Lipoproteins

Question 1

What are chylomicrons and VLDL?

Answer 1

• Both are main transporters of triglycerides.
• Low density die to low protein/lipid.
• Delayed clearance or increased secretion risk factor for CVD.

Question 2

What are LDL and HDL?

Answer 2

• Smaller than VLDL.
• High density. High in protein.
• Involved in the transport of cholesterol to and from the cells.
• 70% total cholesterol in LDL.
• 2-4 days turnover.

Question 3

What is the structure of a chylomicron?

Answer 3

• Mainly composed of triglycerides and lipophilic vitamins.
• Origin is gut and liver.
• Proteins on its surface, important for emzyme interaction.
• Peripheral apoprotein: stabilasation, packaging, hydrolysing enzyme and receptor interaction.
• Transferred from one lipoprotein to another.

Question 4

What is the role of a chylomicron?

Answer 4

• Deposit fat content in adipose tissue, skeletal muscle and cardiac muscle.
• Chylomicron remnants taken up by liver. Remaining trigylcerides metabolised later, or can leave the liver as VLDL.

Question 5

How to chylomicrons give and acquire new apoproteins?

Answer 5

• Chylomicrons with dietary fat are secreted into lymphatic system and enter in the circulation.
• These rapidly acquire apo C2 and apo E from HDL.

Question 6

How are TAGs taken up by the tissues?

Answer 6

• Chylomicrons are too large to move through the capillary wall.
• Adipocytes synthesise and secrete lipoprotein lipase which is attached to endothelial cells.
• Triacylglycerols are then hydrolysed into chylomicrons and the released fatty acids are taken up by tissues.
• Adipose: TAG store as energy reserve.
• Fasting: TAG remobilised and hydrolysed into fatty acids.

Question 7

What happens in the fasting state?

Answer 7

• Lipid mobilisation occurs.
• Activation of adrenoreceptors by catecholamines activates andenyl cyclase, >cAMP levels and activates protein kinase A-dependant phosphorylation of hormone-sensitive lipase.
• Active HSL hydrolyses TAG in adipose droplets, results in release of fatty acids and glycerol.

Question 8

What happens in the fed state?

Answer 8

• Insulin receptor stimulation in adipose cells results in activation of a pathway which decreases dramatically intercellular cyclic AMP and therefore prevents mobilisation of fat.
• LPL hydrolyses to make chylomicron and VLDL on the surface of endothelial cells.

Question 9

How are VLDLs and LDLs generated?

Answer 9

• Assembled with TAG and cholesterol in the liver.
• They require apoproteins from HDLs.
• Interaction with LPL on endothelail cells.
• Hydrolysed VLDLs are called VLDL remnants.
• Remnants absorbed by liver, further hydrolysed by hepatic lipase.
• Further hydrolysis makes low density lipoproteins (LDL), contain a lot of cholesterol.
• These circulate and get absorbed by liver and peripheral cells.

Question 10

How does HDL reverse cholesterol transport work?

Answer 10

• LDL binds to target tissue, absorption occurs through endocytosis.
• LDL particles are hydrolysed with lysosomes which releases lipids and cholesterol.
• Excess cholesterol is transported back in HDL to the liver, can be transported (VLDL-LDL pathway) or excreted as bile.
• Pre-HDL is synthesised in the intestine and liver, then its secreted into the blood and can bind to cholesterol transporter.

Question 11

What factors influence triglyceride response?

Answer 11

Amount of fat and type of fat.