Structure and composition
Like all lipoproteins, chylomicrons are built on a micellar principle. They are spherical particles with a diameter of over 120 nm, featuring two distinct zones:
- Hydrophobic core: contains non-polar molecules, mainly triacylglycerols (accounting for up to 85% of the total particle mass), as well as cholesteryl esters and fat-soluble vitamins.
- Hydrophilic shell: consists of a monolayer of phospholipids, free cholesterol, and protein components—apolipoproteins.
Apolipoproteins play a crucial role in metabolism. The structural framework of the chylomicron is provided by the integral protein apoB-48, which is synthesized directly in the intestine. The surface also contains peripheral proteins (apoC-II, apoE) that the particle acquires later, once in the bloodstream.
Stages of transport and maturation
The life cycle of a chylomicron can be divided into several consecutive stages:
- Synthesis of immature particles. Resynthesized fats are packaged together with the apoB-48 protein within enterocytes. The formed immature chylomicrons enter the lymphatic system.
- Entry into the bloodstream. Particles enter the blood via the thoracic duct. In the postprandial period (after a meal), due to their large size and massive quantity, blood serum may become turbid (opalescent).
- Maturation. In the bloodstream, immature chylomicrons interact with high-density lipoproteins (HDL). HDL act as donors, transferring apoC-II (an activator of the fat-splitting enzyme) and apoE (essential for hepatic uptake) proteins to chylomicrons. After this, the chylomicron is considered mature.
Metabolism and utilization
Mature chylomicrons travel through blood vessels to tissues (predominantly adipose and muscle tissue). The enzyme lipoprotein lipase (LPL) is located on the capillary endothelium of these tissues.
The apoC-II protein on the surface of the chylomicron activates this enzyme. As a result, lipoprotein lipase hydrolyzes the triacylglycerols inside the particle. The breakdown products are distributed as follows:
- Fatty acids enter tissue cells (in muscles, they are oxidized for energy, while in adipose tissue, they are stored as a reserve).
- Glycerol remains in the plasma and is transported to the liver.
As lipids are depleted, the chylomicron decreases in size and transforms into a remnant chylomicron (remnant). It returns apoC-II back to HDL while retaining apoE and cholesterol. The liver recognizes such remnant particles via apoE receptors and takes them up by endocytosis. Inside hepatocytes, lysosomal enzymes ultimately degrade the remnants, releasing cholesterol and remaining fatty acids.
Briefly: answers to topical questions
Which specific receptors do hepatocytes use to capture remnant chylomicrons?
Hepatocytes capture remnant chylomicrons using LDL receptors and LRP receptors.
The process is carried out via specific ligands on the surface of remnant particles:
- apoE — the main ligand recognized by hepatic receptors;
- apoB.
Upon binding to these receptors, remnant chylomicrons are engulfed via endocytosis. Inside hepatocytes, the particles fuse with lysosomes, and their components undergo hydrolysis by intracellular enzymes.
Are chylomicrons present in the blood of a healthy fasting person?
Chylomicrons are absent from the blood of a healthy fasting person.
Normally, mature chylomicrons enter the bloodstream only during the postprandial period—that is, after a meal—to transport exogenous dietary fats. Detecting chylomicrons in fasting blood is a sign of pathology involving impaired triacylglycerol hydrolysis (e.g., lipoprotein lipase or apolipoprotein C-II deficiency). In this case, blood serum appears turbid and may form a "fatty layer" on the surface.
Chylomicron apolipoproteins are easy to remember by their roles: B-48 is the "Internal Builder" (structural), C-II is the "Cutter" (activates lipase, which "cuts off" fatty acids), E is the "Enters the liver" (ligand for hepatic receptors).
Class and exam questions
How do chylomicrons differ from VLDL?
Chylomicrons are formed in the intestine and transport exogenous (dietary) fats. VLDLs are synthesized in the liver and transport endogenous fats produced within the body itself.
Why can blood become turbid after a meal?
In the postprandial period, chylomicrons massively enter the bloodstream. Due to their large diameter (over 120 nm), the serum acquires an opalescent, turbid appearance.
Which enzyme hydrolyzes fats within chylomicrons in the blood?
This is lipoprotein lipase (LPL), located on the capillary walls. Its function strictly requires the activator protein apoC-II.
What else to study on this topic
- Comparison of composition and functions of all lipoprotein classes
- The process of fat resynthesis in enterocytes
- Role and metabolism of high-density lipoproteins (HDL)
- Intracellular degradation of remnant chylomicrons in hepatocytes
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