beta-Casomorphins alter the intestinal accumulation of L-leucine

Everted sacs of the rat jejunum change the accumulation of [3H]leucine when beta-casomorphins (BCMs) or synthetic analogs, in a concentration range of 10(-8) mol/l, are coincubated with the amino acid. BCM5 (BCM fragment 1-5, Tyr-Pro-Phe-Pro-Gly) and [D-Ala2]-BCM5-NH2 (Tyr-D-Ala-Phe-Pro-Gly) increase, whereas [D-Pro4]-BCM5 (Tyr-Pro-Phe-D-Pro-Gly) decreases the leucine accumulation and [Arg8]-vasopressin has no effect. No effect of BCM5 could be observed on the accumulation of the space marker [14C]inulin. Specific binding sites for casomorphins were detected microautoradiographically, exclusively at the epithelial cell layer using [3H][D-Pro4]-BCM5 in competition studies as a model. HPLC analysis revealed that under the experimental conditions about 50% of the studied [D-Pro4]-BCM5 was enzymatically degraded and no intact peptide is accumulated within the samples of everted sacs. From the results we postulate a brush-border receptor contact of the BCMs which induces an alteration of the amino acid uptake. A contraluminal binding of the chemical signals is not likely, because there is no evidence for a transepithelial transport of intact BCMs. The observed effects of the BCMs demonstrate as yet unknown peptide-receptor interactions, probably at the brush-border membrane, with subsequent effects on the nutrient supply. Furthermore, the results support the general hypothesis of distinct peptide-receptor interactions in those types of epithelia in which the cells are connected by tight junctions.