Metallothioneins, zinc

Henkel G, Krebs B. Metallothioneins zinc, cadmium, mercury and copper thilates and selenolates mimicking protein active site features - structural aspects and biological implications. Chem Rev 2004 104 801-24. 

Olsson, P.-E., C. Haux and L. Forlin. Variations in hepatic metallothionein, zinc and copper levels during an annual reproductive cycle in rainbow trout, Salmo gairdneri. Fish Physiol. Biochem. 3 39-47, 1987. 

There are at least three major types of natural proteins and peptides which are rich in cysteinyl residues metallothioneins, zinc finger proteins, and phytochelatins. The biological role and structural characteristics of metallothioneins are dealt with in Chapter 11 of this book. Zinc finger peptides are possible targets for cadmium(ll) toxicity, while phytochelatins play an important role in the defense mechanisms of plants (Chapter 13). The multicysteine peptides are frequently used to mimic the cadmium(ll) binding ability of all these natural substances. 

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. 

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. 

Shaw, C.F. Ill, Laib, J.E., Savas, M.M. and Petering, D.H. (1990) Biphasickinetics of aurothionein formation from gold sodium thiomalate a novel metallochromic technique to probe zinc (2 + ) and cadmium(2 +) displacement from metallothionein. Inorganic Chemistry, 29, 403-408. 

Silver(I) trifluoromethanethiolate (AgSCF3) has been used to prepare trifluoromethyl aryl sulfides by reaction with iodide.996 A mixed silver-zinc thiolate complex [Ag4Zn2(SC6H2-Pr -2,4,6)6(OTf)2] has been prepared by reaction of AgOTf with Zn[N(TMS)2]2 in the presence of the thiol.99 Solid-state 109Ag NMR can be a sensitive environment probe for silver thiolates,998 overall for biological thiolates ligands as cysteine,999 or proteins such as metallothionein.1000,1001... 

It is now established that cadmium, besides zinc, is accumulated in some native cysteine-rich proteins (e.g., metallothioneins) and the binding mode and sites in the protein are studied and largely understood.57 Also, the detection and study of native Cd-enzymes and Cd-substituted Zn-enzymes is just beginning at the time of writing (for a short survey see ref. 58). 

In mammals, as in yeast, several different metallothionein isoforms are known, each with a particular tissue distribution (Vasak and Hasler, 2000). Their synthesis is regulated at the level of transcription not only by copper (as well as the other divalent metal ions cadmium, mercury and zinc) but also by hormones, notably steroid hormones, that affect cellular differentiation. Intracellular copper accumulates in metallothionein in copper overload diseases, such as Wilson s disease, forming two distinct molecular forms one with 12 Cu(I) equivalents bound, in which all 20 thiolate ligands of the protein participate in metal binding the other with eight Cu(I)/ metallothionein a molecules, with between 12-14 cysteines involved in Cu(I) coordination (Pountney et ah, 1994). Although the role of specific metallothionein isoforms in zinc homeostasis and apoptosis is established, its primary function in copper metabolism remains enigmatic (Vasak and Hasler, 2000). 

Cadmium is nutritionally non-essential, toxic and a ubiquitous environmental pollutant. It is found in leafy vegetables, grains and cereals, and since it is present in substantial amounts in tobacco leaves, cigarette smokers on a packet a day can easily double their cadmium intake. It has a long biological half-life (17-30 years in man), accumulates in liver and kidneys and its toxicity involves principally kidney and bone (Goyer, 1997).While Cd interferes primarily with calcium, it also interacts with zinc and can induce the synthesis of metallothionein. Cadmium bound to metallothionein in liver or kidney is thought to be non-toxic, but cadmium in plasma... 

Metallothioneins (MT) are unique 7-kDa proteins containing 20 cysteine molecules bounded to seven zinc atoms, which form two clusters with bridging or terminal cysteine thiolates. A main function of MT is to serve as a source for the distribution of zinc in cells, and this function is connected with the MT redox activity, which is responsible for the regulation of binding and release of zinc. It has been shown that the release of zinc is stimulated by MT oxidation in the reaction with glutathione disulfide or other biological disulfides [334]. MT redox properties led to a suggestion that MT may possesses antioxidant activity. The mechanism of MT antioxidant activity is of a special interest in connection with the possible antioxidant effects of zinc. (Zinc can be substituted in MT by some other metals such as copper or cadmium, but Ca MT and Cu MT exhibit manly prooxidant activity.)... 

Despite the conclusions in the cited literature about direct MT interaction with free radicals, the mechanism of MT antioxidant activity remains obscure. Markant and Pallauf [339] concluded that cysteine groups and not zinc are responsible for the inhibition of lipid peroxidation in hepatocytes. Maret and Vallee [340,341] also questioned the possibility of direct scavenging of free radicals by MT and suggested that zinc release is a major mechanism of antioxidant effects of metallothioneins. 

Birds are comparatively resistant to the biocidal properties of cadmium. Adult drake mallards (Anas platyrhynchos) fed up to 200 mg cadmium per kg diet for 90 days all survived with no loss of body weight (White and Linley 1978). Laying hens fed 200 mg Cd/kg diet also survived egg production was suppressed at that concentration but not at lower concentrations (White and Linley 1978). Marine and terrestrial animals, including ducks, have been shown to be particularly abundant in a wildlife community associated with a marine sewer outfall (Brown et al. 1977). These animals were contaminated with high levels of cadmium, as well as zinc and copper, but were apparently protected from the deleterious effects of high metal body burdens by metallothioneins. Amounts... 

In mammals, cadmium inhibits copper absorption across the intestinal mucosa (Aaseth and Norseth 1986). Intercorrelations of copper with cadmium and zinc in livers of polar bears (Ursus maritimus) are probably mediated by metallothioneins, which may contain all three metals (Braune etal. 1991). In rats, copper protects against nephrotoxicity induced by cadmium, provided that copper is administered 24 h prior to cadmium insult. Specifically, rats given 12.5 mg Cu/kg BW by way of subcutaneous injection 24 h before receiving 0.4 mg Cd/kg BW — when compared to a group receiving Cd alone — did not have excessive calcium in urine and renal cortex or excessive protein in urine. Thus, 2.8 mg Cu/kg BW protects against 0.25 mg Cd/kg BW (Liu et al. 1992). 

Hogstrand, C., G. Lithner, and C. Haux. 1991. The importance of metallothionein for the accumulation of copper, zinc and cadmium in environmentally exposed perch, Perea fluviatilis. Pharmacol. Toxicol. 69 492-501. 

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