Intestinal metallothionein: effect of parenteral and enteral zinc exposure on tissue levels of mice on controlled zinc diets.

An investigation of changes in duodenal metallothionein (MT) levels following zinc exposure was conducted. Mice fed zinc-deficient (less than 1 ppm zinc) or zinc-supplemented (50 ppm) diets showed no increase in duodenal MT levels 24 hours after exposure to 17 mumol/kg zinc by gastric intubation. However, both MT induction and zinc incorporation were measured in the liver indicating transport of zinc via the intestinal mucosa. Only a small increase in [35S]cystine was observed to be incorporated into the MT fractions of duodenal homogenate obtained from zinc-exposed mice. Thus no statistically significant increase in duodenal MT was detected on single oral exposure of mice to zinc sulfate. Repetitive parenteral or enteral doses of zinc sulfate spaced 12 hours apart resulted in significant increases in duodenal MT. However, increases in MT by dietary zinc were not found until a 300 ppm zinc diet was used. No difference could be detected in duodenal MT levels of pair-fed animals fed zinc-deficient or 50 ppm zinc diet for 14 days. Similarly, no statistically significant difference in 65Zn uptake by in situ perfused duodenal sections could be detected in mice maintained on zinc-deficient or 100 ppm zinc diet for 1 month. These findings suggest that intestinal MT functions as a rapidly turned over zinc storage protein rather than a zinc regulatory protein.