Characterization of gastrin binding to colonic mucosal membranes of guinea pigs

Gastrin has significant growth and metabolic effects on colonic mucosal cells. It is, however, not known if gastrin receptors are present on colonic mucosal cells that may directly mediate the reported biological effects of gastrin. In the present studies, the presence of specific gastrin binding sites on colonic mucosal membranes was investigated and the binding sites were further characterized. Crude membranes from colonic mucosa of guinea pigs were analyzed for specific binding to gastrin by our published procedures. A significant number (14.7 ± 1.8 fmoles/mg protein) of high affinity gastrin binding sites (K_d = 0.49 = 0.05 mM) were measured, that were specific for binding gastrin/CCK related peptides and demonstrated negligible binding affinity for all other unrelated peptides examined. In addition a large number of low-affinity (K_d = ∼ μM) binding sites were present. In order to further characterize the molecular size of gastrin binding proteins, we used the chemical cross-linking methods, and observed at least four bands of gastrin binding proteins (GBPs) (∼ 33, 45, 80 and 250 KDa), both under reducing and non-reducing conditions, indicating that these proteins were not sub-units of forms linked by disulfide bonds. Interestingly, majority of the specific gastrin binding sites (∼ 70%) were present on the 45 KDa protein, unlike other target cells of gastrin. The presence of N- and O-linked glycosylated moieties were indicated on the 45 KDa protein, based on enzymatic de-glycosylation studies. The relative binding affinity (RBA) of gastrin and a closely related peptide, cholecystokinin octapeptide (CCK), for GBPs on colonic mucosal membranes was measured in order to determine if GBPs were similar to the CCK-A or CCK-B binding proteins reported in literature. The RBA of gastrin and CCK for displacing the binding of gastrin to the 33, 45, 80 and 250 KDa GBPs on colonic mucosal membranes were calculated to be 39, 100, 78 and 70% (gastrin), and 5.4, 2.9, 3.9 and 2.0% (CCK), respectively, wherein the binding affinity of gastrin for the 45 KDa protein was arbitrarily taken as 100%. Based on the RBA values, it appears more likely that the GBPs on colonic mucosal membranes are more akin to the unique GBPs described on colon cancer cells, and do not represent either the CCK-A or CCK-B binding sites. Based on the cross-linking studies we were not able to determine if the high- and low-affinity binding sites were differentially distributed on the different molecular forms of GBPs measured on the colonic mucosal membranes. The above studies thus indicate for the first time that specific gastrin binding proteins (receptors) are present on colonic mucosal membranes and that these receptor proteins may be directly mediating the observed effects of gastrin on colonic mucosal cells.