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Formation and Catabolism of Chylomicrons and VLDL

Plasma lipids consist of triacylglycerol (16%), phospholipids (30%), cholesterol (14%) and cholesteryl esters (36%), and a much smaller fraction of unesterified long-chain fatty acids (4%). The free fatty acids (FAA), is metabolically the most active of the plasma lipids

 There are two pathways of lipid transport in the body –

  1. Transport of dietary lipid (Exogenous pathway)
    1. It is the transport of lipid from intestine to liver.
    2. Chylomicrons transport the dietry lipid from intestine to liver.
    3. In diet the major lipids are triglyceride and cholesterol.
    4. Cholesterol is absorbed as such in proximal small intestine and is esterified to cholesteryl ester (ChE).
    5. Triglycerides are hydrolysed by lipases to glycerol and fatty acids which are absorbed in intestine. Inside intestinal cells triglyceride is synthesized by fatty acids.
    6. Chylomicrons are synthesized in the small intestine that contain triglyceride, cholesteryl ester, cholesterol, phospholipids, and apoprotein B-48 (apo B-48) and apo-A
    7. These chylomicrons are secreted in the intestinal lymph and reach the systematic circulation via thoracic duct.
    8. In the circulation triglycerides of chylomicrons are hydrolyzed by lipoprotein lipase (LPL) present on endothelial cells of vessels of skeletal muscles, adipose tissue and heart, but not in liver.
    9. The released fatty acids are utilized locally by these tissues.
    10. Adipose tissue uses these fatty acids to restore them as TGs and heart and skeletal muscle use them as source of energy. Thus, function of chylomicrons is to transport exogenous (dietary) triglyceride on to adipose tissue (for storage), heart (for energy) and muscle (for energy).

Chylomicrons are synthesized in the intestine to transport exogenous (dietary) triacylglycerol from intestine to plasma.

On the other hand VLDL transport endogenous triacylglycerol from the liver to extrahepatic tissue.

  1. Chylomicrons and VLDL remnants:
    1. Although triacylglycerol is the main lipid of chylomicrons and VLDL, yet each contains little amounts of cholesterol and cholesterol esters.
    2. By the action of plasma lipoprotein lipase there will be loss of approximately 90% of the triacylglycerol of chylomicrons and also loss of apo-C (which returns to HDL) but not apo-E (which is retained).
    3. The resulting lipoprotein or chylomicron remnant is about half the diameter of the parent chylomicron and becomes relatively enriched in cholesterol and cholesteryl esters.  Similar changes occur to VLDL with the formation of VLDL remnants.
  2. Fate of Chylomicrons and VLDL remnants:
    1. Chylomicrons remnants are taken up by the liver, where its content of cholesteryl esters and triacylglycerol and metabolized.
    2. Up to 50% of VLDL remnants (IDL) are taken into the liver by this route, the rest form LDL.

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