Tripeptides, intestinal uptake

Dalmasso, G., Charrier-Hisamuddin, L., Thu Nguyen, H.T., Yan, Y., Sitaraman, S. and Merlin, D. (2008) PepTl-mediated tripeptide KPV uptake reduces intestinal inflammation. Gastroenterology, 134 (1), 166-178. 

Protein typically is well tolerated as a caloric source in SBS patients. For those SBS patients on EN it is controversial what molecular form of the macronutrient maximizes protein absorption. In the past, EN often was initiated with elemental products that contained free amino acids as the protein source because the efficiency of protein uptake was perceived to be better. However, total protein absorption is faster and more complete with dipeptide and tripeptide formulations. It appears that the absorption of free amino acids by the enteral route is a saturable process, whereas the absorption of small peptides is not. These more complex protein sources also may stimulate intestinal adaptation. ... 

Carrier-mediated transport involves cotransport of the absorbable species with a proton. The required proton gradient is hypothesized to be maintained by a Na+-H+ exchanger. The lumen of the intestine is acidic relative to the epithelial cell cytosol. The low cytosolic sodium concentration, required to produce the transporter driving force, is maintained by the Na K ATPase in the basolateral membrane. The sodium/proton exchanger working in concert with the sodium/potassium ATPase, therefore, results in a transport mechanism for the uptake of di- and tripeptides into the intestinal wall (Ganapthy and Leibach, 1985). 

With the carrier transport pathway restricted almost exclusively to uptake of amino acids, dipeptides, and tripeptides, and passive diffusional uptake also limited mostly to smaller, and generally more lipophilic, species, the principal pathway for uptake of macromolec es in the intestine is by endocytosis. Similarly to passive absorption, endocytotic uptake is truly appreciable only in the immature intestine, decreasing significantly after gut closure (Pusztai, 1989). The capacity for uptake of macromolecules in the mature intestine, however, is still significant enough to allow antigen sampling for the development of mucosal immunity or permit appreciable absorption of various toxins. 

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