Lipid digestion and absorption Flashcards
US adult consumption of lipids
- 81g a day
- 90% are TAG(72g)
- The rest is…
- cholesterol
- cholesterol esters
- phospholipids
- free fatty acids
-most digested are esterified
Acid lipases
- acid-stable lipases function in the stomach at pH 4-6
- Inguinal lipase: secreted from glands at the back of the tongue
- Gastric lipase: secreted from the gastric mucosa
- target TAG containing SHORT AND MEDIUM chain fatty acids(release FA straight into the blood stream)
- milk fat - important in neonates when milk is the primary food source
Emulsification in the small intestine
- two complimentary actions:
1. mechanical agitation
- peristalsis to increase fat droplet surface area
2. bile salts secretion
- made in the liver, stored in the gallbladder
- secreted to small intestine
- have detergent properties that STABILIZE particles as they become smaller and prevent them from COALESCING
pancreatic enzymes in general
- want proper pH and all necessary compnenets, so very coordinated process
- Enzymes secreted from the pancreas digest dietary TAG, cholesteryl esters and phospholipids
Digestion of FA
-removal of specific fatty acids by breaking ester bonds attaching the fatty acids
TAG digestion
-TAG is too big for intestinal mucosal cells to endocytose efficiently
Pancreatic lipase
- an esterase, cleaves fatty acid at Carbon 1 and 3 of glycerol backbone.
- creates 2 fatty acids and a 2-monoacylglycerol
- represents 2-3% total protein in pancreatic secretions
- high catalytic efficiency
If someone has a deficiency of pancreatic enzymes, what should you give them?
- pancreatic lipase
- hella active
What is the maximum number of free fatty acids produced from pancreatic lipas?
-one 2 per TAG because you have to keep one attached to the glycerol backbone
Colipase
- TAG digestion
- binds to pancreatic lipase at 1:1 ratio
- anchors pancreatic lipase at the lipid-aqueous interface to promote pancreatic lipase enzymatic activity when inhibitory bole salts are present.
Cholesteryl ester digestion
- dietary cholesterol is present in two forms
- 90%-nonesterified (free)
- 10%-esterified cholesterol (cholesteryl ester)
Cholesterol esterase
- cholesteryl ester digestion
- pancreatic enzyme secreted to the small intestine to digest esterified cholesterol
- producing cholesterol and fatty acids
- all cholesterol is finally non-esterified
- bile salts promote cholesterol esterase activity
- ONLY FOR ESTERIFIED
Phospholipid digestion
-ingested phospholipids are digested by sequential removal of fatty acids by two different pancreatic enzymes, producing two fatty acids and one glycerylphosphoryl base
Phospholipase A2
- 1 enzyme used in phospholipid digestion
- removes the fatty acid from POSITION 2 to produce lysophospholipid and a free fatty acid
Lysophospholipase
- 1 enzyme used in phospholipid digestion
- removes fatty acid from POSITION 1 to generate one fatty acid and one glycerylphosphoryl base
Glycerylphosphoryl bases
- we need it to get coline to make bile salts
- can be…
- absorbed
- further digested
- excreted into the feces
Coordinating Digestion of dietary lipids
- sensing the presence of dietary lipids and pH (and proteins)
- CCK
- Secretin
Gut endocrine cells
- in the mucosa of the lower duodenum and jejunum
- sense…
- lipids and partially digested proteins
- the low pH of chyme entering the intestine
- they secrete 2 small peptide hormones into the blood
Cholecystokinin (CCK)
-a small peptide hormone secreted to the blood in response to the presence of lipids and partially digested proteins
Positive:
-promote pancreatic enzyme secretion (exocrine cells to the intestine)
-cause gallbladder to secrete bile
Negative:
- decreases gastric motility to reduce the release rate of gastric contents to the small intestine
- gives enzymes time to be able to do their work
Secretin
- peptide hormone secreted to the blood in response to the low pH of chyme entering the intestine
- promotes release of bicarbonate rich solution from pancreas to small intestine
- provides the appropriate pH for optimal pancreatic enzyme function
Lipid digestion in jejunum
3 primary products
- free fatty acid
- free cholesterol
- 2-monoacylglycerol
mechanism for getting fat soluble vitamins into the body
Micelles
- disk-shaped clusters of amphipathic ipids
- formed from 3 products of lipid digestion in J, bile salts, and fat-soluble vitamins
Micelle function
- polar group towards aqueous solution, hydrophobic group toward the center
- as micelle is formed, it recognizes its center and its environment
- water doesnt mix well with chyme so water layer is perfect for micelles and the polar group comes off
- FA tails exposed and the molecule flips into the membrane of enterocytes
- thats how fat soluble molecules get into cells
Chylomicrons
- in ER
- for complex lipid biosynthesis
- Cholesteryl ester and TAG are main things put inside(neutral)
- pocket that forms between 2 layers of ER
- HAVE APOLIPOPROTEIN b-48 ASSOCIATED WITH IT
- excreted into the lymph system
- sealed chromonolipid bilayer
Apolipoprotein B48
- associated with chylomicrons
- NOT apolipoprotein 100 found in liver
Fatty-acyl-CoA synthetase
- thiokinase
- converts long chain fatty acids to their activated form by using energy from ATP–>AMP to form fatty acyl CoA
TAG synthase
-2-monoacylglycerol is converted to TAG by an enzyme complex that sequentially adds activated fatty acids by two enzymatic acyltransferase activities
Acyl CoA: cholesterol acyltransferase
-esterifies cholesterol with a fatty acid
Chyle
- lymph which contains chylomicrons
- passes via the lymphatic system to the thoracic duct, from thoracic duct to subclavian vein, and then enters blood
Lipoprotein lipase
- enzymatically degrades TAG in the circulating chylomicrons to free fatty acids and glycerol
- synthesized and secreted primarily by muscle and adipose tissue(also heart, lung, kidney, liver)
- associates with the lumen endothelial cells of capillary beds
Lipid malabsorption
- problems with gallbladder, or pancreas, or intestinal cells being able to bring materials in
- BIGGEST PROBLEM: pancreatic enzyme issues and gallbladder issues
- cant stabilize smaller droplets, pancreatic enzymes cant remove fatty acids
- cant clip lipids off molecules
Cystic fibrousis
- not all about breathing(long term)
- Short term: children have energy and need to grow, but they cant produce pancreatic enzymes to the small intestine.
- defect chloride ion channel in duct between pancreas and intestine so water cannot get into the mucous and so the duct gets clogged.
- cant digest FA for energy
- cant create micelles and chiromicrons to get fat soluble vitamins into the body
- Treatment: have sufficient energy in their diet.
- increase pancreatic enzymes because they are hella active
