Lipid absorption and intestinal lipoprotein formation

Lipid absorption is a complex process which involves coordinated gastric, intestinal, biliary and pancreatic function. Emulsification of dietary lipid occurs in the stomach and upper intestine where a series of enzymic events also occur. Phospholipids are digested by phospholipases. Colipase anchors lipase to the emulsion surface overcoming the interfering effect of bile salts. The products of lipolysis, monoglycerides and fatty acids, are removed from the emulsion surface by bile salts in the form of mixed micelles which transport lipid digestive products across the unstirred water layer to the epithelial cell. Within the intestinal epithelial cell a series of synthetic events occur resulting in the formation of chylomicrons and very low density lipoprotein (VLDL). Chylomicrons consists of an oily core of triglyceride surrounded by a membrane of phospholipids, free cholesterol and apoproteins which maintain the solubility of the particle in plasma. Chylomicrons from both experimental animals and man have specific apoproteins associated with them. These proteins include apoA-I, the major protein of plasma high density lipoproteins. During chylomicron metabolism, apoA-I and phospholipid from the chylomicron surface contribute to plasma high density lipoproteins. Other chylomicron apoproteins include apoB and apoA-IV, which are synthesized in the intestine, and apoC and apoE which are absorbed onto the chylomicron surface from other lipoproteins. The intestines also synthesizes, very low density sized particles (VLDL) while fasting and during lipid absorption. There is evidence that the intestine also synthesizes high density lipoproteins. The intestine has recently been recognized as a major site of synthesis of plasma lipoproteins and apoproteins, especially apoA-I for plasma high density lipoproteins.