ABSTRACT
References ...........................................................................................................76
Heavy metals such as zinc (Zn) and copper (Cu) are ubiquitous components of the biosphere and are essential elements that participate in a variety of enzymatic reactions (1). Essential heavy metals act in catalytic sites of metalloenzymes and enable specific folding of many other metalloproteins (2). Cadmium (Cd), lead (Pb), and other nonessential metals (as well as the essential metal ions in excess) can exert toxic effects due to their high affinity for amino acid residues involved in important biological functions and their potential to displace essential metals required for biological processes (2). Living organisms have therefore evolved mechanisms that control and respond to the uptake and accumulation of both essential and nonessential heavy metals, including the chelation and sequestering of heavy metals by intracellular ligands (2,3). Some of the best characterized
heavy-metal-binding ligands include metallothioneins (MT), which were first described by Margoshes and Vallee (4) in equine kidney. Since then, several MTs and MT-like proteins, often described as heavy-metal-inducible, heavy-metalbinding, cysteine-rich polypeptides, as well as other heavy-metal-binding proteins have been described and characterized from a number of different organisms (3).
