Amino acid modulation of in vivo intestinal zinc absorption in freshwater rainbow trout

The composition of the intestinal lumen is likely to have considerable influence upon the absorption, and consequently the nutrition and/or toxicity, of ingested zinc in aquatic environments, where zinc is both a nutrient and a toxicant of importance. The effects of amino acids upon intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss) were studied using an in vivo perfusion technique. The presence of histidine, cysteine and taurine had distinct modifying actions upon quantitative and qualitative zinc absorption, compared to perfusion of zinc alone. Alterations in zinc transport were not correlated with changes in levels of free zinc ion. The chemical nature of the zinc-amino acid chelate, rather than the chelation itself, appeared to have the most important influence upon zinc absorption. L-histidine, despite a strong zinc-chelating effect, maintained quantitative zinc uptake at control (zinc alone) levels. This effect correlated with the formation of Zn(His)₂ species. D-histidine at a luminal concentration of 100 mmoll^-1 significantly enhanced subepithelial zinc accumulation, but reduced the fraction of zinc that was retained and absorbed by the fish. The possibility of a Zn(His)₂-mediated pathway for intestinal uptake is discussed. L-cysteine specifically stimulated the accumulation of zinc post-intestinally, an effect attributed to enhanced zinc accumulation in the blood. Taurine increased subepithelial zinc accumulation, but decreased the passage of zinc to post-intestinal compartments. Amino acids are proposed to have important roles in modifying intestinal zinc uptake with potential implications for environmental toxicity as well as aquaculture.