Metal Ions in Biological Systems

Abstract

The high specificity of metal ions in biology has evolved during a period of more than 3.5 billion years. This appears to be the result of environmental factors, such as the concentration of metal ions in sea water and the successively increasing oxidation power of the atmosphere; but also, in particular, this high specificity is a result of a series of random mutations that are expressed in the form of specific metal ion binding proteins. A certain metal ion might not bind with its specific protein binding site unless discrimination systems are also available. One such system depends on the free amino acids present in all tissues in fairly high concentrations (2–10 mM); another discriminating system depends on the existence of both aqueous and lipid media in biology. The latter system can be correlated to the hard/soft character of metal ions. Toxic metal ions that are favoured by the discriminating systems may become extremely harmful; one example is Cd.(II). In human biology there are also specific systems that immobilize and tend to eliminate toxic metal ions. For instance Cd(II) and Hg(II) induce biosynthesis of metallothioneine, and Cr(III)induces an immune response. Details regarding the discriminating systems, the fates of copper, zinc and cadmium ions under various conditions, and the characterization of a Cr(III)-antigen will be discussed.