Characterisation and potential role of metallothioneins in marine mammals

In the framework of a North Sea and adjacent areas monitoring, metallothioneins (MTs) have been studied in livers and kidneys of 8 marine mammals species collected on the Belgian coast and in the Northeast Atlantic between 1993 and 2000: the harbour porpoise Phocoena phocoena , the harbour seal Phoca vitulina, the white beaked dolphin Lagenorhynchus albirostris, the white sided dolphin Lagenorhynchus acutus, the sperm whale Physeter macrocephalus, the fin whale Balaenoptera physalus, the common dolphin Delphinus delphis and the striped dolphin Stenella coeruleoalba. The participation of this protein in metal detoxification has been investigated since high levels of cadmium (Cd) and mercury (Hg) have been measured in livers and kidneys of those marine mammals. It appears that MT concentrations vary widely in marine mammals tissues (from 50 to more than 2000 µg.g–1 dw) underlying the numerous parameters involved: species, tissue, physiological status, pregnancy, age, diet and metal level. The percentage of the cytosolic Cd bound to MTs can reach almost 100%. On the contrary, the percentage of hepatic and renal Hg bound to MT is very low (generally less than 10 %) and this metal is mainly associated with selenium (HgSe) under a detoxified form in the insoluble fraction of the tissues.

Characterisation of renal MTs of white-sided dolphin has been performed: the protein has two isoforms (MT-1 and MT-2) with a molecular weight estimated around 6800. MT-1 can bind Cu, Zn, Hg and Cd while MT-2 only binds Zn, Hg and Cd suggesting different metabolic functions for the two isoforms. MT-2 was also more abundant than MT-1.

To conclude, MTs appear to play a minor role in the binding and detoxification of Hg by marine mammals. On the contrary, close and dynamic interactions occur between Cd and MTs. White-sided dolphin metallothioneins has two isoforms likely characterised by different metabolic functions in relation with cellular homeostasis and cadmium detoxification.