Ovine and bovine metallothioneins: accumulation and depletion of zinc in various tissues.

The accumulation and depletion of zinc in hepatic metallothionein (MT) and the accumulation of zinc with MT in various tissues were studied in the ovine and bovine species. It took 8 weeks for the accumulation of zinc to reach a plateau level of 270 microgram in MT per gram liver when a bull was fed a diet containing 2,000 ppm zinc. The half-lives of zinc in hepatic MT were found to be 24.1 and 22.6 days when a steer and a ewe, respectively, which had been fed the basal diet with 2,000 ppm zinc were changed to the basal diet without added zinc. Although zinc accumulated with all cytosolic zinc proteins, it accumulated to the greatest extent with MT, followed by a low molecular weight (MW) compound. Zinc accumulated with this low MW compound mostly in kidney and pancreas, and this accumulation was much greater in bovine than in ovine tissues. In both lambs and cattle, excess zinc accumulated with MT in liver, kidneys, pancreas and small and large intestinal epithelia, but not with MT in heart, testes, rumen papillae, abomasum mucosa or choroid plexus (cattle) when they were fed high dietary levels of zinc.