Cadmium and copper metallothioneins in the American lobster, Homarus americanus

Abstract

Lobsters were fed cadmium-rich oysters for 28 days, and the induction of cadmium metallothionein and its relation to concentrations of cadmium, copper, and zinc in the digestive gland and gills was determined. A portion of the tissues also was retained for determining the cytosolic distribution of these metals by gel filtration and ion-exchange chromatography. The digestive gland contained a majority of the cadmium, copper, and zinc, and both cadmium and zinc were actively accumulated from the oysters. Gel chromatography of the digestive gland cytosol showed that initially cadmium and zinc were bound to macromolecules with molecular weights of greater than 70,000, approximately 45,000 and less than 5000, and for copper greater than 70,000, 10,000-7,000, and less than 5000. Therefore, only copper was bound to a protein with a molecular weight in the range of metallothionein (i.e., 10,000-7,000). However, after feeding on cadmium-laden oysters for 28 days, both cadmium and copper were bound to the metallothionein-like protein. Further purification of the cadmium/copper protein by ion-exchange chromatography showed that a large portion of the copper and all of the cadmium did not bind to DEAE-Sephacel. The induction of cadmium metallothionein in the digestive gland is correlated with tissue cadmium concentration. There is, however, a tissue threshold concentration of cadmium of 80 to 100 micrograms Cd/g wet weight required for induction. Coincident with the induction of the cadmium metallothionein was a cytosolic redistribution of copper. The distribution of zinc was not affected.