Absorption of Amino Acids and Peptides
Dietary proteins are, with very few exceptions, not absorbed. Rather, they must be digested into amino acids or di- and tripeptides first. In previous sections, we've seen two sources secrete proteolytic enzymes into the lumen of the digestive tube:
- the stomach secretes pepsinogen, which is converted to the active protease pepsin by the action of acid.
- the pancreas secretes a group of potent proteases, chief among them trypsin, chymotrypsin and carboxypeptidases.
Through the action of these gastric and pancreatic proteases, dietary proteins are hydrolyzed within the lumen of the small intestine predominantly into medium and small peptides (oligopeptides).
The brush border of the small intestine is equipped with a family of peptidases. Like lactase and maltase, these peptidases are integral membrane proteins rather than soluble enzymes. They function to further the hydrolysis of lumenal peptides, converting them to free amino acids and very small peptides. These endproducts of digestion, formed on the surface of the enterocyte, are ready for absorption.
Absorption of Amino Acids
The mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides. The lumenal plasma membrane of the absorptive cell bears at least four sodium-dependent amino acid transporters - one each for acidic, basic, neutral and amino acids. These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form.
Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient of sodium across the epithelium. Further, absorption of amino acids, like that of monosaccharides, contributes to generating the osmotic gradient that drives water absorption.
The basolateral membrane of the enterocyte contains additional transporters which export amino acids from the cell into blood. These are not dependent on sodium gradients.
Absorption of Peptides
There is virtually no absorption of peptides longer than four amino acids. However, there is abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed into the small intestinal epithelial cell by cotransport with H+ ions via a transporter called PepT1.
Once inside the enterocyte, the vast bulk of absorbed di- and tripeptides are digested into amino acids by cytoplasmic peptidases and exported from the cell into blood. Only a very small number of these small peptides enter blood intact.
Absorption of Intact Proteins
As emphasized, absorption of intact proteins occurs only in a few circumstances. In the first place, very few proteins get through the gauntlet of soluble and membrane-bound proteases intact. Second, "normal" enterocytes do not have transporters to carry proteins across the plasma membrane and they certainly cannot permeate tight junctions.
One important exception to these general statements is that for a very few days after birth, neonates have the ability to absorb intact proteins. This ability, which is rapidly lost, is of immense importance because it allows the newborn animal to acquire passive immunity by absorbing immunoglobulins in colostral milk.
In constrast to humans and rodents, there is no significant transfer of antibodies across the placenta in many animals (cattle, sheep, horses and pigs to name a few), and the young are born without circulating antibodies. If fed colostrum during the first day or so after birth, they absorb large quantities of immunoglobulins and acquire a temporary immune system that provides protection until they generate their own immune responses.
The small intestine rapidly loses the capacity to absorb intact proteins - a process called closure - and consequently, animals that do not receive colostrum within the first few days after birth will likely die due to opportunistic infections.
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