158 II. DIGESTION AND ABSORPTION OF FATS 



depend not entirely upon the rate at which this foodstuff can be trans- 

 ferred through the cell wall into the epithelial cells, but also upon the speed 

 with which these cells become cleared of fat so that additional quantities 

 can be received from the lumen of the gut. The rate of absorption will 

 thus be controlled by the slower of these two processes. 



a. The Breakdown of the Fatty Acid + Bile Salt Complex in the Intes- 

 tinal Cells. As already discussed in an earlier section (see pages 91 to 

 114), according to the Lipolytic Theory, the fatty acid is carried into the 

 epithelial cells as a fatty acid + bile salt complex. If this theory is the 

 correct one, it is necessary that this combination be broken down to allow 

 the fatty acid to be present in such a form as to facilitate its conjugation 

 with glycerol to form fat. 



Although there is no certain evidence as to the mechanism which may 

 accomplish this reaction, Verzar and Kuthy 745 - 746 suggested two possibilities. 

 In the first place, the fatty acid + bile salt complex would be expected to 

 disintegrate if the interior of the mucosal cells had an acid reaction (i.e., 

 below a pH of 6.2). A clear oleic acid/glycocholic acid solution can be 

 shown by in vitro tests to become turbid when the pH is reduced to 6 by 

 a few drops of 0. 1 A" HC1. A second possibility to account for the disinte- 

 gration of the bile salt + fatty acid complex within the cells is that the 

 complex is negatively charged; the fatty acids may be precipitated by posi- 

 tively charged ions within the cells. Hemoglobin has been shown to pre- 

 cipitate oleic acid at a pH below 6.7, since it has a positive charge. A 

 mechanism of this type is apparently related to the so-called "coacerva- 

 tion" of fatty substances. 746 On the disintegration of the bile salt com- 

 plex, the bile acid is presumably returned to the intestine for further func- 

 tional activity in fat absorption. 



b. The Synthesis of Phospholipids in the Intestinal Mucosa. All evi- 

 dence points to the fact that phospholipids are readily synthesized in the 

 intestinal cells. This synthesis has been demonstrated by histological 

 methods 747 as well as by the use of labeled phosphate 713 - 748 " 752 or fatty 



acids 702,711,713,753-755 



746 F. Verzar and A. Kuthy, Biochem. Z., 225, 267-277 (1930). 

 74 «A. Kuthy, Klin. Wochschr., U, 308-309 (1935). 



747 L. Lison, Histochimie animate, Gauthier-Villars, Paris, 1936. 



748 G. Hevesy and L. Hahn, Kgl. Danske Videnskab. Selskab, Biol. Medd., 15, No. 4, 

 1-60(1940). 



749 C. Artom, G. Sarzana, and E. Segre, Arch, intern, physiol, 47, 245-276 (1938). 



760 C. Entenman, S. Ruben, I. Perlman, F. W. Lorenz, and I. L. Chaikoff, /. Biol. 

 Chem., 124, 795-802 (1938). 



761 B. A. Fries, S. Ruben, I. Perlman, and I. L. Chaikoff, ./. Biol. Chem., 123, 587-593 

 (1938). 



