Metallothioneins

Metallothioneins (MTs) are a superfamily of low-molecular-weight ( 7000-dalton) intracellular metal-binding proteins, which, in many species, play a critical role in (a) the detoxification of nonessential metals such as Cd2+ and Hg2+ and (b) the regulation of intracellular concentrations of essential metals such as Zn2+ and Cu+. In 1957, Kagi and Vallee first purified and characterized MT as a cadmium-binding protein in equine kidney. 

The physiological role of metallothioneins remains unknown, but it may be, in part, to protect cells from metal toxicity by binding metal ions. Metallothioneins may also be important for intestinal and renal absorption of metals and for metal storage and excretion. 

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Otic microorganisms, as well as in some prokaryotes (Nordberg and Kojima 1979, Hamer 1986, Robinson and Jackson 1986). MLPs have not been detected in some mol-lusks, however, and only one MT isoform has been identified in certain other species such as crustaceans, teleosteans and mol-lusks. 

MT have been subdivided into three classes, namely I, II, and III (Fowler et al. 1987), while Binz and Kagi (1999) allocated these compounds to several families. Subsequently, Richards and Beattie (1995), Richards et al. (1996, 1997) and Szpunar (2000) showed capillary zone electrophoresis to be a rapid and reliable method for analysis of this protein. By using a diverse combination of techniques, the metal complexes of metallothionein in rat liver and kidney were first characterized, with two major MT-isoforms (MT-1 and MT-2) being detected in liver, and one MT isoform in kidney (Polec et al. 2002). The order of afHn-ity of metal ions to MTs is Cd Zn, Cu, Ag, Hg Bi Pb, though this may change depending on the tissue involved. MTs are considered to be low molecular-weight pro- 

Their arrangement in the prevailing Cys-X-Cys and Cys-Cys sequences (X = amino acid residue other than Cys) renders the protein a potent metal chelator. Cysteines are implicated in the binding of bivalent metal ions, giving rise to a Cys/metal ratio of about 3. The structures have been established from a large variety of spectroscopic analysis of MTs (Otvos and Armitage 

The maximal UV absorption of MTs occurs at 254 nm, and not at 280 nm as is found with most proteins that contain aromatic amino acids. Specific optical characteristics in terms of the absorption of the metal-thiolate complexes occur at 254 nm with Cd, at 225 nm with Zn, at 275 nm with Cu, and at 300 nm with Hg. MTs bind mineral ions, as both plastic and trace elements, and also toxic heavy metals such as Cd, Hg, and Pb. One of the principal 

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Metallothioneins. The metaHothionekis, a group of low (<10,000) molecular weight proteins containing - 30% cysteine residues, are efficient... 

Metallothionein (from rabbit liver) [9038-94-2], Purified by precipitation to give Zn- and Cd-containing protein fractions and running on a Sephadex G-75 column, then isoelectric focusing to give two protein peaks [Nordberg et al. Biochem J 126 491 1972]. 

The situation is different for other examples—for example, the peptide hormone glucagon and a small peptide, metallothionein, which binds seven cadmium or zinc atoms. Here large discrepancies were found between the structures determined by x-ray diffraction and NMR methods. The differences in the case of glucagon can be attributed to genuine conformational variability under different experimental conditions, whereas the disagreement in the metallothionein case was later shown to be due to an incorrectly determined x-ray structure. A re-examination of the x-ray data of metallothionein gave a structure very similar to that determined by NMR. 

J. H. Beattie, R. Self and M. P. Richards, The use of solid phase concenti ators for online pre-concentration of metallothionein prior to isofom separation by capillary zone electrophoresis , Electrophoresis 16 322-328 (1995). 

Cadmium, 5, 925-1022 acute poisoning, 5,1000 binding to metallothioneins, 6, 673 chronic poisoning, 5, 1000 gravimetry, 1,532 masking, 1,538 metallothioneins, 5,1021 poisoning... 

As more is learned about the chromosomal effects on HS gene expression, it is important to point out that these genes are actually a subset of inducible responses to cellular stress. Not all of these inducible responses involve HSF, and this indicates that cells have diversified transcriptional responses to cope with different types of stress. This diversification is manifested by glucose regulated genes (grp), as well as the metallothionein and oxidant-injury genes (Watswich, 1988 Storz et al., 1990 Devary et al., 1992 Skroch et al., 1993 Xu, 1993). 

Skroch, P., Buchman, C., Karin, M. (1993). Regulation of human and yeast metallothionein gene transcription by heavy metal ions. Prog. Clin. Biol. Res. 380, 113-128. 

Xu, C. (1993). cDNA cloning of a mouse factor that activates transcription from a metal response element of the mouse metallothionein-1 gene in yeast. DNA Cell Biol. 12, 517-525. 

Webb, M. (1979). The metallothioneins. In The Chemistry, Biochemistry, and Biology of Cadmium, ed. M. Webb, pp. 195-266. Amsterdam Elsevier/North Holland. 

The Tissue Levels of Copper of Certain Other Metals Are Regulated in Part by Metallothioneins... 

Metallothioneins are a group of small proteins (about 6.5 kDa), found in the cytosol of cells, particularly of liver, kidney, and intestine. They have a high content of cysteine and can bind copper, zinc, cadmium, and mercury. The SH groups of cysteine are involved in binding the metals. Acute intake (eg, by injection) of copper and of certain other metals increases the amount (induction) of these proteins in tissues, as does administration of certain hormones or cytokines. These proteins may function to store the above metals in a nontoxic form and are involved in their overall metaboHsm in the body. Sequestration of copper also diminishes the amount of this metal available to generate free radicals. 

There is concern over the toxicity of a nnmber of metals and metalloids, and their oxyanions. As for antibiotics, the genes for resistance are often plasmid-bome. There are several mechanisms that may operate—redaction, methylation, efflnx, and the synthesis of metal-binding metallothioneins. The following text illustrates aspects of these mechanisms. 

It is therefore clear that care should be exercised in assigning metallothioneins to a cardinal role in conferring resistance to metals. 

Culotta VC, WR Howards, XE Liu (1994) CRS5 encodes a metallothionein-like protein in Saccharomyces cerevisiae. J Biol Chem 269 25295-25302. 

Gupta A, BA Whitton, AP Morby, JW Huckle, NJ Robinson (1992) Amplification and rearrangement of a prokaryotic metallothionein locus smt in Synechococcus PCC 6301 selected for tolerance to cadmium. Proc Roy Soc (London) SerB 248 273-281. 

Mehra RK, JR Garey, DR Winge (1990) Selective and tandem amplification of a member of the metallothionein gene family in Candida glabrata. J Biol Chem 265 6369-6375. 

Olafson RW, WD McCubbin, CM Kay (1988) Primary- and secondary-structural analysis of a unique prokaryotic metallothionein from a Synechococcus sp. cyanobacterium. Biochem J 251 691-699. 

Palmiter RD (1998) The elusive function of metallothioneins. Proc Natl Acad Sci USA 95 8428-8430. 

Turner JS, NJ Robinson (1992) Cyanobacterial metallothioneins biochemistry and molecular genetics. J Ind. Microbiol 14 119-125. 

Gold(I) thiolates also undergo reactions with disulfides. An example is 2,2-dithio-bis(2-nitrobenzoic acid) (ESSE) used as a kinetic probe of sulfhydryl reactivity in metallothioneins [39-42]. ESSE is readily attacked by metal-bound thiolates. The reaction with the open chain form of AuSTm seems to occur in two stages. The first attack involves the terminal sulfides and the second attack involves the bridging thiolates [19] ... 

Metallothioneins (MTs) are small proteins with an especial affinity for the binding of various heavy metals active in a wide range of reactions [95-97]. Besides their role in... 

Savas, M.M., Petering, D.H. and Shaw, C.F. Ill (1993)Theoxidationofrabbitliver metallothionein-11 by 5,5 -dithiobis (2-nitrobenzoic acid) and glutathione disulfide. Journal of Inorganic Biochemistry, 52, 235—249. 

Suzuki, KT., Imura, N. and Kimura, M. (1993) Metallothionein III Biological Roles and Medical Implications, Birkhaiiser Berlag, Basel, p. 470. 

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