and a T(m) at 48 degrees C. The data suggest that in N. coriiceps metallothionein a particular cysteine is involved in the alpha-helix and in the metal-thiolate complex. Moreover, infrared spectra revealed that both proteins investigated possess a structure largely accessible to the solvent. The time-resolved fluorescence data show that N. coriiceps metallothionein possesses a more flexible structure than mouse metallothionein. The spectroscopic data are discussed in terms of the biological function of the metallothioneins.

Characterization of the zinc binding activity of the rubella virus nonstructural protease

The rubella virus (RUB) nonstructural (NS) protein (NSP) ORF encodes a protease that cleaves the NSP precursor (240 kDa) at a single site to produce two products. A cleavage site mutation was introduced into a RUB infectious cDNA clone and found to be lethal, demonstrating that cleavage of the NSP precursor is necessary for RUB replication. Based on computer alignments, the RUB NS protease was predicted to be a papain-like cysteine protease (PCP) with the residues Cys1152 and His1273 as the catalytic dyad; however, the RUB NS protease was recently found to require divalent cations such as Zn, Co, and Cd for activity (X. Liu, S. L. Ropp, R. J. Jackson, and T. K. Frey, J. Virol. 72:4463-4466, 1998). To analyze the function of metal cation binding in protease activity, Zn binding studies were performed using the minimal NS protease domain within the NSP ORF. When expressed as a maltose binding protein (MBP) fusion protein by bacteria, the NS protease exhibited activity both in the bacteria and in vitro following purification when denatured and refolded in the presence of Zn. Atomic absorption analysis detected 1.6 mol of Zn bound per mol of protein refolded in this manner. Expression of individual domains within the protease as MBP fusions and analysis by a Zn(65) binding assay revealed two Zn binding domains: one located at a predicted metal binding motif beginning at Cys1175 and the other one close to the cleavage site. Mutagenesis studies showed that Cys1175 and Cys1178 in the first domain and Cys1227 and His1273, the His in the predicted catalytic site, in the second domain are essential for zinc binding. All of these residues are also necessary for the protease activity, as were several other Cys residues not involved in Zn binding. Far-UV circular dichroism (CD) analysis of the MBP-NS protease fusion protein showed that the protease domain contained a large amount of alpha-helical structure, which is consistent with the results of secondary-structural prediction. Both far-UV-CD and fluorescence studies suggested that Zn did not exert a major effect on the overall structure of the fusion protein. Finally, protease inhibitor assays found that the protease activity can be blocked by both metal ion chelators and the metalloprotease inhibitor captopril. In conjunction with the finding that the previously predicted catalytic site, His1273, is essential for zinc binding, this suggests that the RUB NS protease is actually a novel virus metalloprotease rather than a PCP.

Circular dichroism, kinetic and mass spectrometric studies of copper(I) and mercury(II) binding to metallothionein

The metalloprotein metallothionein (MT) is remarkable in its metal binding properties: for the mammalian protein, well-characterized species exist for metal to sulfur ratios of M7S20, M12S20, and M18S20, where M = Cd(II), Zn(II), Hg(II), Ag(I), Au(I), and Cu(I). Optical spectra in general, and circular dichroism (CD) and luminescence spectra in particular, provide rich detail of a complicated metal binding chemistry when metals are added directly to the metal-free or zinc-containing protein. CD spectral data unambiguously identify key metal to protein stoichiometric ratios that result in well-defined structures. Electrospray ionization-mass spectrometry data are reported for reactions in which Hg(II) binds to apo-MT 2A as previously described from CD data. Emission spectra in the 450-750 nm region have been reported for metallothioneins containing Ag(I), Au(I), and Cu(I). The luminescence of Cu-MT can also be detected directly from mammalian and yeast cells. We report both steady-state and new dynamic data for titrations of Zn-MT with Cu(I). Analysis of kinetic data for the addition of the first two Cu(I) atoms to Zn-MT indicates a first-order mechanism over a concentration range of 5-50 microM. Three-dimensional modeling was carried out using the results of the CD and EXAFS studies, model calculations for Zn7-MT, Hg7-MT, and Cu12-MT are described.

Comparison of the structures of the metal-thiolate binding site in Zn(II)-, Cd(II)-, and Hg(II)-metallothioneins using molecular modeling techniques

The first fully energy-minimized structures for a series of structurally related metal complexes of the important mammalian metal binding protein metallothionein are described. The structures were calculated based on structural information obtained from existing spectroscopic and crystallographic data, and minimized using molecular mechanics (MM2) techniques. A two domain structure, with stoichiometries of M(II)3-(Scys)9 and M(II)4-(Scys)11 where M = zinc(II), cadmium(II), and mercury(II), was assembled and minimized. The resultant three-dimensional structure closely resembled that of rat liver Cd5Zn2-MT 1 obtained by analysis of x-ray diffraction data [A. H. Robbins, D.E. McRee, M. Williamson, S. A. Collett, N. H. Xuong, W. F. Furey, B. C. Wang and C. D. Stout, J. Mol. Biol. 221, 1269-1293 (1991)]. Minimized structures for Zn7-MT, Cd7-MT, and Hg7-MT are reported. Deep crevices that expose the metal-thiolate clusters are seen in each structure. However, for the mercury-containing protein, much of the mercury-thiolate structure is visible and it is proposed that this provides access for extensive interaction between solvent water molecules and the mercury(II), resulting in the observed distortion away from tetrahedral geometry for Hg7-MT. Volume calculations are reported for the protein metallated with 7 Zn(II), Cd(II), or Hg(II). A series of structural changes calculated for the step-wise isomorphous replacement of Zn(II) by Cd(II) and Hg(II) in the Zn4S11 alpha domain are shown.

Sulfur K-Edge EXAFS Studies of Cadmium-, Zinc-, Copper-, and Silver-Rabbit Liver Metallothioneins

The structures of metal-thiolate clusters in Zn(7)-MT, Cd(7)-MT, Cu(12)-MT, Ag(12)-MT, and Ag(17)-MT from rabbit liver have been investigated by sulfur K-edge X-ray absorption spectroscopy (XAS). In addition to providing metal-cysteinyl sulfur bond lengths, the sulfur K-edge EXAFS data provide the first direct evidence for mixtures of bridging and terminal sulfurs in Cu-MT and Ag-MT. The Zn-S and Cd-S bond lengths for tetrahedrally coordinated Zn(4)(SPh)(10)(2-) and Cd(4)(SPh)(10)(2-) compounds obtained from sulfur K-edge EXAFS data are 2.35 +/- 0.03 and 2.52 +/- 0.02 Å, respectively. Zn-S and Cd-S bond distances of 2.34 +/- 0.03 Å for Zn(7)-MT and 2.54 +/- 0.02 Å for Cd(7)-MT, respectively, calculated from sulfur K-edge EXAFS measurements, are consistent with the previously reported results from metal K-edge EXAFS data. Analysis of the sulfur K-edge EXAFS data for Cu(12)-MT indicates that Cu(I) is trigonally coordinated to sulfurs at a distance of 2.25 +/- 0.01 Å. Significant changes in CD spectra observed between Ag(12)-MT 1 and Ag(17)-MT 1 indicate that the modification of the three-dimensional structure occurs when Ag(17)-MT 1 is formed from Ag(12)-MT 1 as Ag(I) is added to the Ag(12)-MT 1. The Ag-S bond distances of 2.45 +/- 0.02 and 2.44 +/- 0.03 Å in Ag(1)(2)-MT 1 and Ag(1)(7)-MT 1, respectively, calculated from the sulfur K-edge EXAFS measurements, lead us to conclude that the Ag(I) in both Ag(1)(2)-MT 1 and Ag(1)(7)-MT 1 is digonally coordinated by thiolates. The number of metals bonded to sulfur in both model compounds and metal-containing metallothioneins is estimated from sulfur K-edge EXAFS measurements to be in the range 1.2-1.7, depending on the fraction of bridging sulfurs present in compounds. Unlike the spectral data recorded during Cu(I) binding, where sharp changes take place past 12 Cu(I), the CD data for Ag-MT 1 show little variation over the entire range of Ag(I):MT molar ratios. This result, established by both low- and high-energy optical methods, suggests that the three-dimensional structure of the metal-binding sites in metallothioneins is strongly influenced by the fraction of bridging sulfur. This analysis is the first to provide direct support for the presence of a clustered Ag-S structure for the Ag(17)-MT 1 species. These data also suggest that the structures in Ag(I) and Cu(I) metallothioneins are probably quite different.

Copper binding to rabbit liver metallothionein. Formation of a continuum of copper(I)-thiolate stoichiometric species

Circular dichroism and ultraviolet absorption spectral data have been used to probe the binding mechanism for formation and the structure of the copper(I)-thiolate binding clusters in rabbit liver metallothionein during addition of Cu+ to aqueous solutions of Zn7-metallothionein 2 and Cd5Zn2-metallothionein 2. Mammalian metallothionein binds metals in two binding sites, namely the alpha and beta domains. Spectral data which probe the distribution of Cu(I) between the two binding domains clearly show that both the site of binding (alpha or beta), and the structures of the specific metal-thiolate clusters formed, are dependent on temperature and on the nature of the starting protein (either Zn7-metallothionein or Cd5Zn2-metallothionein). CD spectra acquired during the addition of Cu+ to Zn7-metallothionein show that Cu+ replace the bound Zn(II) in a domain-distributed manner with complete removal of the Zn(II) after addition of 12 Cu+. Spectral and metal analyses prove that a series of Cu(I)-metallothionein species are formed by a non-cooperative metal-binding mechanism with a continuum of Cu(I):metallothionein stoichiometries. Observation of a series of spectral saturation points signal the formation of distinct optically active Cu(I)-thiolate structures for the Cu9Zn2-metallothionein, Cu12-metallothionein, and the Cu15-metallothionein species. These data very clearly show that for Cu(I) binding to Zn7-metallothionein, there are several key Cu(I):metallothionein stoichiometric ratios, and not just the single value of 12. The CD spectra up to the Cu12-metallothionein species are defined by bands located at 255(+) nm and 280(-) nm. Interpretation of the changes in the CD and ultraviolet absorption spectral data recorded between 3 degrees C and 52 degrees C as Cu+ is added to Zn-metallothionein show that copper-thiolate cluster formation is strongly temperature dependent. These changes in spectral properties are interpreted in terms of kinetic versus thermodynamic control of the metal-binding pathways as Cu+ binds to the protein. At low temperatures (3 degrees C and 10 degrees C) the spectral data indicate a kinetically controlled mechanism whereby an activation barrier inhibits formation of ordered copper-thiolate structures until formation of Cu12-metallothionein. At higher temperatures (> 30 degrees C) the activation barrier is overcome, allowing formation of new Cu(I)-thiolate clusters with unique spectral properties, especially at the Cu9Zn2-metallothionein point. The CD spectra also show that a Cu15-metallothionein species with a well-defined, three-dimensional structure forms at all temperatures, characterized by a band near 335 nm, indicating the presence of diagonal Cu(I).(ABSTRACT TRUNCATED AT 250 WORDS)

Spectral characteristics of cadmium-containing phytochelatin complexes isolated from Schizosaccharomyces pombe

Phytochelatins, heavy-metal-containing peptides with structures (gamma EC)nG, where n = 2-8, have been isolated from higher plants and the fission yeast Schizosaccharomyces pombe. The present work describes the isolation and characterization of several naturally occurring mixed complexes of these peptides from S. pombe exposed to 1 mM CdCl2. A lower-molecular-mass fraction from Sephadex G-50 chromatography yielded three distinct species on further fractionation. HPLC chromatography revealed the presence of peptides with n = 1-4 in varying amounts in these three complexes, referred to as complexes I, II and III. Stoichiometries are proposed for these complexes, based on [Cd], [SH], [S2-] and the amino acid content. Ultraviolet absorption and magnetic circular dichroism spectra of complexes II and III are similar, whereas the CD spectra of these two complexes are strikingly different. Compared to both complexes II and III, the CD bands of complex I are relatively weak. Ultraviolet absorption, CD and magnetic circular dichroism spectra provide a basis for the discussion of structural differences in these complexes.

Luminescent Ag12-metallothionein: dependence of emission intensity on silver-thiolate cluster formation

We report the observation of emission intensity at 77 K that is a function of Ag(I)-thiolate bonds formation within the protein metallothionein. The emission characteristics (a large, 250 nm, Stokes shift and long emission lifetime) suggests that the transition occurs from the excited triplet state. The emission intensity and circular dichroism both indicate that silver(I) clusters form with stoichiometric ratios of 12 Ag(I) to the 20 thiolate sulfur groups that are present in the protein. These data are the first to show that Ag(I)-metallothionein complexes are luminescent and that a specific Ag12-MT species forms.