Absorption of zinc by the rat ileum: effects of histidine and other low-molecular-weight ligands.

The role of certain amino acids, dipeptides and organic acids as ligands to facilitate the intestinal absorption of zinc was investigated by using an in vivo procedure on ileal segments of adult rats. Ligand:zinc ratios equal to or less than 3:1 were optimal for amino acids and dipeptides such as L-glutamate, glycine, L-histidine, L-tryptophan and glycylglycine. An excess of ligand reduced zinc absorption. At a 130:1 L-histidine:zinc ratio the absorption of zinc was less than one-fourth that obtained at a 3:1 ratio. Picolinate was a less effective ligand. The kinetics of the complex L-histidine:zinc at a 2:1 ratio and at pH 7.5 were determined in the absence and presence of a 20 mM excess of amino acid in a zinc concentration range between 0.038 mM and 6.00 mM. In both cases the Vmax was 2200 pmol/(minute . cm), but the Kt increased from 0.54 mM to 1.46 mM in the presence of the L-histidine excess. These data suggest a competitive inhibition of the L-histidine:zinc complex by the amino acid. Such an effect was dependent on the stereoisomerism of histidine, since the unnatural D-isomer was far less effective than the natural L-isomer in facilitating zinc absorption. The presence of an intact protein (bovine serum albumin) sharply decreased the ileal absorption of the L-histidine:zinc complex.