Structure and Composition
Like all lipoproteins, chylomicrons are built on a micellar principle. They are spherical particles with a diameter greater than 120 nm, divided into two main zones:
- Hydrophobic core: Contains non-polar molecules, predominantly triacylglycerols (accounting for up to 85% of the particle mass), along with cholesterol esters and fat-soluble vitamins.
- Hydrophilic shell: Consists of a monolayer of phospholipids, unesterified cholesterol, and protein components known as apolipoproteins.
Apolipoproteins play a crucial role in metabolism. The structural backbone of the chylomicron is provided by the integral protein apoB-48, which is synthesized exclusively in the intestine. The surface also features peripheral proteins (apoC-II, apoE) acquired later once the particle enters the bloodstream.
Transport and Maturation Stages
The lifecycle of a chylomicron can be divided into several sequential stages:
- Synthesis of nascent particles: Enterocytes package resynthesized dietary fats together with apoB-48. The formed nascent chylomicrons enter the lymphatic system.
- Entry into the bloodstream: Via the thoracic duct, the particles enter the blood. In the postprandial period (after a meal), their large size and high concentration can cause blood serum to appear turbid (opalescent).
- Maturation: In the bloodstream, nascent chylomicrons interact with high-density lipoproteins (HDLs). HDLs act as donors, transferring apoC-II (an activator of fat-cleaving enzymes) and apoE (essential for hepatic uptake) to the chylomicron. Once these proteins are acquired, the chylomicron is considered mature.
Metabolism and Clearance
Mature chylomicrons travel through blood vessels to peripheral tissues (primarily adipose and muscle tissue). The endothelial lining of capillaries in these tissues expresses the enzyme lipoprotein lipase (LPL).
The apoC-II protein on the surface of the chylomicron activates this enzyme. Consequently, lipoprotein lipase hydrolyzes the triacylglycerols within the particle. The breakdown products are distributed as follows:
- Fatty acids enter tissue cells (oxidized for energy in muscles, or stored as fat in adipose tissue).
- Glycerol remains in the plasma and is transported to the liver.
As lipids are depleted, the chylomicron shrinks in size and transforms into a chylomicron remnant. It returns apoC-II back to HDL while retaining apoE and cholesterol. The liver recognizes these remnants via apoE receptors and endocytoses them. Inside hepatocytes, lysosomal enzymes completely degrade the remnants, releasing cholesterol and remaining fatty acids.