Chylomicrons

Chylomicrons

Chylomicrons are lipoprotein particles that play a crucial role in the transport of dietary lipids from the intestines to other locations in the body. They are the largest and least dense of the lipoproteins, primarily responsible for the transport of triglycerides, cholesterol, and fat-soluble vitamins absorbed from the diet.

Structure

Chylomicrons are composed of a core of triglycerides and cholesterol esters surrounded by a monolayer of phospholipids, free cholesterol, and proteins known as apolipoproteins. The primary apolipoprotein associated with chylomicrons is ApoB-48, which is essential for their assembly and secretion from the intestinal enterocytes.

Formation and Secretion

Chylomicrons are formed in the enterocytes of the small intestine. After the ingestion of dietary fats, triglycerides are broken down into free fatty acids and monoglycerides by pancreatic lipase. These components are absorbed by the enterocytes, where they are re-esterified to form triglycerides. The triglycerides are then packaged with cholesterol, phospholipids, and apolipoproteins to form chylomicrons.

Once formed, chylomicrons are secreted into the lymphatic system via the lacteals, small lymphatic vessels in the intestinal villi. They eventually enter the bloodstream through the thoracic duct, allowing for the distribution of dietary lipids to various tissues.

Function

The primary function of chylomicrons is to transport dietary lipids from the intestines to peripheral tissues. In the bloodstream, chylomicrons interact with lipoprotein lipase (LPL), an enzyme located on the endothelial surface of capillaries in adipose tissue, muscle, and the heart. LPL hydrolyzes the triglycerides in chylomicrons into free fatty acids and glycerol, which can be taken up by cells for energy production or storage.

As triglycerides are removed, chylomicrons become smaller and denser, eventually transforming into chylomicron remnants. These remnants are taken up by the liver through receptor-mediated endocytosis, primarily via the LDL receptor and the LDL receptor-related protein (LRP).

Clinical Significance

Abnormalities in chylomicron metabolism can lead to various disorders. For example, chylomicronemia is a condition characterized by elevated levels of chylomicrons in the blood, often due to genetic mutations affecting lipoprotein lipase or its cofactors. This can result in pancreatitis, hepatosplenomegaly, and eruptive xanthomas.

See Also

• Lipoprotein
• Triglyceride
• Apolipoprotein
• Lipid metabolism

References

• Ginsberg, H. N., & Brown, W. V. (2011). Lipoprotein physiology. In: Kronenberg: Williams Textbook of Endocrinology, 12th ed.
• Mahley, R. W., & Rall, S. C. (2000). Type III hyperlipoproteinemia (dysbetalipoproteinemia): The role of apolipoprotein E in normal and abnormal lipoprotein metabolism. In: The Metabolic and Molecular Bases of Inherited Disease, 8th ed.