Zinc intestinal absorption in rats: specificity of amino acids as ligands

The luminal phase of zinc intestinal absorption may be mediated by low-molecular-weight substances originated in digestive, metabolic or secretory processes. Amino acids are considered primary candidates for this role through the formation of complexes with zinc. However, structural characteristics that may be indispensable for this physiological function have not been explored in vivo. We investigated the comparative effectiveness of four amino acids and their respective chemically related homologues on the absorption of zinc by the jejunum, ileum and colon of the rat using a perfusion procedure. L-Tryptophan (Trp) allowed for significantly greater zinc absorption than tryptophol (Tpl) in all areas of the gut. L-Histidine (His) and imidazole (Imd) had similar effects in both the jejunum and the ileum. Imd allowed for much greater zinc absorption from the colon than did His (His, 388 +/- 31; Imd, 937 +/- 107 pmol/min X cm, P less than 0.001). Proline (Pro) was a more effective ligand than pyroglutamate (Pyr) in the ileum (Pro, 559 +/- 19; Pyr, 352 +/- 22 pmol/min X cm, P less than 0.001), but not in the jejunum or the colon. L-Cysteine was superior to N-acetyl-L-cysteine only in the ileum (508 +/- 45 vs. 348 +/- 25 pmol/min X cm, P less than 0.01). The greater zinc absorption achieved by amino acids than by non-amino acid homologues in the small intestine appeared to be due to the presence of both mediated and nonmediated transport mechanisms for amino acids but of only nonmediated zinc uptake for the homologues. In the colon, where amino acid absorption does not take place, high structural affinity for zinc, such as that exhibited by Imd, allowed for considerable absorption of the trace element.