Dietary Lipids Metabolism

Question 1

What are lipids?

Answer 1

Heterogenous group of hydrophobic organic molecules

Question 2

Where are lipids found?

Answer 2

1) Compartmentalized

2) Transported in plasma in lipoprotein or albumin

Question 3

What are some lipid functions?

Answer 3

• Major source of energy
• Provide hydrophobic barriers
• Fat-soluble vitamins
• Regulatory or coenzyme fx
• Prostaglandins (from eicosanoids, mediate pain, BP, cell damage)
• Steroid hormones (derived from cholesterol)

Question 4

What are the power house of fats?

Answer 4

Fatty acids

• Need to be carried (albumin)
• Long hydrocarbon chain + single acidic carboxylic group

Question 5

What is triacylglycerol (TAG)?

Answer 5

FA in an ester bond w/ glycerol in position 1, 2, and 3

-Animals store fat as TAG in adipose cells

Question 6

What is glycerolphospholipid?

Answer 6

Glycerol + FA on C-1 and 2 and phosphate group attached to position #3
-In many membranes

Question 7

What is sphinoglycolipid?

Answer 7

Sphinosine as backbone instead of glycerol + a sugar attached to them
-Think in the CNS

Question 8

In USA, over 90% intake of lipids is from what?

Answer 8

TAG

Question 9

What is the enzyme that begins digestion?

Answer 9

Lingual lipase (glands at back of tongue)
-Mainly milk fat

Question 10

What does pancreatic lipase degrade?

Answer 10

Degrades TAG by removing FA at C1 and C3 -> 2-monoacylglycerol + 2 free FAs
-Substrate = TAG

Question 11

What does colipase do?

Answer 11

Restores activity to lipase in presence of inhibitory bile salts that bind the micelles

Question 12

What does cholesterol esterase (pancreatic cholesteryl ester hydrolase) degrade?

Answer 12

Cholesteryl esters (10-15% of dietary cholesterol) hydrolyzed to remove FA from OH position of cholesterol -> free cholesterol ( w/ 1 OH) + free FA

Question 13

What does phospholipase A2 degrade?

Answer 13

Removes a FA from C2 -> Lysophospholipid

Question 14

What can FA at C1 be removed by?

Answer 14

Lysophospholipase

Question 15

What is Orlistat?

Answer 15

Antiobesity drug inhibits gastric and pancreatic lipases

-Lowers fat absorption -> weight loss

Question 16

What are bile salts conjugated to?

Answer 16

Glycine or taurine

Question 17

Where are bile salts made and stored?

Answer 17

Made in liver; stored in gall bladder

Question 18

Why do conjugated bile salts serve as detergents?

Answer 18

They are amphipathic (hydrophobic and hydrophilic) derivatives of cholesterol

Question 19

What is lipid digestion controlled by?

Answer 19

Hormones

Question 20

What is the role of peptide hormone cholecystokinin in control of lipid digestion?

Answer 20

In response to presence of lipids:

• Gallbladder -> release bile
• Pancreas -> release digestive enzymes
• Stomach -> decrease gastric motility -> slower release of contents into small intestine

Question 21

What is the role of small peptide hormone secretin in control of lipid digestion?

Answer 21

In response to low pH:

• Pancreas -> release bicarbonate -> neutralizes acidity of stomach
• Without bicarbonate none of the enzymes will work

Question 22

How are lipids absorbed by intestinal mucosal cells (enterocytes)?

Answer 22

• In lumen, soluble mixed micelles created that facilitate uptake (micelle = free FAs, cholesterol, 2-monoacylglycerol, bile salts, fat-soluble vitamins A, D, E, K)
• Approach brush border membrane of enterocytes -> absorbed

Question 23

Where are bile salts absorbed? What percentage is lost in feces?

Answer 23

In terminal ileum; 5% lost in feces

Question 24

How are TAGs resynthesized?

Answer 24

By fatty acyl-Coenzyme A (CoA Synthetase)

-Take FA and add a CoA to it to add FA in position 1 and 3 -> TAG

Question 25

What happens to lysophospholipids after absorption by enterocytes?

Answer 25

Reacylated to form phospholipids

Question 26

What happens to cholesterol after absorption by enterocytes?

Answer 26

Esterified w/ a FA by acyl CoA: Cholesterol Acyltransferase

Question 27

How are lipids secreted from enterocytes?

Answer 27

Packaged as chylomicrons (dietary fat = FAT YOU ATE AS FAT) -> lymph -> blood
***different from VLDL b/c they are made in intestinal lumen and they are from dietary fat

Question 28

What are chylomicrons?

Answer 28

Particles of lipid droplets surrounded by a thin layer composed of phospholipids, un-esterified cholesterol and protein apolipoprotein B-48

Question 29

What is steatorrhea?

Answer 29

Lipid malabsorption -> increased lipid in feces

-Can result from cystic fibrosis and short bowel syndrome

Question 30

Where are dietary lipids primarily broken down in?

Answer 30

Capillaries of:

1) Skeletal and cardiac muscle -> energy
2) Adipose tissue -> storage

Question 31

What enzyme in tissues degrades TAG into free FAs and glycerol?

Answer 31

Lipoprotein lipase (LPL)

• Synthesized primarily by adipocytes and muscle cells
• Secreted and associated w/ luminal surface of endothelial cells in capillary beds

Question 32

Where does LPL cleave TAG?

Answer 32

All 3 spots -> 3 FAs and glycerol

Question 33

What is the fate of free FAs?

Answer 33

1) Directly enter adjacent muscle cells (oxidized -> energy)
2) Directly enter adipocytes (reesterify to stored TAG)
3) Transported in blood (albumin)

Question 34

What is the fate of glycerol?

Answer 34

Taken up from blood -> liver and phosphorylated by hepatic glycerol kinase -> glycerol 3-phosphate -> DHAP -> glycolysis or gluconeogenesis

Question 35

What is the fate of remaining chylomicron components?

Answer 35

Cholesteryl esters, phospholipids, apolipoproteins, fat-soluble vitamins and some TAGs -> bind to ApoE receptors on liver -> endocytosed -> hydrolyzed to their component parts -> recycled