150 II. DIGESTION AND ABSORPTION OF FATS 



phosphatides is at the border of the intestinal villi. It is believed that the 

 synthesis of phospholipids from glycerylphosphorylcholine and fatty acids 

 is promoted by pancreatic lecithinase. The suggestion is made that the 

 glycerylphosphorylcholine does not leave the cellular membrane, where it is 

 bound to proteins, but that, through continual synthesis of phospholipids 

 from it, and subsequent hydrolysis in situ to glycerylphosphorylcholine and 

 fatty acids, the latter compounds are transported through the cell mem- 

 brane. This hypothesis presents an explanation for the mechanism of 

 phospholipid in fatty acid absorption analogous to that of Schramm and 

 Wolff, 715 which ascribes this same role to cholesterol, and to that of Ver- 

 zar, 554 who suggested a similar behavior on the part of the bile salts. 



The results of Williams 716 also indicate that phospholipid synthesis is 

 one of the most important considerations involved in fat absorption. 

 On the basis of the histological investigation of frozen sections from differ- 

 ent levels of the intestine of the rat, obtained after olive oil had been fed, 

 it was shown that the first evidence of fat absorption was the deposition of 

 phospholipids between the nucleus and the free border of the columnar 

 epithelial cells. It was reported that, immediately beneath the free border, 

 there was a clear zone containing phosphatase; this is presumably the site 

 of phosphorylation. The next stage in fat absorption involves the enlarge- 

 ment of the vacuoles of the Golgi complex and the deposition on them of 

 the phospholipids. They are then^ transformed into fats and fatty acids. 

 As the fat increases in quantity, it breaks away from the Golgi complex. 

 The cytoplasm becomes filled with fine globules, which apparently coalesce 

 and pass into the core of the villus. 



In spite of these positive results, which indicate the importance of phos- 

 phate in the absorption of fats, the recent studies of Zilversmit, Chaikoff, 

 and Entenman 717 have cast some doubt on the correctness of this assump- 

 tion. These workers found that, in the dog, neither the amount nor the 

 rate of turnover of intestinal phospholipids was influenced by the absorp- 

 tion of cream, corn oil, or corn oil fatty acids. Although some increases in 

 the rate of turnover of the phospholipid fraction were noted in the rat dur- 

 ing fat absorption, they were too small in relation to all the fat passing 

 through this stage. These results lead the authors to state that "fat can 

 pass through the intestinal wall without involving phospholipide as an 

 obligatory intermediate." 



715 G. Schramm and A. Wolff, Z. physiol. Chem., 263, 61-72 (1940). 



716 T. D. Williams, J. Physiol, 108, 30P (1949). 



717 D. B. Zilversmit, I. L. ChaikofT, and C. Entenman, J. Biol. Chem., 172, 637-650 

 (1948). 



