Chylomicron

Chylomicron structure
ApoA, ApoB, ApoC, ApoE (apolipoproteins); T (triacylglycerol); C (cholesterol); green (phospholipids)

Chylomicrons are large lipoprotein particles that transport dietary lipids from the intestines to other locations in the body.

Function

Chylomicrons transport exogenous lipids to liver, adipose, cardiac and skeletal muscle tissue where their triglyceride components are unloaded by the activity of lipoprotein lipase. Consequently chylomicron remnants are left over which are taken up by the liver.

Stages

There are three stages in the chylomicron's "life cycle":

• Nascent chylomicron
• Mature chylomicron
• Chylomicron remnant

Nascent chylomicrons

Chylomicrons are created by the absorptive cells of the small intestine. They are relatively large, having a diameter of 75 to 1,200nm. These nascent chylomicrons are released by exocytosis from enterocytes into lacteals, lymphatic vessels originating in the villi of the small intestine, and are then secreted into the bloodstream at the thoracic duct's connection with the left subclavian vein.

Nascent chylomicrons are primarily composed of triglycerides (85%) and contain some cholesterol and cholesteryl esters. The main apolipoprotein component is apolipoprotein B-48 (APOB48).

Mature chylomicron

While circulating in lymph and blood, chylomicron exchanges components with high-density lipoproteins (HDL). The HDL donates apolipoprotein C-II (APOC2) and apolipoprotein E (APOE) to the nascent chylomicron and thus converts it into a mature chylomicron (often referred to simply as "chylomicron"). APOC2 is the cofactor for lipoprotein lipase (LPL) activity.

Chylomicron remnant

Once triglyceride stores are distributed, the chylomicron returns APOC2 (but keeps APOE) back to the HDL and thus becomes a chylomicron remnant. APOB48 and APOE are important to identify the chylomicron remnant in the liver for endocytosis and breakdown.