10. Digestion and Absorption of Lipids and Iron Flashcards
What lipid digestion occurs in the mouth?
Mastication
Emulsification
Lingual lipase
What do lingual and gastric lipase act on?
TAGs with short to medium fatty acid chains
What emulsifies lipids in the duodenum?
Bile salts
Peristalsis
How does pancreatic lipase degrade lipids?
Binds to the surface of the droplet in the presence of pancreatic colipase
Complex spreads out over the surface of the droplet
TAGs are hydrolysed to monoacylglycerol and fatty acids
What is the function of cholesterol esterase?
Hydrolyses cholesterol esters to cholesterol and free fatty acids
What increases the activity of cholesterol esterase?
Bile salts
What is the function of phospholipase A2?
Digests phospholipid to lysophospholipid by removing a fatty acid
What activates phospholipase A2?
Trypsin
Requires bile salts for optimum activity
What is the function of lysophospholipase?
Removes remaining fatty acid at C1
Leaves a glycerylphosphoryl base
What do the products of fat digestion form?
Mixed micelles
What is contained in a mixed micelle?
Bile salts
Free fatty acids
Free cholesterol
2-monoacylglycerol
How do mixed micelles assist in the absorption of lipids by enterocytes?
Hydrophilic surface of micelles facilitates transport of hydrophobic lipids across the brush border membrane
Where are fatty acids and monoglyceride transported?
Endoplasmic reticulum
Used to synthesise TAGs
What are new TAGs grouped with to form chylomicrons?
Cholesterol esters Free cholesterol Phospholipids Vitamins Apo B48
How do new chylomicrons enter circulation?
Go to golgi apparatus
Extruded from golgi into exocytotic vesicles
Transported to the basolateral aspect of enterocyte
Vesicles fuse with the plasma membrane
Undergo exocytosis
Chylomicrons are dumped into the extracellular space and enter the lymph
What does normal absorption of lipids and fat soluble vitamins depend on?
Bile salts
Normal pancreatic function
Normal intestinal cells
What conditions can result in mal-absorption syndromes?
Pancreatic insufficiency Inadequate mucosal surface Inflammatory states Infection Surgery Cystic fibrosis Disaccharidase deficiency
What are some symptoms of malabsorption states?
Weight loss Weakness Failure to thrive in infants Steathorrhoea Intestinal discomfort Diarrhoea Vitamin deficiencies
How can malabsorption states be detected?
Endoscopy/biopsy Serum enzymes (pancreatitis)
How much iron is contained in the body?
2-4g
What is the average intake of iron?
20mg/day
What happens when iron levels drop?
Storage iron is mobilised first
Haemoglobin synthesis is impaired
Chronic deficiency results in microcytic hypochromic anaemia
What is haemosiderosis?
Accumulation of ferritin and hemosiderin
Leads to free radical generation
What is haemochromatosis?
Progressive hemosiderosis
Results in organ damage
Associated with mutation in HFE gene
How can iron overload be treated?
Repeated phlebotomy
Iron chelator
Name an iron chelator
Desferrioxamine
What change does iron undergo in the stomach?
Fe+++ reduced to Fe++
Favoured by low pH and reducing agents (ascorbic acid)
What change does iron undergo in the small intestine?
Iron dissociates from haem
Fe++ oxidised to Fe+++
Taken up by mucosal cells
What transporter moves haem in the enterocyte?
Haem Carrier Protein 1
What releases Fe++ from haem? (haem iron)
Haem oxidase
What reduces Fe+++? (non-haem iron)
Duodenal cytochrome B
What transports Fe++ into the enterocyte? (non-haem)
Divalent Metal Transporter 1 (DMT1)
What is cytosolic iron stored as?
Ferritin
What transports iron across the basolateral membrane?
Ferroportin
What converts Fe++ back to Fe+++ in the enterocyte?
Hephaestin
What does Fe+++ bind to for transport around the body?
Transferrin
What reduces iron absorption?
Tannins Oxalate Phytate Inorganic phosphates Phosphate-containing antacids
How does iron enter a cell?
Endocytosis via a transferrin receptor
CURL is formed
Fe+++ is released and stored in ferritin in the cytoplasm of the cell
Apotransferrin (transferrin without iron) is recycled to the surface
Which state is most iron stored in?
Fe+++
Stored in ferritin as FeOOH crystals
