lipoprotein metabolism

1. Overview of Lipoproteins

  • Definition: Soluble lipid-protein complexes that transport triglycerides and cholesterol in the blood.

  • Structure:

    • Hydrophobic core: Contains triglycerides and cholesterol esters.

    • Amphipathic monolayer: Composed of phospholipids, cholesterol, and apolipoproteins.

  • Function: Transport lipids to tissues for energy, storage, or synthesis.

2. Classification of Lipoproteins (Based on Density)

  1. Chylomicrons: Lowest density; transport dietary(exogenous) triglycerides.

  2. Very Low-Density Lipoproteins (VLDL): Transport endogenous triglycerides.

  3. Intermediate-Density Lipoproteins (IDL): Transition state between VLDL and LDL.

  4. Low-Density Lipoproteins (LDL): Transport cholesterol to tissues; “bad cholesterol.”

  5. High-Density Lipoproteins (HDL): Transport cholesterol to the liver for excretion; “good cholesterol.”

3. Chylomicrons

  • Composition: 85–92% triglycerides, 6–12% phospholipids, 1–3% cholesterol, and 1–2% proteins.

  • Function:

    • Transport dietary lipids from intestines to tissues.

    • Triglycerides are unloaded by lipoprotein lipase, leaving remnants for liver uptake.

  • Stages:

    • Nascent Chylomicrons: Assembled in intestinal cells.

    • Mature Chylomicrons: Acquire apoproteins in the blood.

    • Chylomicron Remnants: Transport cholesterol and fat-soluble vitamins to the liver.

4. Very Low-Density Lipoproteins (VLDL)

  • Origin: Synthesized in the liver from triglycerides.

  • Function: Transport endogenous triglycerides to tissues.

  • Conversion:

    • VLDL → IDL (loss of triglycerides).

    • IDL → LDL (cholesterol-rich particles).

5. Low-Density Lipoproteins (LDL)

  • Function: Transport cholesterol from the liver to peripheral tissues.

  • Clinical Significance:

    • Excess LDL leads to cholesterol accumulation in arterial walls, forming plaques.

    • Elevated LDL is a significant risk factor for cardiovascular diseases.

6. High-Density Lipoproteins (HDL)

  • Function:

    • Reverse cholesterol transport: Collects cholesterol from tissues and delivers it to the liver.

    • Source of apolipoproteins for chylomicrons and VLDL.

  • Clinical Significance:

    • Elevated HDL levels are associated with decreased cardiovascular risk.

7. Pathways of Lipoprotein Metabolism

  1. Exogenous Pathway:

    • Involves chylomicrons and dietary lipid transport.

  2. Endogenous Pathway:

    • Involves VLDL, IDL, and LDL; focuses on liver-derived lipids.

  3. Reverse Cholesterol Transport:

    • Mediated by HDL, maintaining cholesterol homeostasis.

8. Influences on Plasma Lipoproteins

  • Gender: HDL levels are higher in females; triglycerides are higher in males.

  • Age: LDL and triglycerides increase with age.

  • Exercise: Increases HDL, decreases LDL and triglycerides.

  • Obesity: Decreases HDL, increases triglycerides.