3.01 GI Nutrtion Flashcards
What substance does not need digestion and is directly absorbed into intestinal lumen?
Glucose
Where does the greatest absorption of carbs, proteins, and lipids in small intestine take place?
Duodenum
Where is Ca+, Fe, and folate actively absorbed?
Duodenum
Where are bile acids absorbed in small intestine?
Throughout entire BUT ACTIVELY only at ILEUM*
Where is Vit B12 absorbed ?
Ileum
What carbs are dircetly absorbed by the small intestine?
Glucose, fructose, galactose (monosaccharides)
What initially breakdown carbohydrates aka starch?
A-amylase (acinar cells @ salivary glands + pancreas)
How are the initial oligosaccharides of amylose and amylopectin (starch) broken down?
A- amylase via intralumenal hydrolysis
What digests oligosaccharides further into monosaccharides?
Brush border disaccharidases
Where are the brush border dissachridases?
Integral membrane proteins of intestinal epithelial cells (apical side)
What are the oligosaccharidases?
Lactase
Glucoamylase (aka Maltase)
Sucrose-isomaltase
Lactase substrates and products?
Lactose —> glucose + galactose
Glucoamylase substrate and products?
Maltose —> glucose
Sucrase-isomaltase substrates and products?
Sucrose — > glucose + fructose
Maltose —> glucose
Na+ coupled transporter that mediates uptake of glucose or galactose from lumen on apical membrane?
SGLT1
What kind of transport is SGLT1?
Secondary active transport
Channel that mediates diffusion of fructose into enterocyte?
GLUT5
Channel that mediates facilitated diffusion of glucose, fructose, or galactose across BL membrane into interstitial space —> circulation?
GLUT2
Where are oligosaccharidases generally located in the GI tract/
Small intestine (should not be at the large intestine)
Lactase deficiency impacts what?
Glucose in plasma and H2 in breath
What will you notice in adults with a lactase deficiency?
Glucose levels should be normal but you will notice a higher than normal [H2 ]in their breath
What/Where are the 3 ways/peptidases that breakdown protein?
Luminal peptidases
Brush border peptidases
Cytosolic peptidases
What must proteins be digested into before they are taken up by enterocytes?
Oligopeptides and AA
Where are luminal peptidases secreted and where so they work?
Secreted by stomach and pancrease
(Pepsin; trypsin, chemo trypsin, carboxypeptidases)
Broken down into stomach and small intestine
What do brush border peptidases hydrolyze at apical membrane of enterocytes?
Oligopeptides —> AA and di/tripeptides
Where are cytosolic peptodases found?
Inside enterocytes
Di/tripeptidases —> AA to blood
What are the 5 Inactivated proenzymes/zymogens secreted by the pancreas ? Endo vs. Exo?
(ENDO)
Trypsin
Chymotrypsin
Elastase
(EXO)
Carboxypeptidase A
Carboxypeptidase B
What hydrolyses internal peptide bonds in proteins to make Oligopeptides (chains <6)?
Endopeptidases
What cleaves single AA from Carboxyl end of Oligopeptides?
Exopeptidases
What exopeptidase prefers hydrophobic AA?
Carboxypeptidase A
What exopeptidase targets basic AA like Lys and Arg?
Carboxypeptidase B
Of the ENDOpeptidases which one produces basic Oligopeptides?
Trypsin (onto Carboxypeptidase B next!)
What converts the inactive pancreatic zymogens to its active form?
Enterokinases
What enzyme is essential for kick starting the breakdown of proteins by activating a cascade to activate other pro enzymes?
Trypsin activation cascade
Trypsinogen —> trypsin —> cascade)
What actives protein digestion in the stomach under acidic conditions?
Pepsin
What is activated in the small intestine and works at slightly more alkaline pH than teh stomach?
Pancreatic proteases (trypsinogen, chymotrypsinogen, procarboxypeptidase A & B)
Where are amino acids cleaved from Oligopeptides with carboxypeptidases?
C-terminus
What cleaves aromatic AA writhing peptide chains?
Chymotrypsin
What happens to protein digestion if patients have a total gastrectomies or pernicious anemia?
Protein digestion and absorption remain intact
But will have B12 deficiency
How are small Oligopeptides absorbed into enterocytes on the apical membrane?
H+/oligopeptide cotransporter (PepT1)
How do AA from cytoplasmic hydrolysis exit the enterocyte (via BL membrane) ?
Na+ independent AA transporter
How does AA enter the enterocyte from the apical side?
It needs a Na+ channel (dependent)
What are the lipase sub and products?
Triglyceride —> (2) FA + monoglyceride
What are the cholesterol ester hydrolase sub and products?
Cholesterol ester —> cholesterol + FA
What are the phospholipids A2 sub and products?
Phospholipid —> Lysolecithin + FA
What is the major form of dietary lipid?
TAG (triacylglycerols)
What is the TAG structure?
(3) FA chains Esterified to a glycerol backbone
What are other forms of dietary lipids aside form TAGs?
Phospholipids, cholesterol, free FA
What initiates lipid digestion?
Lingual lipase (not significant)
@ mouth sent to stomach
+
Gastric lipase (partial digestion)
@ G cells gastrin
What is gastric lipase resistant to in an acidic environment?
Pepsin
What is a required to breakdown LCFA (long chain fatty acids) when lingual lipase and gastric lipase can’t finish the job?
Emulsification by bile salts
@ small intestine
+
Pancreatic lipase
What are key hormones to regulate lipid digestion?
CCK and GIP
What is release by duodenal mucosa in responses to FA to help regulate gastric emptying?
GIP (gastric inhib polypeptide)
What is released in response to FA entering duodenum?
CCK (cholecystokinin)
What does CCK do?
Stimulate bile salt release and pancreatic enzyme secretion
What is the pancreatic lipase co-factor?
Colipase
What is the major enzyme needed for TAG digestion in the small intestine?
Pancreatic lipase
What is are the pancreatic lipase sub and hydrolyzed products?
TAG —> 2 MAGs + FFAs
(2 monoacylglycerols + free fatty acids)
What do bile salts do?
Emulsify fat droplets to increase surface area and form micelles that transport digestion products to brush border for absorption
How are bile salts recycled?
Enterohepatic circulation
How are lipid digestion components taken up by enterocytes?
Direct diffusion or specific transporters
What helps solubulize lipid digestion products allowing them to approach the intestinal epithelium?
Mixed Micelles
What are micelles made up of?
Bile salts, fatty acids, 2 MAGs, cholesterol, and fat soluble vitamins
What can diffuse directly across the intestinal epithelium and enter the portal circulation?
SCFA and MCFA (medium)
LCFAs, 2-MAGs, lysophopholipids, and cholesterol need what in order to enter enterocytes?
FATP (FA transport proteins)
What happens to LCFAs, 2-MAGs, and cholesterol once WITHIN enterocytes?
Re-esterification into TAG and Cholesterol esters
OR
Incorporated into chylomicrons
What do chylomicrons do once they are created in the enterocytes?
Exocytosis into lymphatic vessels
What does Vitamin B12 bind to in the stomach?
Haptocorrin
What needs to happen to B12-haptocorrin in order for it to enter the Ilium (absorption) ?
Bind to IF
What is B12 found in diet wise?
Animal products
Meat, fish, shellfish, eggs, milk
Who’s at risk for a dietary cobalamin deficiency?
Strict vegetarians
