Lipid Transport Notes

Lipid Transport

  • Free fatty acids are transported through the blood in association with albumin (carrier protein).
  • Triacylglycerol and cholesterol are transported in the blood as lipoproteins (aggregates of apolipoproteins and lipids).

Lipoproteins

  • Named according to their density, which increases with the percentage of protein.
  • Types (in order of increasing density):
    • Chylomicrons (least dense, highest fat to protein ratio)
    • VLDL (very low density lipoprotein)
    • IDL (intermediate density lipoprotein)
    • LDL (low density lipoprotein)
    • HDL (high density lipoprotein)

Lipoprotein Functions

  • Chylomicrons and VLDL primarily carry triacylglycerols but also contain small quantities of cholesterol esters.
  • LDL and HDL are primarily cholesterol transport molecules.

Chylomicrons

  • Highly soluble in lymphatic fluid and blood.
  • Function: transport dietary triacylglycerols, cholesterol, and cholesterol esters to other tissues.
  • Assembly: occurs in the intestinal lining, resulting in a nascent chylomicron containing lipids and apolipoproteins.

VLDL (Very Low Density Lipoprotein)

  • Metabolism is similar to chylomicrons.
  • Produced and assembled in liver cells.
  • Main function: transport triacylglycerols to other tissues.
  • Also contain fatty acids synthesized from excess glucose or retrieved from chylomicron remnants.

IDL (Intermediate Density Lipoprotein)

  • Formed when triacylglycerol is removed from VLDL (also called VLDL remnant).
  • Fate:
    • Reabsorbed by the liver via apolipoproteins.
    • Further processed in the bloodstream.
  • Can pick up cholesterol ester from HDL to become LDL.
  • Exists as a transition particle between triacylglycerol transport (chylomicrons and VLDL) and cholesterol transport (LDL and HDL).

LDL (Low Density Lipoprotein)

  • Primarily a cholesterol particle.
  • The majority of cholesterol measured in the blood is associated with LDL.
  • Normal role: deliver cholesterol to tissues for biosynthesis.
  • Cholesterol is important for:
    • Cell membranes.
    • Bile acid and salt production in the liver.
    • Steroid hormone synthesis (steroidogenesis) in many tissues.

HDL (High Density Lipoprotein)

  • Synthesized in the liver and intestines and released as dense protein-rich particles into the blood.
  • Contains apolipoproteins used for cholesterol recovery (cleanup of excess cholesterol from blood vessels for excretion).
  • Delivers some cholesterol to steroidogenic tissues.
  • Transfers necessary apolipoproteins to some other lipoproteins.

Apolipoproteins

  • Also referred to as apoproteins.
  • Form the protein component of lipoproteins.
  • Function as receptor molecules and are involved in signaling.

Specific Apolipoproteins

  • APO A-1: Activates LCAT (lecithin-cholesterol acyltransferase), an enzyme that catalyzes cholesterol esterification.
  • APO B-48: Mediates chylomicron secretion.
  • APO B-100: Permits uptake of LDL by the liver.
  • APO C-II: Activates lipoprotein lipase.
  • APO E: Permits uptake of chylomicron remnants and VLDL by the liver.