Lipid Digestion and Transport Flashcards
Mouth
Lingual lipase breaks down fats (TAGs), continuously secreted
Stomach
○ Gastric lipase is continuously secreted
Lipases are stable at a low pH, continue breakdown of TAG (most abundant fat in diet)
Liver
Synthesizes bile acids from cholesterol (essential for absorption of dietary fats)
Gallbladder
○ Stores bile
Releases bile when triggered by hormones (acts as a detergent/emulsifier- solubulizes fats)
Small Intestine
Pancreatic enzymes include pancreatic lipase, cholesterol esterase (targets cholesterol esters- cholesterol with a fatty acid attached)
Mixed Micelles:
• Digested lipids are stabilized by bile acids
• Bile acids are made from cholesterol (structurally similar), act to solubulize lipids
○ Bile salts are the same as bile acids
• Small, spherical complexes containing lipids and bile acids
• Mixed micelle brings digestive lipids into contact with digestive enzymes, and to intestinal cells for absorption (acts as transporters)
• Can access intramicrovillus spaces of intestinal membrane
• Originally thought that digested lipids delivered into intestinal mucosal cells by passive diffusion, but carrier-mediated transporters have been identified
Fat primarily absorbed in the small intestine, leftover bile is reabsorbed (body very efficient at recycling bile acids
Diagram
Enterohepatic Circulation:
• Bile acid comes into the small intestine, reabsorbed in the lower part of the small intestine/large intestine
• 95% reabsorbed
• 5% lost to feces
Can lower recycling efficiency through dietary or pharmaceutical means, more bile acid ends up in the feces, more cholesterol used to remake bile acids, lowers cholesterol
Diagram
Lipid Absorption:
TAGs are broken by lipase into 2 fatty acids and one monoglycerol, brought into the cell and remade, then packaged into a lipoprotein for transport
Diagram
Lipid Transport:
• Different compounds are defined based on density (ratio between lipid and proteins)
• Chylomicrons are high in lipid, low in proteins
• HDL- High density lipoproteins have high density (high protein, low lipid)
• VLDL are made in the liver to transport newly made TAG and cholesterol the to body, as lipids are dropped off, density changes (more protein, less lipid)
○ Protein content doesn’t change, lipid content changes
• Can identify lipoprotein based on density and protein composition
ApoB-48 is only made in the intestine, ApoB-100 is only made in the liver, can differentiate between intestinal and liver lipoprotein
Chylomicrons:
• Made from the intestine
• Increase in blood after a meal, transports lipids to the body
• Enters blood at a slow rate, peaks 30 mins to 3 hrs after eating
• Clearance occurs via Lipoprotein Lipase (LPL), activated by the Apo C in chylomicrons
○ LPL located on endothelial cell surface of small blood vessels and capillaries
○ LPL not expressed in liver, but is expressed by adipose and muscle tissue (chylomicrons drops off lipids at these locations)
○ LPL action hydrolyzes TAGs in chylomicrons
§ Produces Chylomicron remnant (CR)- TAG depleted chylomicron
CR removed from circulation via ApoE interactions with a receptor in the liver
Diagram
Classification of Lipoproteins
• Liver can make TAGS by lipogenesis, which are packaged and sent to the body through VLDL
• Storing fat in the liver leads to fatty liver disease (accumulation of TAG), want TAG stored in adipose tissue instead
• VLDL loses Tag as its deposits it into skeletal and adipose tissue (stimulated by LPL and Apo C)
• VLDL->IDL->LDL as it VLDL continues to lose lipids
HDL is made by the liver but is relatively lipid empty- travels through the body to pick up lipids and return it to the liver, clearing action makes it “good”
Diagram
Low Density Lipoprotein
• Lipogenesis occurs in liver
• Lipids packaged in VLDL, sent throughout body
• LPL allows TAG to be deposited to different cells
• VLDL loses lipids, becomes LDL
• ApoB-100 and Apo E interact with LDL receptors
Delivers cholesterol for essential functions, but can also deposit cholesterol in unwanted places
Diagram
High Density Lipoprotein (HDL):
• Synthesized by the liver
• Low in lipids, goes into body and picks up cholesterol, returning it to the liver for elimination (reverse cholesterol transport)
• Reduced body cholesterol, making it “good” cholesterol as it is going to the liver to be cleared
• ApoA is only found on HDL, interacts with LCAT
○ LCAT- Lecithin-Cholesterol Acyltransferase
○ Enzyme that loads cholesterol on HDL
○ Only HDL has ApoA, so only HDL can pick up cholesterol esters
• Once at the liver, HDL interacts with SRBI (Scavenger receptor class B1), which removes cholesterol esters and brings it into the liver, where the liver processes it for elimination
• CETP (cholesterol ester transfer protein) can transfer cholesterol esters from HDL to VLDL, which interacts with LDL receptors in the liver to dismantle it
Higher HDL levels means more cholesterol is returning to the body
Diagram
Fates of Cholesterol in the Liver:
• Converted into bile acids
• Secreted directly with bile ot be eliminated in feces
• Repackaged into lipoproteins and sent around the body (depending on needs of body)
Can be used to create other hormones (i.e. sex steroid hormones)
Recommended Caloric Intake:
• Protein- 10-35% of daily calories (4kcal/g)
• CHO- 45-65% of daily calories (4 kcal/g)
Fat- 20-35% daily calories (9kcal/g)
