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Protein and lipid digestion

2 Pages

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Physiology 3120
Tom Stavraky

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Human Physiology Friday, April 9, 2010 “Gastro IX” Protein digestion & absorption • Digestion begins in the stomach through the actions of pepsin, which is converted from pepsinogen by the release of acid, which is in turn stimulated by ACh. Pepsin takes care of 15% of the proteolysis, and the rest takes place due to pancreatic proteases. The predominant protease is trypsin, and is release as the inactive precursor trypsinogen for the same reason that pepsin is released as an inactive precursor; the important thing about these proteases is that they are secreted from the pancreas in response to secretin, CCK, or ACh stimulation. The inactive enzymes are activated extracellular-ly, but the mechanism is not by acid because bicarbonate is being released from the pancreatic extralobar duct cells; they are activated by an enzyme in the brush-border enzyme in the duodenum, and this enzyme is called enterokinase (activates MOST proteases). Once trypsin is formed, it can auto-catalyze trypsinogen to make more trypsin, so enterokinase is only required to get the ball rolling. • After the proteases have done their thing, there is a mixture of [mostly] amino acids, and di/tripeptides in the duodenum, so the proteases are quite efficient. Unlike carbohydrates, you can absorb proteins in the form of individual amino acids, di- or tri-peptides. For individual amino acids, there is an apical transporter that is identical to the Na/amino acid transporter just like the Na/glucose transporter except the stoiciometric relationship is 1:1 instead of 2:1. On the basolateral surface, Na is removed by the Na ATPase, and the amino acids are moved out via channels or a facilitated diffusion carrier. The four carriers respond to different types of amino acids (acidic, basic, neutral, imino acids); these carriers are less specific because 4-7 acids can bind to any carriers. There is also a single carrier (with no specificity) that takes every di- and tri-peptide; it probably works the same way as the AA carrier. Lipid digestion & absorption • Digestion  Predominant type of fat in diet is a triglyceride (glycerol backbone + 3 FAs). The major organ for lipid digestion is the pancreas (just like it was for proteins and carbs); it secretes enzymes called pancreatic lipases into the proximal small intestine. Unlike the proteases, the lipases DO st rd NOT have to be activated. They act by chopping off the FA at the 1 and 3 carbon of TG, leaving 2 free fatty acids (FFAs) and 2-monoglyceride (2-MG).  The problem with this mechanism
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