Transport Of Lipids

Question 1

Where does body Cholestrol come from?

Answer 1

Some cholestrol comes from the diet, and the remainder is synthesised by the liver.

Question 2

Why do we need Cholestrol?

Answer 2

Cholestrol is a very versatile molecule. It is an essential component of membranes and also a precursor for steroid hormones and bile salts. It is transported around the body as a cholestrol ester.

Question 3

What are lipoproteins/ apolipoproteins?

Answer 3

A lipoprotein is a phospholipid monolayer which contains small amounts of Cholestrol. Apolipoproteins can be integral (apoA, apoB) or peripheral (apoC, apoE)

Question 4

State the order of density of lipoproteins.

Answer 4

Chylomicrons, VLDL, IDL, LDL, HDL.

Question 5

To what lipoprotein classes are apoA and apoB associated?

Answer 5

Apo A HDL

Apo B VLDL, IDL, LDL.

Question 6

What is the difference between an integral and a peripheral apolipoprotein?

Answer 6

Integral proteins pass all the way through the lipoprotein layer whilst peripheral proteins rest on the surface.

Question 7

What is the function of apolipoproteins?

Answer 7

Apolipoproteins act as enzyme cofactors, and also ligand so for cell surface receptors.

Question 8

Describe the action of a chylomicron.

Answer 8

Chylomicrons are located in the small intestine. Here they pick up TAG and transport them through the lymphatic system to where they enter the blood stream at the subclavanian vein.

Question 9

What two apolipoproteins do chylomicrons acquire in the blood?

Answer 9

ApoC and apoE.

Question 10

What is the name of the enzyme which apoC binds to in the muscle/adipose so that fatty acids are released and chylomicron content decreases?

Answer 10

Lipoprotein lipase.

Question 11

What happens when TaG content in a chylomicron drops below 20%?

Answer 11

At this point apoC dissociates leaving a chylomicron reminant. This then returns to the liver where it is taken up by receptor mediated endocytosis.

Question 12

What is the function of VLDL?

Answer 12

VLDL is made in the liver to carry TAG to tissues of the body.min the blood apoC and apoE are added and this bind to lipoprotein lipase receptors and so TAG content is depleted.

Question 13

What is the difference between IDL and LDL?

Answer 13

When VLDL has got depleted TAG stores, it is named IDL. This is when they are

Question 14

Why is LDL more dangerous than IDL/VLDL?

Answer 14

It does not have apoC/E and is therefore not cleared by the liver meaning it has a longer half life. It is therefore more susceptible to oxidative damage, and can contribute to arteriosclerosis.

Question 15

What is hypercholestrolaemia?

Answer 15

High cholestrol in the blood which leads to Cholestrol deposits around the body. Xanthelasma - yellow patches under eye lids, tendon xanthoma - nodules on tendons and corneal arcus - White around cornea of eye.

Question 16

Explain how high LDL can lead to artherosclerosis.

Answer 16

When there is high Oxidised LDL this is engulfed by macrophages which then become foam cells in the intima of blood vessels. This forms a fatty streak (plaque) which encroaches on the lumen. If this ruptures can cause thrombus to form.

Question 17

Why do statins help to reduce Cholestrol?

Answer 17

Statins inhibit HMG~coA reductase which is a reaction early on in the pathway. This reduces Cholestrol synthesis but also causes many other side effects.

Question 18

What is a bile salt sequestrant?

Answer 18

These bind bile salts in the GI tract. This means that more bile salts have to be produced by the liver which uses up more Cholestrol, lowering blood Cholestrol. Fibre has the same effect because it traps bile salts.

Question 19

In relation to lipid transport, what is receptor mediated endocytosis?

Answer 19

Cells that require Cholestrol have LDL receptors and apoB acts as a ligand to these receptors. The complex enters the cell by endocytosis and then fuses with a lysosomes and releases fatty acids and Cholestrol.

Question 20

What is the function of HDL?

Answer 20

Nascent HDL is synthesised in the liver or buds off VLDL or chylomicrons during their breakdown. This acquires phospholipids and cholestrol from the cells lining blood vessels. As the hollow core fills, it takes on a globular shape.

Question 21

What is the name of the protein which transfers Cholestrol to HDL?

Answer 21

ABAC1 protein. The Cholestrol is then converted to Cholestrol ester by LCAT

Question 22

What happens to mature HDL?

Answer 22

Scavenger receptors can obtain Cholestrol from HDL for cells requiring Cholestrol. CETP allows HDL to exchange Cholestrol ester for TAG for VLDL.

Question 23

What is a hyperlipoproteinaemia and what causes these?

Answer 23

Hyperlipoproteinaemia is where there is excessive lipoproteins in the blood. This is caused by either over production or under removal.

Question 24

How are lipids transported in the blood?

Answer 24

Lipids are insoluble and so must be carried by other molecules. Some are carried bound to carriers such as albumin but the majority are carried in lipoprotein particles.