50: Intestine II: Nutrient Absorption Flashcards
- Explain the mechanism of absorption of amino acids, stereospecificity, concentrative uptake, and saturation kinetics.
- Explain the mechanism of absorption of short peptides.
Absorption of amino acids:
Proteins are digested by pancreatic endo-peptidases (trypsin and chymotrypsin) and by exopeptidases (carboxypeptidases), and by several other proteases.
Enterokinase is the trigger for intestinal protein digestion; enterokinase converts trypsinogen released by the pancreas into trypsin.
Essentially all proteolytic enzymes are secreted in an inactive form, which prevents autodigestion of the secreting organ. Enterokinase is physically attached to the brush border of the enterocytes which line the inner surface of the small intestine. Enterokinase activates trypsinogen to become trypsin in the gut lumen. The trypsin then catalyzes the formation of additional trypsin from trypsinogen as well as several other proenzymes (e.g., chymotrypsinogen, procarboxypeptidase, proelastase, and others). Pepsin is first secreted as pepsinogen, which has no proteolytic activity. However, as soon as it comes into contact with hydrochloric acid, and especially in contact with previously formed pepsin plus hydrochloride acid, it is activated to form pepsin.
Enterokinase converts Trypsinogen -> trypsin
Trypsin then activates endo- and exo-peptidases, yielding amino acids, dipeptides, and tripeptides.
Trypsinogen -> trypsin
Chymotrypsinogen -> chymotrypsin
Proelastase -> elastase
Procarboxypeptidase A -> carboxypeptidase A
Procarboxypeptidase B -> carboxypeptidase B
Normally only individual amino acids and some di- and tripeptides are absorbed via carrier-mediated transporters.
Action of luminal, brush border, and cytosolic peptidases. Pepsin from the stomach and the five pancreatic proteases hydrolyze proteins-both dietary and endogenous-to single amino acids, AA, or to oligopeptides, (AA)n. These reactions occur in the lumen of the stomach or small intestine. Various peptidases at the brush borders of enterocytes then progressively hydrolyze oligopeptides to amino acids. The amino acids are directly taken up by any of several transporters. The enterocyte directly absorbs some of the small oligopeptides through the action of the H+/oligopeptide cotransporter (PepT1). These small peptides are digested to amino acids by peptidases in the cytoplasm of the enterocyte. Several Na+-independent amino acid transporters move amino acids out of the cell across the basolateral membrane
Amino acid transport is stereospecific with L-isomers preferentially absorbed over D-isomers.
Transcellular concentrative uptake of L-amino acids can occur against* their concentration gradients by small intestine.
Most amino acids require an inward sodium concentration gradient for concentrative uptake (they are proportional).
The rate of transport of amino acids reaches a plateau value when the concentration of amino acid is varied. The kinetics of amino acid transport are analogous to Michaelis-Menten enzyme kinetics. Transport mediated by simple diffusion would be linear over the entire range of substrate concentrations. Saturation kinetics implies a limited number of transport sites in the membrane. Vmax is determined by the number of transporters & their turnover rate.
Fructose is the only monosaccharide that is not absorbed by Na+-dependent cotransport; it is transported by facilitated diffusion.
Amino acids are absorbed by Na+-dependent cotransport, but oligopeptides (larger peptide units) are not. Triglycerides are not absorbed without further digestion.
The products of lipid digestion, such as fatty acids, are absorbed by simple diffusion.
Absorption of oligopeptides:
1) The H+/oligopeptide cotransporter PepT1 moves dipeptides, tripeptides, and tetrapeptides into the enterocyte, across the apical membrane. Peptidases in the cytoplasm hydrolyze the oligopeptides into their constituent amino acids, which then exit across the basolateral membrane through one of three Na+-independent amino acid transporters.
2) If glycine is present in the lumen only as a free amino acid, then the enterocyte absorbs it only through apical amino acid transporters. However, if the same amount of glycine is present in the lumen in the form of the dipeptide glycylglycine, the rate of appearance of glycine in the blood is about twice as high. Thus, PepT1, which moves several amino acid monomers for each turnover of the transporter, is an effective mechanism for absorbing “amino acids.“
In Hartnup disease, the system B apical membrane amino acid transporter is defective. As a result, the absorption of neutral amino acids such as L-phenylanine is reduced. The defective transporter controls the absorption of neutral amino acids not only in the intestine but also in the kidneys. Excessive amounts of tryptophan are excreted in the urine. Tryptophan is a precursor for serotonin, melatonin, and niacin.
In cystinuria, the system B0+ apical membrane amino acid transporter is defective. As a result, the absorption of L-cystine and basic amino acids is reduced. In the kidney, the affected patient has inadequate reabsorption of cystine which forms kidney stones.
- Explain the mechanism of absorption of sugars.
- Ingested carbohydrate is 60% starch, 30% sucrose (table sugar) & 10% lactose (milk sugar).
- Salivary amylase (Ptyalin) begins the conversion of starch to sugars.
- Ptyalin has a pH optimum of 6.7 and is inactivated in the stomach.
• Most starch is broken down in the intestine by pancreatic amylase.
Brush-border enzymes: Lactase specifically breaks lactose into glucose (Glc) and galactose (Gal). Maltase, sucrase, and isomaltase are 3 additional enzymes that participate in breaking up oligosaccharides into monosaccharides.
The mechanism of transport for sugars—facilitated diffusion or sodium-coupled cotransport– is kinetically the same as for the transport of amino acids.
SGLT1 is the Na+-coupled transporter that mediates the uptake of glucose or galactose from the lumen of the small intestine into the enterocyte. GLUT5 mediates the facilitated diffusion of fructose into the enterocyte. Once the monosaccharides are inside the enterocyte, GLUT2 mediates their efflux across the basolateral membrane into the interstitial space
• Human GI tract has no cellulase for digesting cellulose or hemi-cellulose, thus accounting for undigested fiber in the diet. Fiber maintains the consistency of the stool.
Lactase Deficiency (Diagnosis is by lactose tolerance test or hydrogen breath test):
- Lack of lactase in the adult is cause of lactose intolerance.
- Lactose is osmotically active, resulting in osmotic diarrhea.
- Lactaid is milk containing lactase.
Lactose tolerance test.
1) Give oral load of lactose
2) Measure blood glucose over time
Without lactase, the lactose in milk remains un-hydrolyzed and unabsorbed. Bacteria in the gut adapt to the relative abundance of lactose and switch over to metabolizing lactose and produce copious amounts of gas by fermentation. The gas causes stomach cramps and bloating. Lactose raises the osmotic pressure of the colon contents, preventing the colon from reabsorbing water and hence causing a laxative effect.
Effects of lactase deficiency on levels of glucose in the plasma and H2 in the breath:
1) In an individual with normal lactase activity, blood glucose levels rise after the ingestion of either glucose or lactose. Thus, the small intestine can split the lactose into glucose and galactose and can absorb the two monosaccharides. At the same time, H2 in the breath is low.
2) In an adult with low lactase activity, the rise in blood levels is less pronounced after ingesting lactose. Because the rise is normal after ingesting glucose, we can conclude that the difference is the result of lactase activity. Conversely, the individual with lactase deficiency excretes large amounts of H2 into the breath. This H2 is the product of lactose catabolism by colonic bacteria
Glucose Galactose Malabsorption, GGM, is characterized by severe diarrhea and dehydration as early as the first day of life and can result in rapid death if lactose (milk sugar), sucrose (table sugar), glucose, and galactose are not removed from the diet.
- Explain the mechanism of digestion and absorption of fats and formation of chylomicrons.
Fats include Neutral fat (triglycerides), Phospholipids, Cholesterol, Fatty acids, Waxes of ingested plant cell walls.
The presence of fats in the duodenum causes the release of GIP (decreases gastric acid) & CCK. This slows gastric motility and emptying, stimulates pancreatic enzyme secretion, stimulates intestinal fluid secretion, stimulates gallbladder contraction, and relaxes sphincter of Oddi.
Cholecystokinin (CCK) is the most important hormone for digestion and absorption of dietary fat. In addition to causing contraction of the gallbladder, it inhibits gastric emptying. As a result, chyme moves more slowly from the stomach to the small intestine, thus allowing more time for fat digestion and absorption.
The digestive fats starts with lingual lipase. It continues with gastric lipase and food bearing lipase with acidic pH optimum. Digestion occurs mostly in the jejunum and is completed by the mid jejunum via pancreatic phospholipase A2, Cholesterol esterase, & Pancreatic lipase.
Lipase is water soluble like all luminal enzymes of digestion.nColipase anchors lipase to micelles and provides access to neutral triglycerides inside micelles.
Pancreatic lipase hydrolyzes only at 1 and 3 position of triglycerides:
Triglyceride -> 2-monoglyceride + FFA
Some 2-monoglycerides isomerize to 1- or 3-monoglycerides which are then converted by lipase to glycerol. Glycerol is absorbed by free diffusion, and some nonsterified glycerol passes across the basolateral membrane into the portal blood.
Amphipathic bile salts (and fatty acids and phospholipids) emulsify fat globules into smaller micelles.Micelles have negative surface charge, mutual charge repulsion stabilizes the emulsion. Large surface area of micelles makes digestion more efficient. Micelles are aggregates of amphipathic molecules including: Long chain fatty acids, Fat soluble vitamines (A, D, E, K), Cholesterol, Monoglycerides, Phospholipids, & Bile salts.
The breakdown of emulsion droplets to mixed micelles:
A, The core of the emulsion droplet contains TAGs, diacylglycerols, and cholesteryl esters. On the surface are fatty acids, MAGs, lysolecithins, and cholesterol. Adsorbed to the surface are pancreatic lipase and possibly bile salts. As the lipases hydrolyze the TAGs at the surface, the TAGs from the core replace them, thus causing the droplet to shrink.
B, A multilamellar liquid-crystalline layer of fatty acids, MAGs, lysolecithins, cholesterol, and bile salts builds up on the surface of the emulsion droplet and causes a small piece to bud off as a multilamellar vesicle.
C, The addition of more bile salts to the multilamellar vesicle thins out the lipid coating and converts the multilamellar vesicle to a unilamellar vesicle.
D, Further addition of bile salts leads to formation of a mixed micelle, in which hydrophobic lipid tails face inward and polar head groups face outward
The mixed micelles carry the major part of all the lipids that are absorbed by the intestinal microvilli. When the lipids of the mixed micelle have diffused into the enterocyte, emulsifying more hydrolysed lipids recycles the empty bile micelle. Neither bile salt micelles nor bile salt molecules diffuse into the enterocyte.
The fatty acids with a short chain (up to 12 C-atoms) are more hydrophilic than the rest. They can diffuse directly to the portal blood as fatty acids. The enterocyte does not re-esterify the medium-short chain fatty acids but instead transfers them directly into the portal blood.
For long-chain fatty acids, fatty acids transport proteins (FATP) such as FATP4 are present at the brush border. Once long-chain fatty acids enter the enterocyte, they bind to a cytosolic protein called fatty acid-binding protein. Fatty acid-binding protein minimizes reflux back into the intestinal lumen and ensures transfer of fatty acids up to the SMOOTH endoplasmic reticulum of the enterocytes for re- esterification. In addition, the enterocyte re-esteifies absorbed monoglycerides, lisophospholipids, and cholesterol and assembles the products with specific apoproteins into emulsion-like particles called chylomicrons.
For absorption of cholesterol, Niemann-Pick C1-like 1 (NPC1L1) serves as a transporter and is required for cholesterol uptake by the enterocyte. The cholesterol-lowering drug Ezetimibe works by inhibiting this transporter.
Bile salts are not absorbed in the jejunum, but rather in the ileum where they enter the enterohepatic circulation and travel back to the liver where they are re-secreted. The bile salts can be re-used several times in the course of a single meal. Bile salts are recirculated to the liver in the enterohepatic circulation via a Na+- bile acid cotransporter located in the ileum of the small intestine.
In the enterocyte the lipids are reformed to triglycerides, cholesterol, phospholipids etc. The reformed triglycerides, cholesterol, phospholipids, fatty acids, esters and fat-soluble vitamins reach the endoplasmic reticulum, where they are packed in another lipid-carrying particle: the chylomicron that also contains apoproteins.
The centre of the chylomicron is a cholesterol ester. Chylomicrons are packed into vesicles in the Golgi-system. These vesicles reach the basolateral membrane, and their contents pass through this membrane by exocytosis and are picked up by the lymphatic system. The lymph delivers the chylomicrons to the blood through the thoracic duct. Plasma is milky following a fatty meal because of the chylomicrons.
Re-esterification of digested lipids by the enterocyte and the formation and secretion of chylomicrons. The enterocyte takes up short- and medium-chain fatty acids and glycerol and passes them unchanged into the blood capillaries. The enterocyte also takes up long-chain fatty acids and 2-MAG and resynthesizes them into TAG in the SER. The enterocyte also processes cholesterol into cholesteryl esters and lysolecithin into lecithin.
See pg. 142.
Non objective important stuff
Soluble lumenal digestive enzymes:
- Salivary: Amylase (CHO), Lingual lipase (Fat)
Stomach:
Gastric chief cells secrete pepsinogen which forms pepsin at low pH. Pepsin is an endopeptidase for aromatic L-amino acids; has a pH optimum of 1- 3; inactive when denatured at pH > 5 in alkaline pancreatic juice in the intestine. Gastric lipase (Fat).
- Pancreas: Amylase (CHO); Endopeptidases (Trypsin, Chymotrypsin, Elastase). Exopeptidase (Carboxypeptidase); Lipase/Colipase–fats; Phospholipase A2–phospholipid; Cholesterol esterase
Intestinal ectoenzymes: membrane-bound in brush border; catalytic site faces lumen. Enterokinase–activates trypsin; Disaccharidases (Maltase, Sucrase, Lactase, Trehalase, Isomaltase); Peptidases (Aminooligopeptidase, Dipeptidase).
