Metabolic fate of earthworm cadmium-binding proteins in rats

Three different molecular weight cadmium-binding proteins isolated from the earthworm, Eisenia foetida, were injected intraperitoneally into young female rats. Cadmium bound to earthworm protein-I (estimated molecular weight, 63,000-70,000 daltons) and -II (estimated molecular weight about 7,000 daltons) was recovered from the kidney supernatants as metallothionein, which indicated that the proteins were degraded and that the cadmium liberated from the degraded proteins induced metallothionein biosynthesis in the kidneys. However, cadmium bound to earth-worm protein-III (estimated apparent molecular weight about 2,000 daltons) was recovered primarily from urine as a form bound to the protein. Compared with cadmium bound to albumin, metallothionein, and the low molecular weight cadmium-binding protein in Chlorella, the metabolic fates of cadmium bound to the three earthworm proteins were explained by the differences of molecular sizes and stability constants. Induced metallothioneins showed different chromatographic properties with time after injections on a Sephadex G-75 column and on a gel permeation column (TSK GEL SW 3,000).