Mammals metallothioneins

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). 

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). 

If animals ingest excessive amounts of Zn(n), Cd (II), Hg(II), or Cu(I) their livers and kidneys accumulate these metals as complexes of proteins called metallo-thioneins/1 e In mammals at least three related genes encode these metal-binding proteins. The best known, metallothionein II, has a highly conserved 61-residue sequence containing 20 cysteine residues and no aromatic residues. 

Metallothionein has been isolated from virtually all of the major mammal organs, including liver, kidney, brain, heart, intestine, lung, skin, and spleen. Nonlethal doses of cadmium, mercury, and lead induce synthesis of metallothionein. In test animals, nonlethal doses of cadmium followed by an increased level of metallothionein in the body have allowed later administration of doses of cadmium at a level fatal to nonacclimated animals, but without fatalities in the test subjects. 

Several classes of molecules are involved in metal homeostasis. Metallothioneins (MTs) are a family of cysteine-rich metal-binding proteins that in mammals appear to function in Zn homeostasis and protect against heavy metal toxicity and oxidative stress60. Glutathione (GSH) is a sulfhydryl-rich tripeptide that is generally involved in the protection of cells against toxicants and in the metabolism of xenobiotics71. MTs and GSH are found in fish, and their expression and functions have been conveniently studied with piscine cell lines. 

The presence of Hg, Cd, Cu and Zn induces the production of thioneins in the liver and kidneys of mammals. Between 4 and 12 metal centres can be bound by one thionein Zn Hg, Cd centres are likely to be in tetrahedral environments, while Cu may be 3-coordinate. The structure of the Cd/Zn-containing metallothionein isoform II from rat liver has been determined by X-ray difiraction, and Figure 28.6a illustrates the folded protein chain consisting of 61 amino acid residues of which 20 are Cys groups. One Cd + and two Zn centres are bound in... 

We also need to know how metals are transported into cells and stored. Iron has been investigated most intensively. Mammals bind and transport iron by the serum protein transferrin and store it in ferritin. One protein can bind around 4500 Fe ions. Copper is taken up by the serum protein ceruloplasmin. Also albumin can bind and transport metal ions. The cystein-rich protein metallothioneine is formed in cells if toxic Cd and are present. This aprotein protects cells against the toxic effects of such metal ions. 

Copper(II) affects gene expression in a wide variety of organisms including yeast, mammals, and bacteria. The best-described copper-regulated genes encode metallothioneins (MTs) the biochemistry and biology of this protein have been extensively reviewed (Chap. 6, this volume Hamer 1986 Andrews 1990). Its role as a model and potential mediator of transcription are discussed here. 

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