Lipoprotein Metabolism

Exogenous Pathway

1. Lipids are digested.
2. Chylomicrons are assembled with apolipoprotein B-48.
3. Chylomicrons move into the liver and subsequently into the bloodstream, where HDL donates apolipoprotein C-II and E, forming a mature chylomicron. ApoC can only bind to receptors found on adipose tissue while ApoE can only bind to receptors on hepatocytes.
4. Mature chylomicrons activate lipoprotein lipase (LPL), an enzyme found on the surface of cells. LPL catalyzes a hydrolysis reaction, releasing glycerol and fatty acids from the chylomicrons. Glycerol and fatty acids can be absorbed by the tissue.
5. Remnants are endocytosed and hydrolyzed within lysosomes. This also releases glycerol and fatty acids in the cell.

Endogenous Pathway

1. In the liver, triacylglycerol and cholesterol are assembled with apolipoprotein B-100 to form VLDL.
2. HDL donates apolipoprotein C-II and E.
3. Apolipoprotein C-II activates LPL, causing hydrolysis of the VLDL particle and the release of glycerol and fatty acids. Glycerol and fatty acids can be absorbed by adipose tissue and muscle.The hydrolyzed VLDL particles are now called IDL.
4. IDLs return to the liver and are further hydrolyzed by hepatic lipase. This releases glycerol and fatty acids, leaving behind IDL remnants, called LDL. After LDLs bind to target tissues, they are endocytosized. The internalized LDL particles are hydrolyzed with lysosomes, releasing mainly cholesterol.