Association of metals (mercury, cadmium and zinc) with metallothionein-like proteins in storage organs of stranded dolphins from the Mediterranean sea (Southern Italy)

Selected toxic and essential metals (mercury, Hg; cadmium, Cd; and zinc, Zn) were determined in the liver and in the kidney collected from 13 dolphins (Stenella coeruleoalba and Tursiops truncatus) stranded along the southern coasts of Italy from 1991 to 1999. Liver samples were also analysed for their methyl mercury (MeHg) and selenium (Se) contents. For subcellular fractionation, liver and kidney samples were homogenized in Tris-HCl buffer; after centrifugation, the supernatant (cytosol) was separated from pellets (insoluble fraction), heated at 80degreesC for 10 min and centrifuged in order to separate the precipitate containing the thermolabile high molecular weight proteins. The cytosol heat-stable fraction, including metallothionein-like proteins (MTLPs), was then purified by gel filtration chromatography on Sephadex G-75 column. The three subcellular fractions collected (insoluble fraction, thermolabile fraction and purified heat-stable fraction) were analysed for their Hg, Cd and Zn contents. The analytical results confirm previous similar studies on toothed whales in showing that: (1) in the liver, as well as in the kidney, Hg was mainly found in the insoluble fraction, therefore, metallothioneins seem to have no role in the Hg detoxification; (2) in the liver, the molar ratio between Se and inorganic Hg was very close to 1; this suggests that the final compound of MeHg detoxification could be HgSe (tiemannite); (3) in almost all the samples, Cd and Zn were detected in the purified heat-stable fraction (including MTLPs). The mechanism of Cd detoxification and Zn homeostasis is also discussed.