P

Peptidases

Basolateral surface

Na+-dependent and -independent

Figure 4 Small intestinal protein digestion and absorption. (Adapted from Shulman RJ (1996) Intraluminal digestion and absorption in the small intestine. In: Gluckman PD and Hayman MA (eds.) Pediatrics and Perinatology: The Scientific Basis, 2nd edn. London: Arnold. Reproduced with permission from Arnold (UK).)

there is some stereospecificity for this transporter because the longer the length of the amino acid side chain on the peptides, the easier the absorption. The transporter system also has greater affinity for dipeptides than tripeptides, and the acidic and basic amino acid residues in dipeptides lower the affinity for the transport system compared with neutral amino acids. In general, the absorption of L-isomers of amino acids in dipeptides is preferred over the d forms. The peptide transport system is coupled to the proton pump system rather than the sodium gradient. The oligopeptide transporter (Pept-1) in the brush border membrane is the major mechanism for protein absorption in the human intestine and is primarily responsible for the transport of di- and tri-peptides. Several factors may determine the levels of Pept-1, such as insulin, which may stimulate membrane insertion of the oligopeptide transporter from a preformed cytoplasmic pool, and cholera toxin, which decreases the activity of Pept-1 through an increase in the intracellular concentration of cyclic AMP.

Once in the absorbing cell, the di- and tripeptides are further hydrolyzed to the constituent amino acids by the cytoplasmic peptidases before absorption. The only small peptides that are known to enter the portal blood directly are those from gelatin that contain proline and hydroxyproline, and those from certain meats containing carnosine and anserine. However, their relative proportion in comparison to amino acids is inconsequential.

Amino Acid Absorption

Although some diffusion of amino acids does occur, they are mostly absorbed by active transport. Unlike peptides, which are absorbed equally well in both proximal and distal small intestine, amino acids are absorbed more rapidly in the duodenum and jejunum. Also in contrast to the parsimonious peptide transport system, there are multiple transport mechanisms for various amino acids at both the luminal end and the basolateral membrane of the enterocyte (Table 3). At the luminal end, the transporters are mostly located at the villous enterocytes. The villous enterocytes utilize approximately 10% of the absorbed amino acids for their own protein production, whereas the crypt cells derive their amino acid supply from the portal circulation. Of the various amino acids, glutamine appears to have a major role in the nutrition and regeneration of enterocytes, and it is now recognized that in the human intestine the predominant mechanism for assimilation of glutamine dipeptides is absorption as intact dipeptide rather than hydrolysis.

Table 3 Major amino acid transport systems in the intestinal

epithelial cells

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