Metabolism of Lipids 9 Sections27 LessonsLifetime
Chylomicrons
Chylomicrons are largest in size and least in density synthesized in epithelial cells of rough ER of intestine.
They are composed of dietary TAGs, phospholipids, cholesterol ester, free cholesterol, Apo Protein B-48 and fat soluble vitamins.
They are called Nascent Chylomicrons.
They go to the lymphatic system and enter in the circulation through left subclavian vein.
In the circulation they take up the ApoC II and ApoE from HDL.
Their structure is completed now.
They have rounded structure with proteins on the outer side while fat is on the inside (protein is hydrophilic while fat is hydrophobic)
In the capillaries of heart, adipose tissues, skeletal muscle, lactating mammary glands lipoprotein lipase is present.
It acts on TAGs of lipoproteins and converts them into 3 fatty acids and glycerol (goes to liver).
Role of Apo C II
ApoC II stimulates Lipoprotein lipase.
Lipoprotein lipase has different isomeric forms in different tissues.
In adipose issue it has high Km value. It will extract TAG when chylomicrons are high in concentration.
But in cardiac muscle it has low Km so it extracts TAGs even in low concentration of chylomicrons
Fate of Chylomicrons
90% of TAG is catabolized and remaining part is called chylomicrons remnants. Their size is reduced.
These ApoC II is given back to HDL but ApoE is retained.
Liver cells have receptors for ApoE.
Chylomicrons when reach liver through the circulation, they are recognized by ApoE by these receptors – endocytosed and then enter lysosomes where degradation occurs.
Cholesterol ester is converted to free cholesterol and fatty acid.
TAG is converted to fatty acid plus glycerol.
