Cadmium accumulation and Cd-binding proteins in the bivalve Scapharca inaequivalvis

Abstract

In the arcid clam, Scapharca inaequivalvis, cadmium was accumulated in the kidneys, gills, viscera, mantle, foot, muscle and red blood cells. Maximum levels were reached in the kidney (725.70 μ/g dry weight) after 4 weeks exposure to 0.5 μg Cd/ml sea-water. The accumulation was higher during the warm rather than in the cold season and in animals exposed to 0.5 as compared with 0.05 μg Cd/ml sea-water. Concentrations of zinc and copper were unaffected by cadmium treatment, whereas iron levels changed during cadmium exposure. When molluscs were returned to natural sea-water, cadmium levels fell slowly. The metal was found in the cytosolic fraction, bound to a low molecular weight ligand, shown by gel filtration to be a Cd-protein with an apparent molecular weight of 10,000 Da, possibly a metallothionein. Ion exchange chromatography of this protein isolated from the viscera revealed the presence of at least two Cd-binding isoforms; the -SH: Cd ratio of the main isoform was 2.8, the UV spectra presented a strong absorption band at 250 nm.