Cadmium accumulation and metallothionein-like proteins in the sea starAsterias rubens

Sea stars were exposed for four months to cadmium at 50 g Cd/L. The cadmium content of stomachs, pyloric caeca, gonads and body wall was determined after 2, 4, 6, 8, and 16 weeks of exposure. The highest accumulation rates were found for body wall and pyloric caeca. The lowest accumulation rate was found for the gonads. In all tissues cadmium accumulation showed saturation kinetics. The body wall contained the largest part of the accumulated cadmium.Sephadex G-75 gel filtration experiments demonstrated the presence of metal-binding, metallothionein (MT)-like proteins with a molecular weight of 10.8 kD in cytosolic fractions of pyloric caeca from unexposed sea stars. The MT-like proteins contained mainly zinc and a small amount of copper. During exposure, the MT-like proteins accumulated cadmium at a relatively high rate, but a steady state was reached after 8 weeks, indicating thatin vivo, the Cd-binding capacity of these proteins was limited. When pyloric caeca cytosol from unexposed sea stars was incubated with cadmiumin vitro, the MT-like proteins could bind larger amounts of cadmium thanin vivo during the Cd exposure. After 16 weeks of Cd exposure, thein vitro Cd-binding capacity had hardly increased.Low-molecular-weight metal-binding proteins were absent in cytosolic fractions of the testes, ovaries and oocytes from Cd-exposed sea stars, even after four months of exposure. The cadmium accumulated in the gonads was bound exclusively to high-molecular-weight proteins, which accumulated cadmium at a relatively low rate, compared to the MT-like proteins in the pyloric caeca. The absence of MT-like proteins in the gonads may explain the low cadmium accumulation rates found for these organs, but as a consequence, the accumulated cadmium is not detoxified.