Studies on the Distribution and Chemical Form of Cadmium in the Liver of Whelk, Buccinum tenuissimum

Abstract

In order to clarify the chemical form and role of cadmium in tissues which contain high levels of the metal, the present study was carried out on the subcellular distribution of cadmium and characterization of cadmium-containing protein fraction from watersoluble fraction in the liver of whelk, Buccinum tenuissimum.The concentration of cadmium accumulated by whelk liver was found often more than 1mM and about 30% of which distributed to the 105, 000×g supernatant, about 60% to the nuclear and cellular debris fractions, and residual small amount of the element to the mitochondrial and microsomal fractions.When the liver homogenate was treated with high ionic strength such as 2.0M NaCl and 10mM Na2-EDTA, cadmium content of the soluble fraction increased in both cases by 35% compared with control, which appeared to be explained by ionic adsorption.Cadmium in the soluble fraction did not markedly increase by boiling the liver homogenate for 25min, and also by digestion with proteases; pepsin and trypsin. On the other hand, cadmium in the soluble fraction under the acidic pH, whereat concentration of the metal increased 4-fold compared with that extracted with 0.25M sucrose, was found to be inorganic free state by gel filtration, but under the alkaline pH, cadmium in the soluble fraction almost was found to be bound to proteins.By means of gel filtration over Sephadex G-75, cadmium in the soluble fraction was found to be principally bound to fraction F-I of >75, 000 molecular weight and fraction F-III of <1, 500 molecular weight and a significant amount of cadmium was also associated with fraction F-II of 9, 000 to 13, 000 molecular weight. Under the presence of 2-mercaptoethanol, cadmium in the soluble fraction was almost associated with fraction F-II.Cadmium-binding protein F-I and F-II equilibrated with free cadmium ion, and the equilibrium was influenced with pH and 2-mercaptoethanol. At the acidic pH under pH 5.0, protein bound cadmium decreased sharply, but over pH 7.0, cadmium-protein complex was very stable, and therefore it was difficult to remove completely cadmium from F-I or F-II by dialysis with 1mM Na2-EDTA.These evidences show that cadmium in the liver of whelk are bound firmly to be the nucleus and protein in the soluble fraction.