Pancreatic lipase, triacylglycerol digestion

Phospholipids are digested and absorbed in a similar manner to that of triacylglycerol. Pancreatic lipase has some hydrolytic activity towards phospholipids and removes the fatty acid from the 1-position. The product is a lysophospholipid such as lysolecithin (Figure 4.12). It also acts as a detergent and contributes to the stability of the mixed micelles. 

In neonate, suckling mammals, short- and medium-chain fatty acids are preferentially split at the sn-3 triacylglycerol position by oral and gastric lipases and are absorbed in the stomach, while the long-chain fatty acids are hydrolyzed at the sn- and sn-2 positions and by pancreatic lipases and are absorbed in the small intestine (50, 51). With growth, the neonate fat digestion system becomes less active, and is replaced by the small intestine-pancreatic lipase pathway. But residual oral and gastric lipase activities and direct absorption of short-chain fatty acids in the... 

The digestion of triacylglycerols in adult nonruminant mammals has been described as initiated in the mouth by hngual lipase released in the sahva at the base of the tongue (52). Up to 6% of the fatty acids are hydrolyzed and initiate emulsion formation in the stomach. The digesta (called chyme at this location) is released from the stomach slowly into the duodenum to ensure complete mixing with the bile salts and emulsification. Lipolysis occurs by association of pancreatic lipase and co-lipase at the surface of the bile salt-stabihzed emulsion. Amphipathic molecules (fatty acids, sn-2 monoacylglycerols, and lysolecithins) are produced and associate with the bile salts to form water-soluble micelles from which absorption occurs. 

Pancreatic lipase is one of the mammalian key digestive enzymes. It completes the dietary triacylglycerol breakdown initiated by preduode-nal lipases, including lingual and gastric enzymes, (see below). The enzyme is inhibited in the intestine by bile salts, but the activity is restored in the presence of colipase (CLP), a relatively short (95 residues) heat-stable polypeptide secreted by the pancreas (Semeriva and Desnuelle, 1979 Borgstrbm and Erlanson-Albertsson, 1984). The structural details of the interaction of colipase with lipase are described in Section III,C. 

Because they are water-insoluble, triacylglycerols are emulsified in the intestine by bile salts. They are digested by pancreatic lipase to 2-monoglycerides and free fatty acids. 

Pancreatic lipase, with the aid of colipase, digests the triacylglycerols to 2-monoacylglycerols and free fatty acids, which are packaged into micelles. The micelles, which are tiny microdroplets emulsified by bile salts, also contain other dietary lipids such as cholesterol and the fat-soluble vitamins. 

A. Pancreatic lipase catalyzes the breakdown of dietary triacylglycerols into free fatty acids and 2-monoacylglycerols, an essential step in the digestion of dietary lipids. Since prostaglandins are produced from linoleate, an essential fatty add, a deficiency of pancreatic lipase would eventually cause a prostaglandin defidency. 

Bernback, S., Blackberg, L. and Hernell, O. (1989) Fatty acids generated by gastric lipase promote human milk triacylglycerol digestion by pancreatic colipase-dependent lipase. Biochim. Biophys. Acta 1001, 286— 291. 

Digestion and absorption of triacylglycerols with medium-chain fatty acids 12 carbons) proceed by a different pathway. Medium-chain triacylglycerols are partly water-soluble, are rapidly hydrolyzed by lingual and pancreatic lipases, and do not require the participation of bile acids. Some are absorbed intact and hydrolyzed inside the absorptive cell. Medium-chain fatty acids enter the portal... 

After triacylglycerols are emulsified (solubilized) by mixing with bile salts, they are digested by intestinal lipases, the most important of which is pancreatic lipase. The products, fatty acids and monoacylglycerol, are transported into enterocytes and resynthesized to form triacylglycerol. The triacylglycerol molecules, along with newly synthesized phospholipid and protein, are then incorporated into chylomicrons. After the chylomicrons are transported into lymph, via exocytosis, and then blood, they are taken up by peripheral tissues. 

Pancreatic lipase is an enzyme found in intestinal mucosal cells that digests triacylglycerols to a mixture of free fatty acids, glycerol, monoacylglycerols, and diacylglycerols. The enzyme requires calcium and is unusual in catalyzing its reaction at an oil-water interface (Figure 18.6). 

The major enzyme that digests dietary triacylglycerols is a lipase produced in the pancreas. Pancreatic lipase is secreted along with another protein, colipase, along with bicarbonate, which neutralizes the acid that enters the intestine with partially digested food from the stomach. Bicarbonate raises the pH of the contents of the intestinal lumen into a range (pH 6) that is optimal for the action of all of the digestive enzymes of the intestine. Bicarbonate secretion from the pancreas is stimulated by the hormone secretin, which is released from the intestine when acid enters the duodenum. 

There is evidence that the configuration of the constituent triacylglycerols of fats can influence the extent to which they are digested.Thus, palmitate (hexadecanoate) distributed randomly throughout the 1, 2 and 3 positions was found to be less digestible than that which occupied position 2, the favoured position for attack by pancreatic lipase. 

They act as emulsifying agents in preparing dietary triacylglycerols for hydrolysis, by pancreatic lipase, in the process of digestion. 

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