Cadmium-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... 

For -t-2 cations such as zinc(II) and cadmium(ll) each metallothionein molecule contains up to seven metal atoms. X-ray studies indicate that the metal atoms are in approximately tetrahedral sites bound to the cysteine sulfur atoms. The soft mer-cury(II) ion has a higher affinity for sulfur and will displace cadmium from metallothionein. At first the mercury ions occupy tetrahedral sites but as the number increases, the geometries of the metal sites and protein change until about nine Hg(Il) atoms are bound in a linear (S—Hg—S) fashion.92 Up to twelve + 1 cations such as copper(l) and silver(I) can bind per molecule, indicating a coordination number lower than four, probably three (see Problem 12.34),... 

Zinc can play a structural role in proteins and it does this by binding sulfur and/or nitrogen (in cysteine or histidine residues, respectively) [90, 97]. For example, the zinc finger motif which is found in several structures of zinc-binding proteins, contains a motif of Cys and His side chains, four in all, that bind the zinc ion. This motif is also used to bind certain other metal ions such as iron (in rubredoxin), copper (in plastocyanin and azurin), and cadmium (in metallothionein) and so is not entirely specific to zinc. On the other hand, it is an entirely different binding site from that preferred by magnesium ions. 

Metallothioneins (MTs) are small (6000-7000 Da), intracellular, cysteine-rich ( 33% of the amino acids are cysteines) metal binding proteins that were first discovered in 1957 in equine kidney cortex. The subsequent purification of this protein identified it as the only known native cadmium-containing protein. Further studies showed this protein to bind both essential (e.g., Cu and Zn) and nonessential (e.g., Cd and Hg) metal ions and to be truly ubiquitously distributed in nature [215-218]. Additionally, MT biosynthesis is induced at the transcriptional level by a wide range of factors, which includes heavy metal ions that were subsequently found bound to the protein. All of the above factors suggest a role for this protein in heavy metal homeostasis, transport and detoxification [215-218]. Despite 55 years of extensive studies on MTs, which has included the high resolution characterization of the 3D stmcture and metal binding properties [134,136,138,151,154,218-222], the essential physiological functional role(s) of MT remains elusive. Of particular note in these studies was the determination of the NMR solution structure of a mammalian MT in a tour deforce effort by the Wiithrich lab which resulted in the reinterpretation and correction of the mily mammalian MT crystal structure currently available [223,224]. 

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). 

One example of the use of semiempirical methodology is provided in an article detailing a molecular-dynamics simulation of the beta domain of metallothionein with a semiempirical treatment of the metal core.73 The beta domain of rat liver metallothionein-2 contains three-metal centers. In this study, three molecular variants with different metal contents—(1) three cadmium ions, (2) three zinc ions, and (3) one cadmium ion and two zinc ions—were investigated using a conventional molecular dynamics simulation, as well as a simulation with a semiempirical quantum chemical description (MNDO and MNDO/d) of the metal core embedded in a classical environment. For the purely classical simulations, the standard GROMOS96 force-field parameters were used, and parameters were estimated for cadmium. The results of both kinds of simulations were compared to each other... 

Kidney. Since the kidneys receive 20 to 25% of the cardiac output, they will be exposed to a relatively large amount of any systemically administered drug. The kidney also contains a protein, metallothionein, that has a high affinity for metals. This protein is responsible for the renal accumulation of cadmium, lead, and mercury. 

Tolerance to heavy metals, specifically mercury and cadmium, has been associated with the induction of kidney metallothionein, a protein rich in sulfhydryl groups which protects by chelation (102). The synthetic antidote dimercaprol, introduced after World War I for arsenic-containing gases, works by a similar mechanism (103). 

The two clusters differ in their metal-binding properties. Mammalian Cd, Zn metallothionein contains four Cd ions in cluster A and two Cd ions and one Zn ion in cluster B. 1157,1159 Calf liver metallothionein contains three Cu ions in cluster B with Cd-displaceable zinc ions in cluster A.1160 Studies on the binding of zinc and cadmium to human liver metallothionein show that binding occurs first in cooperative fashion to cluster A, followed by cooperative binding to cluster B. Incubation of the 7Cd metallothionein resulted in loss of Cd ions from cluster B initially.1161... 

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... 

K2. Kagi, J. H. R., and Vallee, B. L., Metallothionein A cadmium- and zinc-containing protein from equine renal cortex. J. Biol. Chem. 235, 3460-3465 (1980). 

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