A spectroscopic study of rat liver and Scylla serrata crab metallothioneins

Purification, secondary structure and antioxidant activity of metallothionein zinc-binding proteins from Arca subcrenata

Zinc-binding proteins named MT-M-I and MT-M-II were obtained after purification from metal-exposed hairy clams (Arca subcrenata) using gel permeation and ion-exchange chromatography. MT-M-I and MT-M-II were resolved by ion-exchange chromatography, and they were found to have similar molecular weights. MT-M-I and MT-M-II can bind 6 and 7 equivalents of Zn²⁺ in vitro, and they showed unusual migration behaviors in Tricine sodium dodecyl sulfate polyacrylamide gel electrophoresis (Tricine-SDS-PAGE). Such migration behaviors may be due to themetal thiolate clusters in these proteins. In terms of amino acid composition, the proportion of cysteine in MT-M-I and MT-M-II was approximately 30%, and glycine accounted for approximately 15%, where as aromatic amino acids were absent. Considering the performance in Tricine-SDS-PAGE and the amino acid compositions, MT-M-I and MT-M-II conform to the molecular characteristics of the metallothionein proteins. The structures were explored using circular dichroism (CD) and Fourier-transform infrared spectroscopy (FTIR). Also determined the antioxidant activities in terms of DPPH radical scavenging ability, hydroxyl radical (·OH) scavenging ability, and ferric-reducing/antioxidant power. The antioxidant activities of MT-M-I were found to be stronger than those of MT-M-II.

Cadmium binding studies to the earthworm Lumbricus rubellus metallothionein by electrospray mass spectrometry and circular dichroism spectroscopy

The earthworm Lumbricus rubellus has been found to inhabit cadmium-rich soils and accumulate cadmium within its tissues. Two metallothionein (MT) isoforms (1 and 2) have been identified and cloned from L. rubellus. In this study, we address the metalation status, metal coordination, and structure of recombinant MT-2 from L. rubellus using electrospray ionization mass spectrometry (ESI-MS), UV absorption, and circular dichroism (CD) spectroscopy. This is the first study to show the detailed mass and CD spectral properties for the important cadmium-containing earthworm MT. We report that the 20-cysteine L. rubellus MT-2 binds seven Cd(2+) ions. UV absorption and CD spectroscopy and ESI-MS pH titrations show a distinct biphasic demetalation reaction, which we propose results from the presence of two metal-thiolate binding domains. We propose stoichiometries of Cd(3)Cys(9) and Cd(4)Cys(11) based on the presence of 20 cysteines split into two isolated regions of the sequence with 11 cysteines in the N-terminal and 9 cysteines in the C-terminal. The CD spectrum reported is distinctly different from any other metallothionein known suggesting quite different binding site structure for the peptide.

A new insight into metallothionein (MT) classification and evolution. The in vivo and in vitro metal binding features of Homarus americanus recombinant MT

We report the synthesis and characterization of a Homarus americanus MT-cDNA (MTH) through retrotranscription of MTH-mRNA from metal-injected lobsters. Heterologous Escherichia coli expression in zinc- and copper-supplemented medium was achieved for MTH, the two domains betabetaMTH and betaalphaMTH and three site-directed mutants, betabetaC9H, betaalphaC37H, and betaalphaE31C/T34C. The in vivo conformed metal complexes and the in vitro substituted cadmium aggregates were characterized. Major stoichiometries of M(II)6-MTH for the entire MTH and M(II)3-betabetaMTH and M(II)3-betaalphaMTH for the independent domains fully validated our expression system. A low affinity binding site for a seventh Zn(II) in the in vivo synthesized MTH was located in the betaalpha domain. Additionally, minor M(II)4 species were found for each domain. Both single Cys to His mutations exhibited a similar reduction of their in vivo zinc binding ability but differed in their cadmium binding behavior when compared with the wild-type forms. Conversely, the double mutant showed an enhanced zinc and cadmium binding capacity. In vivo synthesis of MTH and of its independent domains in the presence of copper only afforded heterometallic copper-zinc species. These findings allow consideration of MTH as a zinc thionein and question the view of all crustacea MT structures as copper thioneins. Furthermore, a new approach for the evolutionary and functional classification of MT is proposed, based on the stoichiometry of metal-MT species and molecular phylogenetic analysis.

Expression, purification and characterization of beta domain and beta domain dimer of metallothionein

In order to examine the independent self-assembly of the beta fragment of metallothionein and the interaction between two domains with the linker sequence, Lys-Lys-Ser, the chemically synthesized genes of the beta domain and its dimer (beta-KKS-beta) were cloned into vector pGEX-4T-1 and expressed as carboxyl terminal extension of glutathione-S-transferase (GST). After the GST fusion proteins had been digested with thrombin on a glutathione-Sepharose 4B affinity chromatography column, the beta domain and its dimer were purified with gel filtration and analyzed for their biochemical and spectroscopic properties. Amino acid composition and molecular mass are determined to be consistent with the expected value. The analysis of metal content shows that the beta domain and its dimer can bind with about 3eq and 6eq divalent metals, respectively. The characteristic peak presented around 254 nm in the UV and CD spectrum indicated that both the beta domain and its dimer are able to form the cadmium-thiolate clusters without the aid of the alpha domain. Furthermore, the absorption peak of the beta domain dimer is much higher than that of the beta domain, which suggested that there is an interaction between two beta domains. Finally, the metal-binding ability was determined by DTNB competitive reaction and the value of half dissociation pH, the results reveal that the beta domain dimer has stronger metal-binding ability than the single beta domain, which provides further evidence of the interaction between the two domains.

Fluorometric study of the isoform difference of mammalian metallothionein

A fluorometric method for studying the isoform difference of mammalian metallothionein (MT), which lacks aromatic amino-acid residues, is reported. Cadmium-induced rabbit and hedgehog liver MT exhibited a strong luminescence signal in the range of 335 to 340 nm when excited at 285 nm in aqueous solution. The differences in emission intensity of the two major isoforms of MTs are significant. When titrated with chloride acid, which is believed to proton the metal-thiolate coordination bound to the -SH group in MT, a 10-nm red-shift property of the emission spectrum was observed, and the red-shift properties of the isoforms varied with the species. The observed fluorescence property of MT was considered to be the result of its polypeptide chains, which was confirmed by comparing the luminescence and absorption spectra during the titration of MT with diethylenetriaminepentaacetic acid. The new luminescence property of MT should be useful in studying the isoform and function difference of MT.

Studies on plantacyanin. IV. Reconstitution with Cu-thionein, oxidation by cytochrome oxidase and autooxidation in the presence of cardiolipin

Cu-thionein isolated from cucumber roots was used for reconstitution of plantacyanin from cucumber. The rate of the copper transfer from Cu-thionein to apoplantacyanin was found to depend on pH, ionic strength and concentrations of the proteins. The rate of reconstitution with Cu-thionein was 10-times higher than with copper ions. No intermediate was observed during reconstitution with Cu-thionein. The incubation of oxidized holoplantacyanin with Cu-thionein or apothionein brings about the reduction of plantacyanin copper. This process, however, was found to be slow as compared to the rate of copper transfer from Cu-thionein to apoplantacyanin. Cytochrome oxidase from heart mitochondria was detected to possess some plantacyanin oxidase activity with the turnover number 5 min-1. The activity of the enzyme towards plantacyanin as well as with cytochrome c as a substrate was established to be lipid and ionic strength-dependent, and it was inhibited by CN- and N3-. Lineweaver-Burk plots show that the inhibitory effect of ionic strength on plantacyanin oxidase activity is connected with changes of Michaelis constant rather than of the maximal rate. Plantacyanin which is known to be very resistant towards many cationic, anionic and nonionic detergents, becomes, as well as cytochrome c, autooxidable in the presence of cardiolipin.

Spectroscopic studies on cadmium (II)- and cobalt(II)-substituted metallothionein from the crab Cancer pagurus. Evidence for one additional low-affinity metal-binding site

The binding of diamagnetic Cd(II) and paramagnetic Co(II) ions to the metal-free form of crab, Cancer pagurus, metallothionein (MT) was studied by various spectroscopic techniques. Both reconstituted and native Cd(II)-MT containing 6 mol Cd(II)/mol protein display electronic absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra which were indistinguishable. The stoichiometric replacement of Cd(II) ions in native Cd(II)6-MT by paramagnetic Co(II) ions enabled the geometry of the metal-binding sites to be probed. The electronic absorption and MCD spectra of Co(II)6-MT revealed features characteristic of distorted tetrahedral tetrathiolate Co(II) coordination for all six metal-binding sites. The stepwise incorporation of Cd(II) and Co(II) ions into this protein was monitored by electronic absorption and CD, and by electronic absorption and EPR spectroscopy, respectively. The results indicate that the metal-thiolate cluster structure is generated when more than four metal ions are bound. Below this titration point separate tetrahedral tetrathiolate complexes exist. This suggests that the cluster formation occurs in a two-step process. Furthermore, the spectroscopic features in both Cd(II)- and Co(II)-metal derivatives above the full metal occupancy of six suggest the existence of one additional metal-binding site. The subsequent loss of one Cd(II) ion from crab Cancer Cd(II)7-MT in the gel filtration studies demonstrate the low metal-binding affinity of the latter site. While the spectroscopic properties indicate an exclusively tetrahedral type of metal-thiolate sulfur coordination for the binding of the first six metal ions, they suggest that the seventh metal ion is coordinated in a different fashion.

Dogfish metallothionein--I. Purification and characterization and comparison with rat metallothionein

A low-molecular weight metal-binding protein has been found in the liver of the dogfish (Scyliorhinus canicula) in both control and cadmium-treated animals and in both male and female animals. This protein is present as Zn, Cu-protein in control and as Cd, Zn, Cu-protein cadmium-treated (both water and i.p. administration) animals. This hepatic protein was characterized as metallothionein from several properties related to mammalian metallothionein. A different response to cadmium administration between sexes is suggested.

Cadmium binding to metallothioneins and the estimation of protein concentration using cadmium-saturation methods

The detailed spectral changes observed in the absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra upon addition of Cd2+ to rat liver Cd,Zn-metallothionein (MT) are reported. Results from dialysis experiments clearly demonstrate that up to 8.6 mole equivalents of Cd2+ can be bound to this protein. The excess Cd2+ ions bound appear to have lower binding constants than those of the first seven Cd2+ ions bound. Red blood cell hemolysate (RBC) can compete with the metallothionein for all Cd2+ bound in excess of seven mole equivalents. Thus the RBC hemolysate method of estimating protein concentrations is shown to be correct when based upon complete loading of all binding sites in MT with Cd2+.

Absorption, circular dichroism, magnetic circular dichroism and emission study of rat kidney Cd,Cu-metallothionein

Absorption, circular dichroism (CD), magnetic circular dichroism (MCD) and emission spectra of rat liver and rat kidney cadmium-, zinc- and copper-containing metallothioneins (MT) are reported. The absorption, CD and MCD data of native rat kidney Cd,Cu-MT protein closely resemble data recorded for the rat liver Cd,Zn-MT. This suggests that the major features in all three spectra of the native Cd,Cu-MT are dominated by cadmium-related bands. The CD spectrum of the Cd,Cu-MT recorded at pH 2.7 has the same band envelope that is observed for a Cd,Cu-MT formed in vitro by titration of Cd,Zn-MT with Cu(I), suggesting that the copper occupies the zinc sites in Cd,Cu-MT formed both in vivo and, at low molar ratios, in vitro. Remetallalion of the metallothionein from low pH in the presence of both copper and cadmium results in considerably less cadmium bound to the protein than was present in the native sample. It is suggested that this is due to the effect of the distribution of the copper amongst all available binding sites, thus inhibiting cluster formation by the cadmium. Emission spectra are reported for the first time for a cadmium- and copper-containing metallothionein. An emission band at 610 nm is shown to be a sensitive indicator of Cu(I) binding to metallothionein. Both the native Cd,Cu-MT and a Cd,Cu-MT formed in vitro exhibit an excitation spectrum with a band in the copper-thiolate charge-transfer region.