Copper-containing metallothioneins

The metallothioneins have been found in several vertebrate species and in marine invertebrates.1452 Prinz and Weser purified a copper-containing metallothionein from Saccharomyces cerevisiae.1453 Another copper-binding protein was isolated from Neurospora crassa.1452 The first unequivocal demonstration of a metallothionein in a vascular plant was recently reported.1455 The amount of metallothionein in different species and tissues is variable. The concentration has been reported to increase up to 40-fold by the induction of its biosynthesis by certain metals such as cadmium or zinc. In new-born rat liver (one to four days old) the concentration of Zn- and Cu-metallothionein is 20 times that in 70-day-old adult rats.1456 There are several recent reports and reviews in this active area.1243,1467a k... 

Fig. 1. Schematic overview of copper trafficking and homeostasis inside the yeast cell. The actions of Mad and Ace 1, copper-dependent metalloregulatory transcription factors, control the production of copper import [copper transporter (Ctr) and reductase (Fre)] and detoxification/sequestration [metallothionein (MT)] machineries, respectively. Three chaperone-mediated delivery pathways are shown. Atxl shuttles Cu(I) to the secretory pathway P-type ATPase Ccc2 (right). CCS delivers Cu(I) to the cytoplasmic enzyme copper-zinc superoxide dismutase (SOD) (left). Coxl7 shuttles Cu(I) to cytochrome c oxidase (CCO) in the mitochondria (bottom). Mitochondrial proteins Scol and Sco2 may also play a role in copper delivery to the CuA and CuB sites of CCO. Copper metabolism and iron metabolism are linked through the actions of Fet3, a copper-containing ferroxidase required to bring iron into the cell (lower right) (see text).

Copper is a cofactor in several enzymes, including lysyl oxidase and superoxide dismutase. Ceruloplasmin, a deep-blue glycoprotein, is the principal copper-containing protein in blood. It is used to transport Cu2+ and maintain appropriate levels of Cu2+ in the body s tissues. Ceruloplasmin also catalyzes the oxidation of Fe2+ to Fe3+, an important reaction in iron metabolism. Because the metal is widely found in foods, copper deficiency is rare in humans. Deficiency symptoms include anemia, leukopenia (reduction in blood levels of white blood cells), bone defects, and weakened arterial walls. The body is partially protected from exposure to excessive copper (and several other metals) by metal-lothionein, a small, metal-binding protein that possesses a large proportion of cysteine residues. Certain metals (most notably zinc and cadmium) induce the synthesis of metallothionein in the intestine and liver. 

Historically, the term metallothionein designates the cadmium-, zinc- and copper-containing sulfur-rich protein from horse renal cortex,and must satisfy several criteria. 

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.)... 

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). 

Foetal and neonatal livers contain exceptionally high levels of copper compared to the adult organ. Thus, the livers of new-born rats contain as much as 20 times the level of copper and zinc metallothionein as that found in 70-day-old rats.1150 Again, a metallothionein from foetal bovine liver contained eight copper and two zinc atoms per molecule of protein.1151 These proteins can only be isolated with difficulty under oxygen-free conditions. It appears then that large amounts of copper (and zinc) are stored in the liver bound to metallothionein, and are mobilized as required for enzyme synthesis after birth. 

Pseudomonas putida growing on 3 mM cadmium synthesizes three cysteine-rich proteins of molecular weight 4000 to 7000, containing four to seven cadmium, zinc and copper atoms per molecule. The use of I13Cd NMR on the major cadmium protein shows it to be related to cadmium metallothionein, but with some significant differences.I223b... 

High levels of zinc stimulate the synthesis of metallothionein in the small intestines. The elevated levels of metallothionein then serve as a depot for the binding of high levels of zinc consumed in subsequent meals. The induced protein has been shown to limit the amount of zinc entering the bloodstream with consumption of a high-zinc diet (Menard ef o/., 1981). High doses of copper can induce metallothionein synthesis to the same extent as can zinc. At levels near those found in the diet, zinc is a potent inducer while copper is only a weak inducer. Normally, hepatic metaiiothionein contains mainly zinc, whereas kidney metallothionein contains copper and, when present in the diet, cadmium. The copper entering the liver may be stored in hepatic metallothionein and released into the plasma in ceruloplasmin or secreted in the bile later. 

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