Accumulation and depletion of zinc in chick tissue metallothioneins

Two experiments were conducted with Hubbard broiler chicks on the metabolism of high levels of dietary zinc. In the first experiment, chicks were fed a basal diet or the basal diet plus 500, 1,000, 2,000 or 4,000 ppm zinc, and in the second experiment chicks were fed the basal diet or the basal diet plus 1,000, 2,000, 4,000, 8,000 or 16,000 ppm zinc, using zinc acetate. Zinc was found to be associated with 4 peaks when the cytosols from tissues (liver, kidney, pancreas and intestinal mucosa) were eluted through columns of G-75 Sephadex. Although the zinc content of all four peaks increased with increased zinc content of the diet, the greatest elevation occurred in metallothionein (MT). This protein was purified from all four tissues (liver, kidney, pancreas and intestinal mucosa) and the amino acid analysis revealed the presence of about 30% cysteine. Zinc accumulated to the greatest extent (microgram in MT/g tissue) in MT of the pancreas. The zinc in MT disappeared very rapidly when chicks which had been fed a diet containing high levels of zinc were fed a low zinc diet, indicating the extreme lability of this metal in this protein. The results suggest the involvement of MT in zinc homeostasis.