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Lipid Transport

  • Key enzymes in lipid transport
    • Cholesterol ester transfer protein mediates transfer of cholesterol esters to other lipoprotein particles.
    • Lecithin-cholesterol acyltransferase: Catalyzes esterification of 2⁄3 of plasma cholesterol.
  • Hepatic lipase: Degrades TGs remaining in IDL.
  • Hormone-sensitive lipase: Degrades TGs stored in adipocytes.
  • Lipoprotein lipase: Degrades TGs circulating chylomicrons and VLDLs. Found on vascular endothelial surface.
  • Pancreatic lipase: Degrades dietary TGs in small intestine.
  • Lipoprotein functions
    • Lipoproteins are composed of varying proportions of cholesterol, TGs, and phospholipids. LDL and HDL carry the most cholesterol.
    • LDL transports cholesterol from liver to tissues.
    • HDL transports cholesterol from periphery to liver.
    • LDL is Lousy. HDL is Healthy.
    • Cholesterol: Needed to maintain cell membrane integrity and synthesize bile acid, steroids, and vitamin D.
    • Chylomicron: Delivers dietary TGs to peripheral tissues. Delivers cholesterol to liver in the form of chylomicron remnants, which are mostly depleted of their TGs. Secreted by intestinal epithelial cells.
    • VLDL: Delivers hepatic TGs to peripheral tissue. Secreted by liver.
    • IDL: Formed in the degradation of VLDL. Delivers TGs and cholesterol to liver.
    • LDL: Delivers hepatic cholesterol to peripheral tissues. Formed by hepatic lipase modification of IDL in the liver and peripheral tissue. Taken up by target cells via receptor-mediated endocytosis.
    • HDL: Mediates reverse cholesterol transport from periphery to liver. Acts as a repository for apolipoproteins C and E (which are needed for chylomicron and VLDL metabolism). Secreted from both liver and intestine. Alcohol ↑ synthesis.
      • Direct pathway: HDL delivers cholesterol esters directly the liver via scavenger receptor (SCARB1) on the hepatocyte cell membrane.
      • Indirect pathway: cholesterol in HDL is transferred to LDL and VLDL via cholesterol ester transfer protein.

 

 

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