Lipids 2: Lipid Digestion, Absorption and Transport Flashcards
What is the length of most FAs found in food?
Most abundant is the long chain FAs >12 carbons long
* short chain FAs of C2-C4 are not a major component
* medium chain FAs are also not a major component
What is the most common form of ingested FAs?
> 90% is in the form of TG
How much ingested fat in is excreted in feces?
Only 4%
* Highly saturated fat is the expection because it is harder to solubilize
What is a critical biological property for digestion and absorption of fats?
The solid-liquid phase change of fats
What are the three classes of dietary lipids?
- triacylglycerides
- phospholipids
- cholesterol
How are ingested lipids presented for digestion?
Presented as micelles or tiny fat droplets (containing all 3 lipid classes) via bile and then specific enzymes break it down more individually.
What is the major and minor enzymes for TG digestion?
- Pancreatic lipase (major)
- gastric lipase (minor)
- lingual lipase (minor)
What are the following properties for pancreatic lipase?
* dietary lipid it acts on
* preferred environment
* specificity
* site of action
* cofactors
- TGs
- preferred environment: pH 5-6
- specificity: sn-1 and sn-3 positions
- site of action: SI lumen oil and aqueous phase interface
- cofactors: colipase
What are the products of pancreatic lipase?
2-monoglycerols + free fatty acids
What are the following properties for gastric lipase?
* Dietary lipid it acts on
* preferred environment
* specificity
* site of action
- Dietary lipid it acts on: TGs
- preferred environment: acidic medium optimal pH 4
- specificity: medium chain FAs on TG therefore milk fat hydrolyzed efficiently in infacts; FAs at sn-3
- site of action: stomach, jejunum
What is the product after gastric lipase?
1,2-diacylglyceride + free fatty acids
What are the following properties for lingual lipase?
* Dietary lipid it acts on
* preferred environment
* specificity
* site of action
- Dietary lipid it acts on: TGs
- preferred environment: acid stable
- specificity: hydrolyzes at sn-3; milk fats?
- site of action: stomach
What are the following properties for cholesterol esterase?
* Dietary lipid it acts on
* preferred environment
* specificity
* site of action
* cofactors
- Dietary lipid it acts on: cholesterol
- preferred environment: mixed micells
- specificity: non-specific
- site of action: SI
- cofactors: stimulated by bile salts
What are the major locations for digestion and absorption of fats?
- mouth and salivary glands
- stomach
- Small intestine
- Large intestine
Describe digestion and absorption of fats in the mouth
Some hard fats begin to melt as they reach body temperatures. Sublingual salivary gland in the base of the tongue secretes lingual lipase
Describe digestion and absorption of fats in the stomach
~ 15% of fats broken down here, most digestion and absorption occurs in the SI
* Once the fat reaches the stomach, acid-stable lingual lipase initiates lipid digestion by hydrolyzing one bond of TG to produce diglycerides and FAs. More prominent for milk fats
* The stomach churning action mixes fat with water and acid and gastric lipase can access and hydrolyze a small amount of fat
Describe digestion and absorption of fats in the small intestine
The presence of chyme in to SI leads to the release of CCK from the intestinal mucosa.
* CCK stimulates the release of bile acids from the gall bladder via the common bile duct into the SI which emulsifies the fat in order for enzymes to be able to work on them.
* CCK also stimulates the release of pancreatic lipase from the pancreas via pancreatic duct which turns the emulsified fats of TGs into monoglycerides, glycerol and fatty acids which can then be absorbed
Describe digestion and absorption of fats in the large intestine
Some fat and cholesterol trapped in fibre, exit in feces
what enzymes are important in the digestion of TGs for infants?
gastric lipase and ligual lipase because they have very little pancreatic lipase
Describe the emulsification process of lipids
- in the stomach, the fat and watery GI juices tend to separate hence enzymes cant really get at the fat.
- When fat enters the SI, the gallbladder secretes bile which has an affinity for both fat and water so can bring fat into the water
- Biles emulsifying action converts large fat globules into small droplets that repel each other
- After emulsification more fat is now exposed to the enzymes making fat digestion more efficient
What is the role of the cofactor colipase?
Colipase is a cofactor for pancreatic lipase which binds to both bile salts as well as lipase, thus assisting the enzyme in breaking down TGs
How does pancreatic lipase break down TGs?
- the TG (sn-1 & sn-3) and two molecules of water are split.
- The H and OH from water complete the structures of two fatty acids and leave a monoglyceride (FA left on sn-2)
What happens to the monoglyceride and FA products after pancreatic lipase?
These products (FAs, monoglycerides, glycerol) may pass into the intestinal cells
* sometimes the monoglyceride is split with another molecules of water to give a third FA and glycerol
Summary of major events and secretions of TG digestion and absorption
What are the products of phosphatidylcholine after phospholipase A2?
lysophosphatidylcholine + free fatty acid
How do lipids cross the unstirred water layer to be absorbed by the enterocyte?
- free fatty acids can pass right through to be absorbed
- mixed micelles are more soluble in the unstirred warer layer and can easily cross the unstirred water and once in contact it can transport content through the cell via different transport mechanisms
How does NPC1L1 work?
major sterol transporter
* micelle comes in contact with the transporter on the membrane and allows for the sterols to move in then an endosome separates the different sterols such that cholesterols move forward with absorption and plant sterols are sent back out.
Describe the absorption of phytosterols
The are absorbed with cholesterol from the micelle via the NPC1L1 transporter and then an endosome sorts the sterols and plant sterols are pumped back out via ATP-binding cassette sub-family G member 5/8 (ABCG5/8) transporters to be excreted into the feces
What is enterhepatic circulation of bile acids?
Essentially the recycling of bile acids whereby 90% is reabsorbed.
What is the monoacylglycerol pathway?
Major reformation of TGs in the enterocyte using enzymes found on the ER.
* monoacylglycerol transferase (MGAT) adds an acyl group to the monoacylgycerol to form a diacylglycerol
* diacylglycerol transferase (DGAT) addis a secondary acyl group to diacyglycerol to form TG
* Once TG is formed it is packaged into chylomicron before leaving cell
How does the glycerol 3-phosphate pathway form TGs?
- glycerol phosphatase acyltransferase (GPAT) adds 2 acyl groups to the glycerol 3-phosphate
- phosphatidate phosphatase removes the phosphate group forming a diacylglycerol
- diacylglycerol acyltransferase (DGAT) adds another acyl group to the diacylglycerol forming TG
How are cholesterol ester reformed in enterocytes?
Cholesterol binds to Acyl-CoA via enzyme Aceyl-CoA cholesterol acyltransferase (ACAT)
How is phosphotidylcholine reformed in enterocytes?
lysophospatidylcholine is reacylated via some sort of acyltransferase enzyme
