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

Abstract

The subcellular distribution of the naturally occurring radionuclide "' PO was investigated in the Liver of the Atlantic mackerel Scomber scombrus. The majority of the 2'0Po 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 210Po was bound to ferritin and approximately 28% to metallothioneins. The affinity of 210Po for these proteins was confirmed by a similar binding of the artificial Z08Po isotope incubated in vitro with the cytosolic proteins. Two other proteins, likely selenium-and zinc-containing enzymes, may also bind smaller amounts of 210Po, about 8 % each. The extensive binding of 'lOPo to ferritin and metallothioneins is not accompanied by a similar strong binding of 21uPb, the radioactive grandparent of "OPo, which explains the generally very high 210Po:210Pb ratio observed in fish tissues and, in particular, fish liver KEY WORDS: Polonium. Fish. Metallothionein. Ferritin INTRODUCTIONPolonium-210, a high energy D-particle emitter in the uranium decay chain, is the major source of internal radiation dose to marine organisms (Cherry & Heyraud 1982, Carvalho 1988. This naturally occurring radionuclide is also responsible for a significant proportion of the radiation exposure of humans to background radiation, in particular, through consumption of seafood (Parfenov 1974, Hunt & Allington 1993, Yamamoto et al. 1994, Carvalho 1995a. 210Po is naturally occurring and ubiquitous in our environment, but, due to its radiotoxic properties, special attention has been paid to the possible enhancement of 210Po environmental concentrations which may occur from the inputs of effluents of phosphate-ore processing industries (Germain et al. 1992, Pennders et al. 1992, Carvalho 1995b, Swift et al. 1995.It is well known that 210Po is accumulated particularly by marine organisms and transferred through 'E-mail: jpdurandmnat svt sciences.univ-nantes.fr marine food chains (Heyraud & Cherry 1979, Carvalho 19881, with ingestion being the major route of entry (Carvalho & Fowler 1994). The tissues of some organisms, such as the hepatopancreas of crustaceans and fish liver may contain particularly high concentrations of 210Po. Furthermore, they display 210Po:2'0Pb ratios of 10' to 103 (Carvalho 1988), which correspond to an enhanced ratio in the food chain compared to the 210Po:210Pb ratio in sea water, which is around unity (Heyraud & Cherry 1979). The reason for an elevated 210Po:210Pb ratio in marine food chains is not known. Furthermore, several studies have suggested that the subcellular localization of 2'0Po, in particular in the cell nucleus, may have important implications with respect to the genetic radiotoxic effects of the 210Po U particles (Howell et al. 1990).Some attempts have been made previously to establish the subcellular localization of 2'0Po in tissues of marine invertebrates and small mammals. The aim of this study was to investigate the subcellular distribution of 210Po a...

Two ferritin subunits from disk abalone (Haliotis discus discus): Cloning, characterization and expression analysis

“…Many reports have been published with respect to the tissue-specific function of ferritin, such as the mantle ferritin involved in shell formation in pearl oyster [18]. It has also been suggested that ferritin has other functions, including roles in invertebrate immune responses [35], protection against free radicals [36], cell proliferation [37], and high level accumulation of natural radionuclide 210 Po [17,38,39]. Therefore, having two different ferritin subunits, as well as high levels of expression in certain important organs, gives us a high degree of confidence stating that abalone ferritin has other, unknown functions which warrant future investigation.…”

Ferritin and hemocyanin: 210Po molecular traps in marine fish, oyster and lobster

“…This distribution may be different in crustaceans; other authors have shown an important fixation of 210 Po in the microsomal fraction in the lobster hepatopancreas (Heyraud et al 1987). On the other hand, in mackerel liver at the cytosolic level, the distribution of 210 Po is approximately equivalent between metallothioneins and ferritin (28 and 30% of fixation, respectively), so obviously ferritin is not the only binding site at the cellular level (Durand et al 1999).…”

“…metallothioneins and ferritin (Durand et al 1999). The principal aim of this study was to compare the ability of ferritin from different marine organisms to accumulate 210 Po.…”

Experimental studies on the accumulation of polonium‐210 by marine phytoplankton

“…210 Po is quickly scavenged out of the surface layer, leading to estimated residence times in surface waters of approximately 0.6 yr (Bacon et al 1976;Cochran et al 1983), a value that is about half its residence time in deeper waters (Kadko et al 1987). 210 Po concentrates appreciably in marine animals (Heyraud and Cherry 1979;Bulman et al 1995;Dahlgaard 1996;Durand et al 1999), providing the largest radiation doses to aquatic organisms. The concentration of 210 Po is Ն148 mBq g Ϫ1 in phytoplankton (Cherry 1964), 3,145 mBq g Ϫ1 in shrimp hepatopancreas (Cherry and Heyraud 1981), Յ700 and Յ1,026 mBq g Ϫ1 in mussel soft tissues (Germain et al 1995) and hepatopancreas (Stepnowski and Skwarzec 2000), respectively, and Յ262 mBq g Ϫ1 in fish (pyloric caecum, Clulow et al 1998).…”