Lipid Transport and Classification

Lipid Transport

• Lipids are insoluble in water, so they associate with proteins forming water-soluble lipoproteins
• The protein part of lipoproteins is called apolipoprotein
• Lipoproteins transport lipids between tissues
• Total plasma lipid is 400-600 mg/dL, including triacylglycerol (20%), phospholipids (30%), cholesterol (40%), and unesterified fatty acids
• Lipoproteins consist of:
  • A lipid core of nonpolar TAG and cholesterol esters
  • A surface layer of phospholipids and cholesterol
  • Proteins, called apolipoproteins (APOs)
  • APOs range from 1% in chylomicrons to 60% in some HDLs
  • Some APOs are peripheral, while others are integral

Classification of Lipoproteins

• Lipoprotein density increases with the protein component
• Lipoproteins can be separated by ultracentrifugation and electrophoresis
  • Chylomicrons
  • Very low-density lipoproteins (VLDL)
  • Intermediate-density lipoproteins (IDL)
  • Low-density lipoproteins (LDL)
  • High-density lipoproteins (HDL)
  • Lipoprotein (a) [Lp(a)]: similar lipid composition to LDL; competes with plasminogen for tissue plasminogen receptors
• Lipids in lipoproteins include triacylglycerol (TAG), phospholipids (PL), cholesterol, cholesterol esters, and free fatty acids (FFAs)
• TAG is predominant in chylomicrons and VLDL. Cholesterol is predominant in LDL, and phospholipids in HDL

Apo-lipoproteins (Apoproteins)

• Apoproteins are the protein part of lipoproteins, mainly synthesized in the liver and small intestine
• Five major classes (from A to E) are known
  • Apo B exists in two forms: Apo B100 (in LDL) and Apo B48 (in chylomicrons)
  • Apo(a) is present in Lp(a) with Apo B100 and has a high carbohydrate content
• Functions of Apoproteins
  • Increase lipid solubility in plasma
  • Enhance lipoprotein stability
  • Act as ligands for interaction with lipoprotein receptors, regulating tissue uptake.
    • Apo B-100 and Apo E bind to LDL receptors
    • Apo E binds to remnant receptors
    • Apo A-I binds to HDL receptors

Some Apolipoproteins as Enzyme Activators or Inhibitors

• Some APOs act as enzyme activators or inhibitors
• Activators:
  • Apo C-II activates lipoprotein lipase
  • Apo A-I activates lecithin-cholesterol acyltransferase (LCAT)
• Inhibitors:
  • Apo A-II and Apo C-III inhibit lipoprotein lipase
  • Apo C-I inhibits cholesteryl ester transfer protein

Metabolism of Chylomicrons

• Site of synthesis: intestinal mucosa
• Function: transport dietary lipids to peripheral tissues
• Structure:
  • 98% lipids (mainly triacylglycerol)
  • 1-2% proteins (including apo B-48, apo E, and apo C-II)
• Half-life: approximately 1 hour

Metabolism of Very-Low-Density Lipoproteins (VLDL)

• Site of synthesis: liver
• Function: transport endogenously synthesized lipids from the liver to peripheral tissues
• Structure:
  • Primarily triacylglycerol
  • 10% proteins (including apo B-100, apo E, and apo C-II)
• Half-life: 1-3 hours

Formation of IDL (Intermediate-Density Lipoproteins)

• LpL activity reduces TAG by 70-90%, and Apo C, but not Apo E, leading to VLDL remnants (IDL)
• IDL contains less TAG and more cholesterol
• IDL fate:
  • Taken up by the liver via LDL receptors (Apo B-100, E)
  • Converted to LDL (mostly)

This quiz covers the essentials of lipid transport and the classification of lipoproteins. It explores the roles of apolipoproteins, the composition of lipoproteins, and how they transport lipids in the body. Test your knowledge on the different types of lipoproteins and their densities.