Metallothioneins in marine mammals.

Metallothioneins (MTs) have been detected in livers and kidneys of 10 marin e mammals species (Pinnipeds and Odontocetes). Characterization of renal MT s of striped dolphin has shown that the protein has two isoforms (MT-I and MT-2) with a molecular weight estimated around 6800, MT concentrations also vary widely in marine mammals tissues (from 58 to 1200 mu g.g(-1) ww) unde rlying the numerous parameters involved: physiological status, pregnancy, a ge, diet. The participation of this protein in metal detoxification has bee n investigated since high levels of cadmium (Cd) and mercury (Hg) have been measured in livers and kidneys of marine mammals, It has been suggested th at those animals can mitigate at least in part, the toxic effects of Cd and Hg through binding to MTs. The percentage of the cytosolic Cd bound to MTs can reach almost 100%. On the contrary, the percentage of hepatic and rena l Hg bound to MT is very low (generally less than 10%) and this metal is ma inly associated with selenium (HgSe) under a detoxified form in the insolub le fraction of the tissues. MTs appear to play a minor role in the binding and detoxification of Hg by marine mammals. On the contrary, close and dyna mic interactions occur between Cd and MTs. Cytosolic MTs appear as a potent ial short term way of detoxification of Cd accumulated from diet. Long-term detoxification would imply a sequestration of the metal under a precipitat ed form (e.g. in lysosomes).