Cadmium in Metallothioneins

1 Divalent Metal-Binding Sites and Their Organization. 342 

Abstract Metallothioneins (MTs) are low-molecular-mass cysteine-rich proteins with the ability to bind mono- and divalent metal ions with the electron configuration in form of metal-thiolate clusters. MTs are thought, among others, to play a role in the homeostasis of essential Zn(ll) and Cu(l) ions. Besides these metal ions also Cd(ll) can be bound to certain MTs in vivo, giving rise to the perception that another physiological role of MTs is in the detoxification of heavy metal ions. Substitution of the spectroscopically silent Zn(ll) ions in 

Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland 

Sigel (eds.), Cadmium From Toxicity to Essentiality, Metal Ions in Life Sciences 11, DOI 10.1007/978-94-007-5179-8 ll, 

Keywords circular dichroism electronic absorption electrospray ionization mass spectrometry magnetic circular dichroism metal-thiolate cluster structures metallothionein perturbed angular correlation of y-rays spectroscopy X-ray 

---

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. 

The objective in this section of this chapter is to illustrate how " Cd NMR methods were used to provide the first definitive evidence for the existence of polynuclear Cd clusters in MT, which was followed by the elucidation of the 3D solution structure of a variety of MTs and the establishment of the relative affinities of the multiple metal binding sites for Cd, Zn and Cu [132,136,138,143,154,219,220,225]. For a more in depth discussion on metallothioneins in general, the reader is referred to Chapter 11, Cadmium in Metallothionein by Freisinger and Vas in this volume. 

Avdeef, A. Zelazowski, A. J. Garvey, J. S., Cadmium binding by biological ligands. 3. Five- and seven-cadmium binding in metallothionein A detailed thermodynamic study, Inorg. Chem. 24, 1928-1933 (1985). 

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... 

Jessen-Eller, K. and J.F. Crivello. 1998. Changes in metallothionein mRNA and protein after sublethal exposure to arsenite and cadmium chloride in juvenile winter flounder. Environ. Toxicol. Chem. 17 891-896. 

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. 

Sato, M. and Y. Nagai. 1989. Effect of zinc deficiency on the accumulation of metallothionein and cadmium in the rat liver and kidney. Arch. Environ. Contam. Toxicol. 18 587-593. 

There seems to be a critical level of cadmium in the kidney when the kidney metallothionein is saturated and the free cadmium causes toxicity. The damage to the kidney occurs in the first and second segments of the proximal tubule. This can be detected biochemically as glucose, amino acids, and protein in urine. The proteins are predominantly of... 

Cadmium is a cumulative toxicant with a biologic half-life of up to 30 years in humans. More than 70% of the cadmium in the blood is bound to red blood cells accumulation occurs mainly in the kidney and the liver, where cadmium is bound to metallothionein. In humans the critical target organ after long-term exposure to cadmium is the kidney, with the first detectable symptom of kidney toxicity being an increased excretion of specific proteins. 

Metallothionein-bound cadmium and ionic cadmium were separated by ion exclusion on a short chromatographic column. Cadmium in the effluent was detected by coupled ICP-MS (Szpunar et al., 1997). The authors constructed an interface between the column and detector that allowed gradients up to 30% methanol to be used in the eluents. The rapid method of analysis was applied to studies on distributions of cadmium species in mussels. 

Goyer RA, Cherian MG. Role of metallothionein in human placenta and rats with cadmium exposure. In Cadmium in the human environment toxicity and carcinogenicity. Nordberg G, Herber R,Alessio E (editors). International Agency for Research on Cancer (lARC) Scientific Publications, Vol 118, Eyon 1992 p. 239-247. 

Trace metal disturbances may be due to the uremia per se. Indeed, as the urinary excretion route is an important pathway of elimination of many trace elements, i.e. silicon, strontium, aluminum,... impairment of the kidney will be an important determinant of their accumulation, whilst in the presence of a reabsorptive defect a number of trace elements, especially those that are reabsorbed because of their essential role, be lost resulting in a deficient state. The presence of proteinuria may reasonably result in losses of protein bound elements. It has also been shown also that residual renal funchon may importantly alter the accumulation and hence toxic effects of aluminum [2]. In uremia translocation of a particular metal from one tissue to another may also occur. As an example, under normal circumstances the kidney is an important target organ for cadmium. In chronic renal failure however, possibly as a consequence of a reduction in binding proteins (e.g. metallothionein), the concentrahon of cadmium in this tissue decreases to extremely low levels which... 

Liu J, liu Y, Habeebu SM, Waalkes MP, Klaassen CD. Chronic combined exposure to cadmium and arsenic exacerbates nephrotoxicity, particularly in metallothionein-l/ll null mice. Toxicology 2000,147 157-166. 

---