VLDL is formed in
Chylomicrons are found in "chyle" formed only by the Lymphatic system draining the intestine. However, it is now realised that a smaller and denser paicle similar to VLDL is now known to be synthesised in small quantity in intestinal cells also. Chylomicrons formation fluctuates with the load of TG absorbed, whereas VLDL formation is quantitatively less, but is more constant and occurs even in the fasting state. However, bulk of plasma VLDL is of hepatic origin, being the vehicle for transpo of TG from Liver to extrahepatic tissues (carrier of "endogenous" TG). There are many similarities in the mechanism of formation of chylomicrons by intestinal epithelial cells and VLDL by hepatic parenchymal cells. Salient Features * Chylomicrons and VLDL are synthesised in intestinal mucosal cells and Liver cells respectively. Polysomes on rough endoplasmic reticulum (ER) of these tissues synthesise apo-B48 and apo-B100 respectively. * TG is synthesised in smooth endoplasmic reticulum (ER) of both tissues. * Microsomal and cytoplasmic enzymes paicipate in cholesterol synthesis. * In the smooth endoplasmic reticulum (ER) of intestinal mucosal cells and Liver cells, the lipids are incorporated with apo-B48 and apo-B100 to form chylomicrons and VLDL respectively. * B-apoproteins are finally glycosylated in the Golgi- complex, which then packs the lipoproteins into secretory vesicles and budded off from Golgi cisternae. Release: Chylomicrons and VLDL are released from either the intestinal or hepatic cell by fusion of the "secretory vacuole" with the cell membrane, i.e. by reverse Pinocytosis. Chylomicrons pass into the spaces between the intestinal cells, eventually making their way into the Lymphatic system ("Lacteals") draining the intestine. VLDL are secreted by hepatic parenchymal cells into the "Space of Disse" and then into the hepatic sinusoids.Ref: Textbook of Medical Biochemistry, Eighth Edition, Dr (Brig) MN Chatterjea, page no: 446-447
Intestine
Liver
From chylomicrons
Blood
Metabolism of lipid