Lipid Digestion And Absorption Flashcards
Major lipid components of the diet
How many grams per day?
90% triacylglycerol; rest is cholesterol, cholesterol esters (cholesterol with fatty acid associated), phospholipids, and FFAs
81 grams per day
- Name the two acid lipases
- What do they target
- Where do they function
- What do they release/digest
- Lingual lipase and gastric lipase
- TAG containing short and medium chain fatty acids
- In the stomach at pH 4-6
- Milk fat; especially important in neonates when milk is primary food source (gastric lipase specifically)
- Lingual lipase is secreted from?
2. Gastric lipase is secreted from?
- Glands at the back of the tongue (does not digest FAs in the mouth)
- Gastric mucosa
Emulsification requires two complementary actions:
Mechanical agitation and bile salts secretion
- Purpose of mechanical agitation?
2. Purpose of bile salt secretion?
- Increases lipid droplet SA
- stabilize particles as they become smaller, prevents them from coalescing (Made in liver, stored in gallbladder, secreted to small intestine); also helps keep things somewhat water soluble
~making components smaller increases SA
Pancreatic enzymes digest what 3 things
Process of digestion?
Dietary TAG, cholesteryl esters and phospholipids
Removal of specific FAs by breaking ester bonds attaching the fatty acids
Pancreatic lipase
- Target substrates
- Relative efficiency
- Products produced
- TAG digestion: Fatty acids at carbon 1 and 3 of glycerol backbone
- High catalytic efficiency
- 2 FFA, a 2-monoacylglycerol
Function of colipase
Binds to pancreatic lipase to anchor it at the lipid-aqueous interface to promote its activity when inhibitory bile salts are present
~bile salts inhibit pancreatic lipase; colipase will help prevent that
Cholesterol esterase
- Target substrates
- Products produced
- Secreted to
- Cholesteryl esters (mostly nonesterified cholesterol, but also esterified cholesterol)
- Cholesterol and FFAs (nonesterified)
- From pancreas to small intestine
~bile salts promote cholesterol esterase activity
Phospholipase A2
- Target substrates
- Important for
- Products produced
- Phospholipids
- Releasing arachnoidic acid from phospholipid bilayer
- Lysophospholipid and a FFA
Lysophospholipase
- Target substrates
- Products produced
- Lysophospholipid formed from phospholipase A2 activity
2. One fatty acid and one glycerylphosphoryl base
What do gut endocrine cells (in mucosa of small intestine) sense? (2 things)
What two hormones do these cells release?
Lipids and partially digested proteins, and the low pH of chyme entering the intestine
Cholecystokinin (CCK) and secretin
- When will CCK be secreted to the blood?
- What two things does this hormone cause?
- What does this hormone inhibit?
- In response to the presence of lipids and partially digested proteins
- Promotes pancreatic enzyme secretion and causes gallbladder to release bile
- Decreases gastric motility, reducing release rate of gastric contents to small intestine
~CCK stimulates digestion of fat and protein
- When will secretin be secreted into the blood?
2. Secretin promotes the release of?
- In response to low pH of chyme entering the intestine
- Bicarbonate rich solution from pancreas to small intestine, providing the appropriate pH for optimal pancreatic enzyme function (neutralizes the acid)
Lipid digestion in the jejunum generates 3 primary products:
What do these 3 products form?
FFAs, free cholesterol, and 2-monoacylglycerol
Micelles
Micelles:
- Structure
- Formed by
- How are they absorbed?
- Function
- Disk shaped clusters with hydrophobic interior and hydrophilic exterior
- FFAs, free cholesterol, 2 monoacylglycerol, vitamins ADEK, and bile salts
- Associate with the brush border membrane of enterocytes where they are absorbed
- Necessary for cholesterol to be absorbed (since cholesterol is hydrophobic); short/medium chain FAs do not need micelles)
What happens if you are not eating a lot of dietary fat?
You will have a problem making micelles, and will have a hard time absorbing fat soluble vitamins into your body
Lipids are processed where in the cell?
Smooth ER (site of lipid biosynthesis)
- What enzyme converts long chain fatty acids to their activated form
- Function of TAG synthase?
- Function of acyl CoA:cholesterol acyltransferase
- Thiokinase (fatty acyl-CoA synthetase)
- Converts 2-monoacylglycerol to TAG (by adding activated FAs one at a time)
- Esterifies cholesterol with a fatty acid
What is the fate of short and medium chain fatty acids?
They do not become activated; they are released into portal circulation (likely carried by serum albumin to the liver)
Since TAG and cholesteryl esters are hydrophobic, what has to happen in order for them to be secreted by intestinal enterocytes
They need to be packaged into stable lipid droplets called chylomicrons
Chylomicrons:
- Composition
- Where are they produced?
- Fate?
- TAG and cholesteryl esters surrounded by a single layer of phospholipids, cholesterol and apolipoprotein B-48
- Produced at ER of the enterocytes
- Exocytosed into the lacteals (lymph vessels that originate at the villi of small intestine)
What is chyle
Lymph which contains chylomicrons
Lipoprotein lipase
- Origin
- Localization
- Action
- Associates with lumen endothelial cells of capillary beds
- Synthesized and secreted primarily by muscle and adipose (but also heart, lung, kidney, and liver)
- Enzymatically degrades TAG in the circulating chylomicrons to FFAs and glycerol
Fate of glycerol
Taken up by the liver and used to produce glycerol-3-phosphate which can enter glycolysis or gluconeogenesis or be used to make TAG (storage)
- Lipid digestion and absorption problems can cause?
2. Causes of this (4)
- Increased excretion of essential dietary lipids in the feces
- Pancreatic insufficiency, cystic fibrosis, defective mucosal cells, or shortened bowel
Cystic fibrosis
- This disease affects
- How is that relevant to lipid digestion?
- How does it present in patients
- Treatment?
- Cl- ion channel called CFTR that is important for hydration (viscosity of mucus)
- Reduced function of CFTR in secretory ducts of pancreas results in thick mucus that blocks secretion of pancreatic enzymes to small intestine (pancreatic insufficiency)
- Delayed growth and energy deficiency (due to reduction in dietary calories and decreased fat soluble vitamin uptake)
- Dietary needs assessment; enzyme replacement and fat soluble vitamin supplements
