Lecture 12/13 Flashcards
What are the 4 major sites of absorption for nutrients in the GI tract?
- Macro nutrients such as carbohydrates, proteins, and lipids are absorbed in the small intestine, with highest absorption in the duodenum and jejenum
- Iron and folate are absorbed in the duodenum, and calcium is absorbed throughout the small intestine
- Net absorption of bile acids mainly occurs in the distal jejunum, ileum, and ascending colon
- Cobalamin (vitamin B12) is absorbed in the ileum
Slide 4
What are the 6 enzymes used in digestion of carbohydrates?
- Amylase- reduce polysaccharides for oligosaccharides and brush border enzymes complete the digestion into monosaccharides
- Lactase- digests lactose
- Maltase, sucrase, isomaltase- xan digest α1-4 linkages of maltose, maltotriose, and α-limit dextrins
- Maltase- digests straight chain oligosaccharides 9 monomers long
- Sucrase- digests sucrose
- Isomaltase- only enzyme to digest α1-6 linkages and α-limit dextrins
Slide 6
What are the 3 channels that mediate carbohydrate absorption?
- Sodium/glucose cotransporter (SGLT1) mediates absorption of glucose & galactose on the apical surface
- Glut5 mediated absorption of fructose on the apical surface
- GLUT2 mediated the transport of monosaccharides to the bloodstream on the basolateral side
Slide 6
What is lactose intolerance?
Caused by lactose deficiency
Occurs naturally after weaning in many mammals, including some humans
When occurs, consumption of lactose (milk) will commonly lead to diarrhea and increased flatulence
Hydrogen gas can be observed in the breath of lactase deficient person who recently drank milk
Treatment- eliminate lactose from diet
Slide 7
What is glucose-galactose malabsorption?
Caused by SGLT1 deficiency
Can lead to diarrhea whenever they consume sugar or lactose
Treatment- eliminate lactose, glucose & galactose from the diet and consume only fructose as a source of carbohydrate
Slide 7
How are proteins digested?
2 ways
- Gastric (pepsin) and pancreatic proteases (trypsin) reduce proteins into oligopeptides, and in some cases into amino acids
- Brush border proteases have a high affinity for 3-8 amino acid length oligopeptides and digest these into smaller peptides and amino acids
Slide 8
How are proteins absorbed?
3 ways
- Amino acids in the intestinal lumen are absorbed by sodium/amino acid contransporter, 7 types of transporters that sever different types of amino acids
- Tetrapeptide, tripeptides, and dipeptides are absorbed by a H/oligopeptides contransporter james PepT1
- Transport of amino acids at the basolateral surface from the cytoplasm into the bloodstream is mediated by at least 3 different types of sodium in dependant transporters
Slide 8
Slide 10
What is the kinetic advantage of absorbing oligopeptides, as opposed to single amino acids?
It is thought that the reason an oligopeptide absorption mechanism exists is for a higher efficiency in absorption
Evidence for higher efficiency is shown
Slide 9
What is hartnup disease?
What is cystinuria?
What is lysinuric protein intolerance?
Hartnup disease- absorption of neutral amino acids is defective, can result in mental disorders
Cystinuria- absorption of cationic amino acids is defective, can result in kidney stones
Lysinuric protein intolerance- impaired basolateral transport of cationic amino acids
Slide 11
How are lipids digested?
Dietary lipid is an essential component of the human diet
This is the only source of polyunsaturated fats
Certain vitamins can only be absorbed in fats
Majority of ingested lipid is triacylglycerols with varying degrees of saturated and unsaturated fatty acid composition, small but significant portion will be phospholipids, cholesterol, and cholesterol ethers
Slide 12-13
How are lipids absorbed?
Lipolytic products must still cross the mucous later lining the intestine and the apical membrane before they can be absorbed by enterocytes
Mixed micelles are thought to be able to diffuse through the mucous layer and interact with the apical surface of cells
Fatty acids are thought to be neutral at the microvilli surface, and that this helps with cellular uptake
If bile acids are absorbed they are secreted back into the lumen by the ABCG1/ABCG8 transporter to contribute furthermore to mixed micelles formation
Slide 14-15
What is the digestion/absorption of folate?
Dietary folate is essential to produce tetrahydrofolate, which is involved in the synthesis of DNA
Folate deficiency can lead to metablastic anemia, and defects in a developing fetus
Foods such as spinach, beans, and liver supply is with this nutrient in the form of folate polyglutamate
To absorb folate, PteGlu7 is deconjugated by a brush border enzyme (folate conjugase)
Once inside the cell, it is converted to the active form of tetrahydrofolate, which is transported into the bloodstream through an unknown mechanism
Slide 16
What is the digestion/absorption of cobalamin?
Dietary cobalamin can be found in meat, fish, shellfish, eggs
Critical for DNA synthesis
It is bound to haptocorrin in stomach
Must bind to intrinsic factor for absorption, this occurs in intestine because proteases digest haptocorrin
Intrinsic factor carries cobalamin into enterocytes through receptor mediated uptake
Then transferred to transcobalamin which carries it to bloodstream
Slide 17
How is calcium absorbed?
Numerous cellular processes and the formation of healthy bone depends on calcium
The absorption of calcium can occur through paracellular passive uptake in the small intestine
Can also occur through a transcellular active transport mechanism (regulated by vitamin D)
Vitamin D stimulates the uptake of calcium via the TRPV6 calcium channel down its electrochemical gradient, as well as the production of calbindin, and production of a calcium pump and sodium/calcium exchanger
Slide 18
What is the absorption of iron?
Iron Is also critical for numerous cellular processes and must be carefully balanced as too much causes hemochromatosis and too little causes anemia
Iron absorption comes from heme and non heme sources
Slide 19
