Chilomicroni

Chilomicroni, or chylomicrons in English, are the largest lipoprotein particles in human plasma. They are formed in the enterocytes of the small intestine from dietary fats absorbed from the gut lumen and are secreted into the lymphatic system before reaching the bloodstream, contributing to postprandial lipid transport.

Chylomicrons are triglyceride-rich particles with a core of triglycerides and cholesterol esters surrounded by a surface of phospholipids, cholesterol, and apolipoproteins. They contain apolipoprotein B-48 as the essential structural protein and acquire ApoC-II and ApoE from HDL in circulation. They are the least dense of the lipoproteins and are large in size, reflecting their high triglyceride content.

Formation and secretion: dietary lipids are emulsified and hydrolyzed in the intestine; long-chain fatty acids and monoglycerides are re-esterified into triglycerides within enterocytes. These triglycerides are packaged with cholesterol esters and ApoB-48 by microsomal triglyceride transfer protein into prechylomicrons, which mature and are secreted into the lymphatic system via the basolateral membrane, eventually entering the bloodstream after the thoracic duct. They acquire ApoC-II and ApoE from circulating HDL.

Metabolism: in capillaries of adipose and muscle tissue, lipoprotein lipase (LPL) hydrolyzes chylomicron triglycerides to free fatty acids, enabling tissue uptake. Chylomicron remnants, enriched in cholesterol esters and ApoE, are cleared by the liver through receptor-mediated endocytosis, primarily via ApoE receptors.

Clinical relevance: defects in chylomicron metabolism cause hyperchylomicronemia (type I hyperlipoproteinemia), marked by elevated postprandial triglycerides and pancreatitis risk. Secondary factors include diabetes, obesity, and excessive alcohol intake.