MT isoform

The half-life of M-MT is dependent on the binding affinity of thionein for different metal ions. For instance, upon oxidation, Cu-MT forms insoluble polymers which are biologically unavailable and are eventually eliminated via biliary secretion. In contrast, thionein has lower affinity for Zn, making it more easily released from the protein and rendering the ion available for cellular processes. Furthermore, the rate of degradation may be influenced by differences in metal distribution between MT isoforms. It has been determined that MT degradation can occur in lysosomal and nonlysosomal (cytosolic) compartments. 

Four major MT isoforms, MT-1, MT-2, MT-3, and MT-4, have been identified in mammals. The most widely expressed isoforms in mammals, MT-1 and MT-2, are rapidly induced in the liver by a wide range of metals, drugs, and inflammatory mediators. In the gut and pancreas, MT responds mainly to Zn status. A brain isoform, MT-3, has a specific neuronal growth inhibitory activity, while MT-1 and MT-2 have more diverse functions related to their thiolate cluster structure. These include involvement in Zn homeostasis, protection against heavy metal (especially Cd) and oxidant damage, and metabolic regulation via Zn donation, sequestration, and/or redox control. A possible role for MT-4 is related to copper requirements in epithelial differentiating tissues. 

Otic microorganisms, as well as in some prokaryotes (Nordberg and Kojima 1979, Hamer 1986, Robinson and Jackson 1986). MLPs have not been detected in some mol-lusks, however, and only one MT isoform has been identified in certain other species such as crustaceans, teleosteans and mol-lusks. 

MT have been subdivided into three classes, namely I, II, and III (Fowler et al. 1987), while Binz and Kagi (1999) allocated these compounds to several families. Subsequently, Richards and Beattie (1995), Richards et al. (1996, 1997) and Szpunar (2000) showed capillary zone electrophoresis to be a rapid and reliable method for analysis of this protein. By using a diverse combination of techniques, the metal complexes of metallothionein in rat liver and kidney were first characterized, with two major MT-isoforms (MT-1 and MT-2) being detected in liver, and one MT isoform in kidney (Polec et al. 2002). The order of afHn-ity of metal ions to MTs is Cd > Zn, Cu, Ag, Hg > Bi > Pb, though this may change depending on the tissue involved. MTs are considered to be low molecular-weight pro-... 

Table 1 Comparison of representative amino acid sequences of the mammalian MT isoforms. The conserved Cys residues are in bold...

MTs were first isolated from equine kidney, but they have widely been found throughout the animal kingdom. In typical mammals, there are two main isoforms, MT-1 and MT-2, named in the order of their elution from an anion-exchange column. Several experiments under Cd exposure, as well as under natural conditions, have demonstrated that Cd binds to both MT isoforms, in combination with copper and zinc. The relative affinity of such metal ions for MTs is in the following order Zn [Pg.327]

Once a fraction is separated (e.g., MT-1), further separation of MT isoforms is possible using reverse phase (RP)-LC, which is the preferred technique in terms of resolution because the packing material is usually free of ligands for metals. The hybrid technique RP-LC-ICP-MS has been used, in fact, for such speciation purposes in biological samples as liver, kidney, and brain MTs, the main limitation being the loss of sensitivity in the ICP-MS due to the presence of the organic modifiers. 

Rgure 2 Electrophoretic separation of fish Cd-MTs complexes (real sample) by CE-ICP(Q)MS. Signals for " Cd and " Cd were monitored showing that Cd binds two main MT isoforms. (Alvarez-Uamas G, Fernandez de la Campa MR, and Sanz-Medel (2003) Sample stacking capillary electrophoresis with ICP-(Q)MS detection for Cd, Cu, and Zn speciation in fish liver metallothioneins. Journal of Analytical Atomic Spectrometry 18 460-466 reproduced from The Royal Society of Chemistry.)... 

In most of the mammalian tissues studied, there are two major isoforms of MT (termed MT-1 and MT-2), which differ slightly in amino acid sequence and net charge. These MT isoforms bind metals identically but are encoded by two distinct genes. Recently two other mammalian MT genes (termed MT-3 and MT-4) have been identified in certain mammal tissues (Palmiter et al. 1993), but their significance is not yet understood. In contrast, only one MT gene has been isolated from avian species, chicken, and turkey (Fernando and Andrews 1989). 

Schaumlofel et al. have characterized and quantified metallothionein (MT) isoforms. " ... 

Metallothionein (MT) is a low molecular mass protein (6-10 kDa) that was firstly isolated from horse kidneys by Margoshes and Vallee in 1957. This protein contains cysteine moieties and lacks aromatic amino acids in the structure (Figure 1). Four MT isoforms (MT-l to MT-4) have been foimd in mammals [1]. Expression and localization of individual MT isoforms are variable at intracellular level (cytosol, nucleus, lysosomes and mitochondria) and in individual tissues. MT-l and 2 are present in tissues of kidneys, liver and intestine. MT-3 is located in brain and MT-4 in skin [2]. MT-l and MT-2 primarily provide protection against the effects of heavy metals, but also participate in the maintenance of intracellular homeostasis of zinc [2-7]. 

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