Amino acids are taken up by enterocytes via secondary active transport. Six major amino acid carriers in the small intestine have been identified; they transport related groups of amino acids. The amino acid transporters favor the L form over the D form. As in the uptake of glucose, the uptake of amino acids is dependent on a Na+ concentration gradient across the enterocyte brush border membrane.
The absorption of peptides by enterocytes was once thought to be less efficient than amino acid absorption. However, subsequent studies in humans clearly demonstrated that dipeptides and tripeptide uptake is significantly more efficient than the uptake of amino acids. Dipeptides and tripeptides use different transporters than those used by amino acids. The peptide transporter prefers dipeptides and tripeptides with either glycine or lysine residues. Furthermore, tetrapeptides and more complex peptides are only poorly transported by the peptide transporter. These peptides can be further broken down to dipeptides and tripeptides by the peptidases (exopeptidases) located on the brush border of the enterocytes. Dipeptides and tripep-tides are given to individuals suffering from malabsorption because they are absorbed more efficiently and are more palatable than free amino acids. Another advantage of peptides over amino acids is the smaller osmotic stress created as a result of delivering them.
In adults, a negligible amount of protein is absorbed as undigested protein. In some individuals, however, intact or partially digested proteins are absorbed, resulting in ana-phylactic or hypersensitivity reactions. The pulmonary and cardiovascular systems are the major organs involved in anaphylactic reactions. For the first few weeks after birth, the newborn's small intestine absorbs considerable amounts of intact proteins. This is possible because of low prote-olytic activity in the stomach, low pancreatic secretion of peptidases, and poor development of intracellular protein degradation by lysosomal proteases.
The absorption of immunoglobulins (predominantly IgG) plays an important role in the transmission of passive immunity from the mother's milk to the newborn in several animal species (e.g., ruminants and rodents). In humans, the absorption of intact immunoglobulins does not appear to be an important mode of transmission of antibodies for two reasons. First, passive immunity in humans is derived almost entirely from the intrauterine transport of maternal antibodies. Second, human colostrum, the thin, yellowish, milky fluid secreted by the mammary glands a few days before or after parturition, contains mainly IgA, which is poorly absorbed by the small intestine. The ability to absorb intact proteins is rapidly lost as the gut matures—a process called closure. Colostrum contains a factor that promotes the closure of the small intestine.
After dipeptides and tripeptides are taken up by the ente-rocytes, they are further broken down to amino acids by pep-tidases in the cytoplasm. The amino acids are transported in the portal blood. The small amount of protein that is taken up by the adult intestine is largely degraded by lysosomal proteases, although some proteins escape degradation.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.