Metalloprotein metallothionein

Metallothionein was first discovered in 1957 as a cadmium-binding cysteine-rich protein (481). Since then the metallothionein proteins (MTs) have become a superfamily characterized as low molecular weight (6-7 kDa) and cysteine rich (20 residues) polypeptides. Mammalian MTs can be divided into three subgroups, MT-I, MT-II, and MT-III (482, 483, 491). The biological functions of MTs include the sequestration and dispersal of metal ions, primarily in zinc and copper homeostasis, and regulation of the biosynthesis and activity of zinc metalloproteins. 

SEC has been used extensively to separate proteins containing different elements such as Zn (II), Cd (II), Cu (H), and Hg (II) in metallothionein [78-80]. These studies show the potential of SEC-ICP-MS for metalloprotein separations but are not specifically concerned with elemental speciation. 

Like pesticides, heavy metals are traditionally tested by enzyme inhibition or modulation of catalytic activity. Several metalloproteins behave as chelators for specific metals with no known catalytic reactions. Such heavy metal binding sites exist in metallothioneins and in various protein elements of bacterial heavy metal mechanisms and have been exploited for specific detection through affinity events. Nevertheless and as previously mentioned, bacterial resistance mechanisms can also be linked to catalytic pathways. For instance, c5rtochromes c3 and hydrogenases from sulfate and sulfur reducing bacteria [284,285] are well suited for bioremediation purposes because they can reduce various metals such as U(V) and Cr(VI) [286,287]. Cytochrome c3 has been reported to catalyse Cr(VI) and U(VI) reduction in Desulfovibrio vulgaris [288,289], suggesting... 

Capillary electrophoresis has proven to be useful in characterizing different molecular forms of various metalloproteins like metallothionein, transferrin, and conalbumin [2-5]. Molecular forms arise from differences in the amino acid sequence of proteins (isoforms), differences in the amount or type of metal bound (metalloforms), or from differences in the type and amount of carbohydrate side chains linked to the protein (glycoforms). CZE was used to follow the formation of the oligomeric iron core and its incorpora-... 

The most definitive assessment of the metal composition of metalloproteins comes from the application of element-specific detection methods. CE-ICP-MS provides information not only about the type and quantity of individual metals bound to the proteins but also about the isotopes of each element as well [11,12]. Elemental speciation has become increasingly important to the areas of toxicology and environmental chemistry. Such analytical capability also opens up important possibilities for trace element metabolism studies. Figure 1 depicts the separation of rabbit liver metallothionein containing zinc, copper, and cadmium (the predominant metal) using CE-ICP-MS with a high-sensitivity, direct injection nebulizer (DIN) interface. UV detection (200 nm) was used to monitor the efficiency of the CE separation of the protein isoforms (MT-1 and MT-2), whereas ICP-MS detection made it possible to detect and quantify specific zinc, copper (not shown), and cadmium isotopes associated with the individual isoform peaks. 

Gold sodium thiomalate provides a stimulus for liver, kidney and possibly other cells to change the body distribution of zinc and copper. Proteins such as metalloproteins, superoxide dismutase and metallothioneins help the cell carry out this task. These essential metals are important in the physiological processes relevant to rheumatoid arthritis, and thus it also appears possible that the gold complexes may mediate their antiarthritic activity through an effect on the metabolism of zinc and copper. 

Cadmium metallothionein has also been studied extensively. This metalloprotein is high in the amino acid cysteine ( 30%) and is devoid of aromatic amino acids. Metallothionein itself may function to help detoxify cadmium. For some experimental tumors, cadmium appears to be anticarcinogenic (e.g., it reduces the induction of tumors). 

With few exceptions, metallothioneins consist of relatively simple amino acids, aromatic amino acids and histidine only being found in a small number of species [329]. This amino acid composition suggests that metallothioneins evolved early in the evolution of life, probably even before the oxygenation of the atmosphere. A further clue is one of their functions. As metal-transport and storage proteins, thioneins are capable of binding metal ions but release them relatively easily as well. Metallothioneins can therefore be considered a transition from non-metal to metalloproteins. It is improbable, however, that the known copper proteins evolved from copper metallothioneins as there are no homologies between them and other copper proteins or enzymes. 

Kagi JHR, Kojima Y, Berger C, Kissling MM, Lerch K, Vasak M (1979) Metallothionein structure and evolution. In Weser U (ed) Metalloproteins. Georg Thieme, Stuttgart New York, p 194... 

Binding those metal ions in a metalloprotein usually prevents them from entering into these types of reactions. For example, transferrin, the iron-transport enzyme in serum, is normally only 30 percent saturated with iron. Under conditions of increasing iron overload, the empty iron-binding sites on transferrin are observed to fill, and symptoms of iron poisoning are not observed in vivo until after transferrin has been totally saturated with iron. Ceruloplasmin and metallothionein may play a similar role in preventing copper toxicity. It is very likely that both iron and copper toxicity are largely due to catalysis of oxidation reactions by those metal ions. 

Fig. 4.5.6. An example of the combination of gel permeation (ion exchange) chromatography of metalloproteins with atomic absorption spectrometry for evaluation of fractions I96], Rat liver supernatant (0.2 ml) obtained after injection of cadmium chloride was applied to a TSK Gel SW 3000 column 600x21.5 mm, and eluted with 50 mM Tris-HCl buffer solution (pH 8.6 at 25°C). Absorbance at 280 nm (lower curves) and concentration of cadmium (A) or zinc (B) (upper curves) were continuously monitored. I and II indicate metallothionein-l and -II, respectively. This chromatogram indicates the contribution of ion exchange to TSK Gel SW gel-permeation chromatography, because both separated proteins have the same molecular weight, and the sequence of peaks emerging corresponds to the ion exchange chromatography separation.

Metalloproteins, proteins with bound metal ions. To this family of transport and storage proteins belong, e.g., ferritin, transferrin, iron-sulfur proteins, metallothioneins [D. P. Ballou, Metalloproteins, Princeton University Press, 1999 A. Messerschmidt et al. (Eds.), Handbook of Metalloproteins, Vols. 1-4, John Wiley Sons, 2001-2004]. 

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