Response to heavy metals of uni‐ and multicellular organisms: Homologies and analogies∗

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Abstract
The homologies and analogies of the biological systems involved in the effects of heavy metals are examined in a phylogenetic perspective. The different effects of essential and non essential metals on the metabolism of organisms are reported, as well as the metal accumulation at body and subcellular levels and their relationship with absorption and excretion. Both organs and tissues involved in these functions, are particularly considered and the accumulation of metals at subcellular level in paniculate or soluble fractions. The compartimentalization in cytoplasmic granules or membrane bound vesicles seems to be a very widespread phenomenon. The soluble fraction is involved in the primary mechanisms of tolerance and detoxification of metal ions, being characterized by the presence of specific chelating proteins. These are also involved in the excretion of metals, in both invertebrates and vertebrates, and prokaryotic and eukaryotic unicellular organisms. These proteins, generally named metallothioneins (MTs), are distributed throughout the different kingdoms. They have some characteristic features, but their differences, particularly in the amino acid sequence, permit us to subdivide them in three classes, according to the their current characterization. However, the presence of homologous MTs in many Phyla is noteworthy. Their composition is slightly modified from unicellular organisms to man. This conservatism is good evidence of effectiveness of such a structure in the metal detoxification system. In conclusion, the biological response to metals varies according to their essentiality and subsequently according to the regulation or non regulation of their content in organisms. Analogous or in many cases homologous structures and molecules are at the basis of the mechanisms involved in these responses.