3b. Metabolism of plasma lipoproteins

Where are chylomicrons formed?

In enterocytes of the small intestine.

What is the main role of chylomicrons?

Transport of exogenous dietary lipids from the intestine.

Which apolipoprotein is essential for chylomicron formation?

Apo B-48.

Where does chylomicron formation begin?

In the rough endoplasmic reticulum of enterocytes.

What happens in the smooth endoplasmic reticulum during chylomicron formation?

Apolipoproteins bind with absorbed exogenous lipids.

How are chylomicrons released from enterocytes?

By exocytosis.

Where do chylomicrons go after leaving enterocytes?

First into lymphatics, then into the bloodstream.

Do nascent chylomicrons contain apo C and apo E?

No, they acquire apo C and apo E from HDL in circulation.

Which enzyme is most important for chylomicron catabolism?

Lipoprotein lipase.

Where is lipoprotein lipase located?

Bound to capillary walls.

Which apolipoprotein activates lipoprotein lipase?

Apo C-II.

What does lipoprotein lipase hydrolyze?

Triglycerides in chylomicrons and VLDL.

What are triglycerides hydrolyzed into?

Glycerol and free fatty acids.

How much of chylomicron triglycerides can be hydrolyzed by lipoprotein lipase?

Up to 90%.

What remains after chylomicron triglyceride removal?

Chylomicron remnants.

Which apolipoproteins are present in chylomicron remnants?

Apo B-48 and apo E.

What is the function of apo E on chylomicron remnants?

It allows uptake by liver receptors.

Where are chylomicron remnants metabolized?

In the liver.

What does chylomicron presence after 12 hours fasting suggest?

A pathological finding.

What is the main role of VLDL?

Transport of endogenous lipids from the liver to tissues.

Where is VLDL formed?

In hepatocytes of the liver.

Which apolipoprotein is essential for VLDL formation?

Apo B-100.

Where is apo B-100 synthesized?

On ribosomes of the rough endoplasmic reticulum of hepatocytes.

Do nascent VLDL particles contain apo C and apo E?

No, they acquire apo C and apo E from HDL in circulation.

Which enzyme catabolizes VLDL?

Lipoprotein lipase.

What does VLDL become after lipoprotein lipase action?

IDL.

Which apolipoproteins are found in IDL?

Apo B-100 and apo E.

What can IDL be transformed into?

LDL.

Which enzyme helps transform IDL into LDL?

Hepatic lipoprotein lipase.

Which receptor is important for LDL uptake?

LDL receptor.

Which apolipoprotein binds LDL to the LDL receptor?

Apo B-100.

What happens to LDL inside cells?

It is hydrolyzed by lysosomal enzymes.

What does LDL hydrolysis release?

Amino acids, glycerol, free fatty acids, and free cholesterol.

What does intracellular cholesterol inhibit?

New LDL receptor synthesis and HMG-CoA reductase activity.

Why does cholesterol inhibit HMG-CoA reductase?

To reduce endogenous cholesterol synthesis.

What can free cholesterol be used for?

Cell membranes and steroid hormone synthesis.

Which enzyme esterifies excess free cholesterol inside cells?

ACAT.

What is the main protective function of HDL?

Reverse cholesterol transport from tissues to the liver.

Where is HDL synthesized?

In the liver and small intestine.

What shape does nascent HDL have?

Discoidal shape.

Which transporter helps move cholesterol to HDL?

ABCA1.

Which enzyme is important for HDL maturation?

LCAT.

Which apolipoprotein activates LCAT?

Apo A-I.

What does LCAT do?

It esterifies free cholesterol.

What happens after cholesterol is esterified in HDL?

Cholesterol esters move into the core and HDL becomes spherical.

What does the liver take up from spherical HDL?

Cholesterol esters.

Which additional protective properties does HDL have?

Antioxidant, anti-inflammatory, and anticoagulant properties.