Characterization of binding between the rat small intestinal brush-border membrane and dietary proteins in the sensory mechanism of luminal dietary proteins

Dietary proteins are recognized by the gastrointestinal tract to display physiological functions, however, the sensory mechanism of the intestinal mucosa is not known. We examined binding properties between the rat small intestinal brush-border membrane (BBM) and proteins by using a surface plasmon resonance biosensor. BBM and solubilized BBM prepared from the rat jejunum bound to casein immobilized on the sensor surface, but not to bovine serum albumin. The ileal BBM showed less binding to casein than the jejunal BBM. Solubilized BBM binding to immobilized ol-casein was slightly inhibited by aminopeptidase inhibitors, but still more inhibited by addition of casein with the inhibitors. Guanidinated casein inhibited the solubilized BBM binding to ol-casein more strongly than casein (casein sodium and ol-casein) inhibited. Trypsinization of solubilized BBM abolished its binding activity to or-casein. These results indicate that some membrane protein, but not aminopeptidases, contained in BBM interacts with dietary proteins, and that guanidinated casein has a higher affinity for BBM than intact casein. These binding intensities for proteins were closely correlated to physiological responsiveness, and are possibly involved in a sensory system for dietary protein in the intestine.