Metallothioneins (MT) are the widespread proteins in animal world. These proteins are characterized by the great invariability of their structure. While isolated from the different organs of different animals they only slightly differ in the aminoacid composition from one another. Studies on the structure and biological functions of MT have identified four major isoforms (MT-I, -II, -III and ?IV) in mammals. Both MT-I and MT-II genes are expressed in various organs and they are inducible by a variety of metals, hormones, cytokines and xenobiotics. MT-III is expressed only in brain and no specific environmental regulatores have been identified. MT-IV is only expressed in differentiating cells of stratified squamous epithelia.
Metallothioneins are intracellular proteins binding metals. There proteins are involved in zinc and copper homeostasis as well as in organism detoxication of heavy metals. The induction of metallothioneins synthesis is caused not only by metal ions but also by the interleukins, by tumor necrosis factor and by glicocortycoids or the stressing factors. Metallothioneins prove the protective action in cadmium toxication and prevent interaction of cadmium with thiol groups -SH proteins. In neoplasms the increase of metallothionein level occurs both in tumors and in liver, the level of these proteins is correlated with the size of the tumor.
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