130 



II. DIGESTION AND ABSORPTION OF FATS 



(2) Digestion in the Small Intestine 



Conditions are all normally favorable for a rapid digestion of fats in the 

 small intestine. The chyme reaches the small intestine from the stomach 

 in small installments. When the food being digested is low in fat, the quan- 

 tities of chyme passed through the pylorus are rather large and the passage 

 into the intestine occurs at frequent intervals; the small percentage of 

 fat in the mixture prevents overtaxing of the fat-digesting and fat-absorb- 

 ing mechanisms. On the other hand, when the quantities of fat in the food 

 are high, the passage of the chyme into the small intestine is extended over 

 a long period, and the quantities of chyme passing through the pylorus 

 at any one time are small and the amount of fatty material present in 

 the small intestine is not large enough to overburden the organism. 



a. The Action of Lipases. Lipase is supplied not only by the pancreatic 

 juice but likewise by the intestinal secretion. The intestinal lipase is suf- 

 ficiently potent so that, even in the absence of the more powerful pancreatic 

 lipase, steapsin, most of the fat will be digested. In fact, Angelico and 

 Ligori 639 report that the external pancreatic secretion does not influence 

 the intestinal absorption of lipids. As was indicated earlier, emulsifica- 

 tion is a prerequisite for fat splitting. Since bile salts apparently have 

 the property of lowering surface tension, the formation of an emulsion is 

 facilitated by the presence of these substances. 



It is thought that, as a result of the lipolytic action of the fat-splitting 

 enzymes, a series of intermediate products may arise, namely the di- and 

 monoglycerides. One might assume that the breakdown of 2-oleyl-l,3- 

 dipalmitin could take place in the following stages: 



2-01eyl-l,3-dipalmitin 



1,3-Dipalmitin + Oleic acid 



-> 2-Oleyl-l-palmitin + Palmitic acid 



2-Monoolein + Palmitic acid 



Palmitic acid + 1-Monopalmitin -*• 2-Monopalmitin 1-Monoolein 



Glycerol + Palmitic acid Glycerol + Oleic acid 



Possible Mechanisms of Hydrolysis of 2-01eyl-l,3-dipalmitin (9) 



639 R. Angelico and M. G. Ligori, Bull. soc. chim. biol., SI, 540-543 (1949). 



