Reception, emulsification, lipolysis, solubilization, and absorption
secreted by the palate, although its contribution to lipolysis in adults is questionable and thought to be more important in young suckling infants, in which its release is stimulated by suckling and the presence of milk. It is possible that this lingual lipase is carried into the stomach, where it acts as a human gastric lipase (HGL) that has been shown to degrade up to 10% of ingested fat. Although these early products of fat digestion, fatty acids and monoacylglycerols, rep-resent a relatively minor component of fat digested, their entry into the duodenum is believed to supply a major stimulus for the production of the hormone cholecystokinin (CCK), which inhibits gut motility.
The stomach serves mainly as an organ of mechan-ical digestion and, by churning its contents, produces a coarse creamy emulsion known as chyme. The cir-cular pyloric sphincter muscle that separates the stomach from the duodenum and, with other factors, controls the rate of gastric emptying opens twice a minute to release approximately 3 ml of chyme. Since emulsified fat in chyme is less dense than the aqueous material, the two fractions separate with the fat col-lecting above the aqueous layer. As a result, the entry of emulsified fat into the duodenum is delayed, allow-ing sufficient time for the minor breakdown products to act on CCK.
The duodenal phase involves the breakdown of the emulsified fat by a process known as lipolysis and the solubilization of the products of lipolysis. The entry of chyme containing minor lipolytic products into the duodenum stimulates the:
●release of CCK, which inhibits gut motility
●secretion of bile acids from the gall bladder
●release of pancreatic juice containing a battery of lipases.
Figure 6.4 Reception, emulsification, lipolysis, solubilization, and absorption of fats. ACAT, acyl-CoA-cholesterol acyltransferase; MAG, mono-acylglycerol; TAG, triacylglycerol; PL phospholipid; P, phosphate.
Lipolysis is an enzyme-catalyzed hydrolysis that releases fatty acids from lipids (TAGs, phospholipids, and CEs). It involves the hydrolytic cleavage of bonds between a fatty acid and the glycerol backbone of TAGs and phospholipids, and cholesterol in CEs, and occurs not only in the digestive tract but also in cir-culating and intracellular lipids (Figure 6.4). The hydrolysis of emulsified dietary fat entering the duo-denum is catalyzed by a battery of pancreatic enzymes including a pancreatic lipase that acts chiefly on TAG and phospholipase A2 and a cholesterol ester hydro-lase acting on phospholipids and CEs. The hydrolysis of TAG by pancreatic lipase occurs in a sequential fashion with the initial removal of a fatty acid from position 1 and then position 3 from the glycerol back-bone, generating a 2,3-diacylglycerol, followed by a 2-monoacylglycerol (2-MAG).