CHAPTER II 



THE DIGESTION AND ABSORPTION OF 

 FATS IN THE GASTROINTESTINAL TRACT 



1. Introduction 



The media in which biochemical transformations take place in the animal 

 organism are almost exclusively aqueous. Moreover, the two most im- 

 portant fluids concerned in the transport of nutrients and waste products, 

 namely the blood and urine, dissolve only water-soluble substances. It is 

 therefore apparent that, in contrast to carbohydrates and proteins, which 

 are readily soluble in water as such or as their digestion products, it is 

 only with difficulty that lipids can undergo metabolic changes or be trans- 

 ported in solution by such a medium as the blood. However, the blood 

 does contain small microscopic particles, the chylomicrons, which are dis- 

 crete lipid droplets. It is believed that these elements may offer an ex- 

 planation for one mechanism of transfer of lipid materials which are not 

 capable of transformation into water-soluble compounds. Another pos- 

 sible avenue of transport of such hydrophobic lipids as vitamin A alcohol 

 and ester, the carotenoids, 1 and also the sterols, is in the form of protein 

 complexes. These proteins should probably all be classed as lipoproteins; 

 there is evidence that a variety of such proteins exist which present marked 

 differences as regards solubility in ammonium sulfate solutions. It seems 

 that a considerable degree of specificity exists between the lipid component 

 involved and the protein with which it is combined. 



2. Enzymes Concerned with the Digestion of Lipids 



The predominant enzymes involved in the several phases of lipid metab- 

 olism are classed as ester-hydrolyzing enzymes. Table 1 lists the most 

 common representatives, together with the sites of occurrence, the sub- 

 strates on which they act, and the reaction products which result. 



1 J. Ganguly, N. Krinsky, J. W. Mehl, and H. J. Deuel, Jr., Arch. Biochem. Biophys., 

 38, 275-282 (1952). 



