The functional significance of the presence of opioid peptides in enzymatic digestion of food proteins remains uncertain. The effect of natural beta-casomorphins (beta-CMs), beta-CM-5 and beta-CM-4-NH2 (morphiceptine), and the analogue beta-[D-Ala2,4,Tyr5]CM-5-NH2 were studied in isolated rabbit ileum mounted in Ussing chambers. All three peptides caused a naloxone-reversible reduction in short-circuit current (Isc) after addition at a concentration of 10(-4) M to the serosal side of the tissue. After addition to the mucosal side, only the analogue beta-[D-Ala2,4,Tyr5]CM-5-NH2 reduced Isc. Natural beta-CMs were degraded by the intestinal mucosa, and no intact transepithelial passage was detected for these peptides, whereas beta-[D-Ala2,4,Tyr5]CM-5-NH2 was demonstrated to cross the epithelium intact when added at a concentration of 10(-3) M in the mucosal reservoir (mucosal-to-serosal flux = 3.5 +/- 1.2 nmol.h-1.cm-2). These results show that both natural beta-CMs and the protected analogue have an opiate activity on intestinal electrolyte transport. Their action from the luminal side of the intestine seems to depend on the transfer of the intact peptides from the luminal to the blood side of the tissue where opiate receptors are located. This action is prevented by luminal hydrolysis of the natural peptides.

Intestinal transepithelial transport constitutes a major limiting step in the transfer of food protein antigens to the blood. This transport was studied in isolated rabbit ileum in Ussing chamber in vitro for the milk protein antigen beta-lactoglobulin (beta-Lg). The transepithelial passage of beta-Lg was measured by enzyme-linked immunosorbent assay (ELISA) and radiolabeled protein transfer and compared with that of the nonmetabolizable marker polyethylene glycol (PEG)-4000. When 1 mg/ml of beta-[14C]Lg or [3H]PEG was added to the mucosal side of the tissue, the total uptake, measured as the transfer of radiolabeled material across the ileum, was significantly higher for beta-Lg than for PEG (5.46 +/- 1.75 vs. 1.43 +/- 0.26 micrograms.h-1.cm-2). Measured by ELISA, 6-9% of the total amount of beta-Lg transported was absorbed in an intact antigenic form. This transport of intact beta-Lg was inhibited by the metabolic inhibitors 50 mM 2-deoxyglucose and 1 mM azide added simultaneously, was reduced by the microtubule assembly inhibitor 0.05 mM colchicine, and was enhanced by 20 mM ammonia, which inhibits lysosomal proteolytic activity. These results indicate that beta-Lg is efficiently absorbed by the intestinal mucosa of adult animals, partly in intact antigenic form and that beta-Lg transport is probably transcellular, as observed for other proteins. The finding that beta-Lg is absorbed in intact antigenic form agrees with other reports implying that beta-Lg is the main factor responsible for milk protein immunoreactivity and intolerance.

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