210Po binding to metallothioneins and ferritin in the liver of teleost marine fish

The subcellular distribution of the naturally occurring radionuclide Po-210 was investigated in the Liver of the Atlantic mackerel Scomber scombrus. The majority of the Po-210 was found in the cytosol of the liver cells. Fractionation of the cytosol proteins by high performance size-exclusion and ion-exchange chromatography indicated that about 30% of Po-210 was bound to ferritin and approximately 28 % to metallothioneins. The affinity of Po-210 for these proteins was confirmed by a similar binding of the artificial Po-208 isotope incubated in vitro with the cytosolic proteins. Two other proteins, likely selenium- and zinc-containing enzymes, may also bind smaller amounts of Po-210, about 8 % each. The extensive binding of Po-210 to ferritin and metallothioneins is not accompanied by a similar strong binding of Pb-210, the radioactive grandparent of Po-210, which explains the generally very high Po-210:Pb-210 ratio observed in fish tissues and, in particular, fish liver.