The human plasma lipoprotein containing the highest percentage of triacylglycerol by weight is
CHYLOMICRONS Synthesis of chylomicrons Chylomicrons are formed in the intestinal mucosal cells, and secreted into the lacteals of lymphatic system (Chapter 11). They are rich in triglyceride (Fig. 12.9). If lipemic serum is kept overnight in the refrigerator, chylomicrons rise as a creamy layer to the top, leaving the subnatant clear. When the chylomicrons are synthesised by the intestinal mucosa, they contain only apo-B-48 and apo-A but apo-C and apo-E are added from HDL in blood during transpo (Fig. 12.10). Metabolism of Chylomicrons i. Main sites of metabolism of chylomicrons are adipose tissue and skeletal muscle. The half-life of chylomicrons in blood is about 1 hour. ii. The enzyme lipoprotein lipase (LpL) is located at the endothelial layer of capillaries of adipose tissue, muscles, and hea; but not in liver. Apo C-II present in the chylomicrons activates the LpL (Table 12.4). The LpL hydrolyses triglycerides present in chylomicrons into fatty acids and glycerol. Muscle or adipose tissue cells take up the liberated fatty acids (Fig. 12.10). iii. Following injection of heparin, the LpL is released from the tissues and lipemia is thus cleared. This is called post-heparin lipolytic activity. Lack of C-II leads to decreased activity of LpL and consequent accumulation of chylomicrons and VLDL in blood. Insulin increases LpL activity. Liver Takes up Chylomicron Remnants As the TAG content is progressively decreased, the chylomicrons shrink in size. These remnants containing apo-B-48 and apo-E are taken up by hepatic cells by receptor-mediated endocytosis. Apo-E binds the hepatic receptors (Fig. 12.10). Function of Chylomicrons Chylomicrons are the transpo form of dietary triglycerides from intestines to the adipose tissue for storage, and to muscle or hea for their energy needs.Ref: DM Vasudevan - Textbook of Biochemistry, 8th edition, page no: 153
VLDL
Chylomicrons
HDL
LDL
Metabolism of lipid