METABOLISM OF CHYLOMIKRONS AND VERY LOW-DENSITY LIPOPROTEINS

The liver supplies peripheral tissues with lipoproteins containing triglycerides, a source of energy, and cholesterol, a structural component of cell membranes. The liver is also capable of taking up lipoproteins and excreting cholesterol in the bile. Because of this unique metabolic role the endocytosis of lipoproteins by the liver was studied. Lipoproteins bind through their component proteins to the low density lipoprotein-receptor of hepatocytes. Apolipoprotein B of low density lipoproteins exhibits fairly low affinity, whereas apolipoprotein E of the postlipolytic remnants of very low density lipoproteins and chylomicrons bind with much higher affinity, which results in a much shorter residence time of these lipoproteins in plasma. Premature uptake is prevented by the C-apolipoproteins, yet. Changes in the phospholipid composition of the lipoproteins during the intraplasmatic lipolysis lead to loss of C-apolipoproteins enabling interaction of apolipoprotein E with the receptors of the liver. Binding is followed by endocytosis into endosomes, which fuse to multivesicular bodies. The contents gets catabolized by hydrolytic enzymes of lysosomes thus becoming available to the cell.