Long chain triglycerides (LeT)
The mechanical and chemical actions ofthe stomach release the food fat from the protein and carbohydrate. Fat enters the duodenum in a coarse, unstable emulsion. Two digestive juices essential for normal lipolysis of fat are secreted into the upper duodenum, bile and the pancreatic juice. The mechanical action of the intestine and the emulsifying capacity of the bile salts and phospholipids of the bile allow formation of a finely divided stable emulsion. The emulsified fat droplets consist mainly of triglycerides and of lesser amounts of diglycerides and fatty acids. They form what is known as the oil phase, which is dispersed in the bulk of the intestinal contents, the water phase.
The enzymatic hydrolysis oftriglycerides takes place at the oil-water interphase by pancreatic lipase (glycerol ester hydrolase) present in the water phase. The finer the emulsion, the larger the surface area available for the enzyme action; an ample supply of lipase is normally available, making the accessibility to the substrate molecules the determining factor for the rate of lipolysis. Pancreatic juice also contains a peptide molecule known as colipase. Colipase increases the activity of pancreatic lipase, probably by strengthening its binding to the substrate. The role of colipase appears to be especially important in the presence of a high concentration of bile salt which, although essential for the emulsification of dietary fat, also weaken or prevent the binding of lipase to its substrate at the water-oil interface and thereby inhibit triglyceride hydrolysis. This inhibition is prevented by colipase.
The intestinal digestion of long chain triglycerides consists of two essential steps: hydrolysis and solubilization ofthe end products. The pancreatic lipase releases the fatty acids esterified at carbons 1 and 3 of the triglyceride glycerol (also known as a-positions) .The resulting free fatty acids and 2-monoglycerides (remaining fatty acid in ,B-position) are solubilized and removed from the site of hydrolysis through formation of micelles with conjugated bile salts. Small amounts of other lipids are also found in these micelles, including cholesterol, phospholipids, and the fat-soluble vitamins. Though the micelles are aggregates of molecules, they are much smaller than the emulsified triglyceride droplets, and they form a clear dispersion in the aqueous medium in the lumen (water phase). The major luminal events of LCT digestion and absorption.
Micelle formation allows the monoglycerides and fatty acids to make close contact with the absorptive surfaces of the epithelial brushborder by facilitating their diffusion through the unstirred wa ter layer. The minimun level of bile salts required to accomplish complete solubilization of the insoluble monoglycerides and fatty acids is known as the critical micellar concentration (CMC) of bile salts. Below this level some of the end products ofhydrolysis remain in the oil phase, and the entire process of digestion and absorption is slowed down.
LCTs are absorbed into the epithelial cell as monoglycerides, fatty acids, and glycerol. Glycerol is produced by limited hydrolysis of micellar monoglycerides, probably by intestinal lipase. About two-thirds to threefourths of the dietary LCTs are absorbed as mono glycerides. The exact mechanism by which they enter the epithelial cell is not known, but the available evidence points to passive diffusion across the lipoprotein membrane.
At the site of absorption the bile salts separate from the rest of the micellar components and move farther down the intestinal lumen to the ileum, where about 95% are reabsorbed. They pass through the mucosal cell and into the capillaries of the portal system, which carries them back to the liver. If the bile salts are deconjugated in the intestine (by bacteria) before reabsorption, they, reconjugated in the liver and, along with the newlysyntl sized bile salts, are secreted into the gallbladder ready be released once more into the duodenum as needed. Trecycling is known as the enterohepatic circulation oft salts.
The small fraction of bile salts lost daily in the fece: replaced by synthesis from cholesterol in the liver. Activi ty represen ts the major means of cholesterol remo from the body and can be increased by administration certain drugs, such as cholestyramine, which binds 1 bile salts, preventing their reabsorption and thereby creasing the conversion of cholesterol in to bile acids in 1 liver. The net result is a reduction in blood cholesterol hypercholesterolemic people.
The intracelluar phase in the absorption ofLCT is major importance. The entering fatty acids a monoglycerides become mixed with those already present.In the cell and lose their "identity." They also must undergo reconversion into triglycerides before leaving the cell. The major pathway involves the direct acylation of the monoglycerides, and its activity responds to the changes in the dietary in take of fa 1.
The newly synthesized triglycerides are "packaged" in to lipoprotein particles known as chylomicrons. The core of these particles consists of triglyceride and some cholesterol. They are covered with a protein-phospholipid "wrapping," which allows them to be dispersed in water. The Golgi complex of the mucosal cell appears to be the site for the completion of chylomicrons before their release. Some of this dietary or exogenous triglyceride is incorporated into very low densi ty lipoproteins (VLDL) by the epithelial cells, although this blood lipoprotein fraction is mainly knOVII1 as the carrier of endogenous triglycerides synthesized in the liver.
Both lipoprotein particles are then released from the cells and enter the lacteals, the small vessels of the lymphatic system in the lamina propria. The contents of the lymph reach the left subclavian vein through the thoracic duct and, from there, the various sites ofutilization in the body.
