Peptide binding to intestinal epithelium: distinct sites for insulin, EGF and VIP

Several peptides, including insulin, epidermal growth factor and vasoactive intestinal polypeptide bind to intestinal epithelial cells. However, it is unclear whether one binding site binds several peptides or whether separate sites exist for each peptide. These studies were designed to examine the specificity of peptide binding sites on intestinal epithelial cells. Peptide binding was measured directly with [125I]radiolabelled peptides to isolated enterocytes prepared from rabbit ileum. The characteristics of insulin and epidermal growth factor binding were similar. Both insulin and epidermal growth factor specific binding was saturable, directly correlated to cell concentration and temperature and pH dependent. The total number of insulin binding sites per cell was 4500, that for epidermal growth factor was 2280. Scatchard analysis for both peptides produced curvilinear plots. Dissociation of both peptides from the binding site was increased in the presence of their respective unlabelled peptide. However, insulin specific binding was not altered by epidermal growth factor, and epidermal growth factor specific binding was unaffected by insulin. Further, both insulin and epidermal growth factor failed to inhibit the specific binding of vasoactive intestinal polypeptide to ileal enterocytes, and vasoactive intestinal polypeptide did not inhibit insulin or epidermal growth factor specific binding. These studies demonstrate that insulin, epidermal growth factor and vasoactive intestinal polypeptide interact with three distinct membrane binding sites on the enterocyte.