ABSORPTION AND TRANSPORT OF FATS 163 



sumed 679 - 766 " 768 necessarily proves the synthesis of triglycerides in the in- 

 testinal cells. It is suggested that the synthesis may as well be assumed 

 to take place in the lumen of the intestine under the influence of lipase as 

 within the epithelial cell. Several investigators 33 - 34 - 59 have proved the 

 in vitro synthesis of fat from glycerol and fatty acid in the presence of lipase ; 

 it would therefore be reasonable to assume a similar in vivo synthesis. 645 

 The evidence 677 of the appearance of fatty acids in the intestinal cells within 

 30 minutes after absorption starts, followed after six hours by the presence 

 of neutral fat alone in these cells, might as well be indicative of an immediate 

 absorption of fatty acids followed by a later absorption of neutral fat as of 

 the conversion of the fatty acids to triglycerides within the cell during this 

 period. 623 On the other hand, Reiser and Williams 768 reported that 73% 

 of the monoglycerides administered were hydrolyzed; they suggested that 

 the hydrolysis of monoglycerides and the synthesis of triglycerides both 

 occur in the intestinal mucosa. These workers 768 observed that palmitoxy- 

 hydroxyacetone and monopalmitin were hydrolyzed in the gut in a similar 

 manner. The fatty acid esters of dihydroxyacetone did not appear in the 

 lymph as such, but only after reduction of the ketone group and conversion 

 of the molecule to a triglyceride. It is postulated that esterification of the 

 fatty acid with dihydroxyacetone, followed by its reduction and esterifica- 

 tion, may be the normal method of triglyceride synthesis during the ab- 

 sorption of fatty acids. An enzyme which oxidizes glycerol to dihydroxy- 

 acetone was isolated from the intestinal bacillus, Escherichia coli, by Asnis 

 and Brodie. 769 This glycerol dehydrogenase is relatively heat-stable, and 

 requires diphosphopyridine nucleotide. 



Even if one accepts the Particulate Theory as the explanation for fat 

 absorption, one must still assume the synthesis of triglycerides in the epi- 

 thelial cell under certain conditions. For example, mono- and diglycerides, 

 which are formed in the small intestine and which are presumably absorbed 

 as such, must be reconverted to triglycerides. The experiments of Argyris 

 and Frank 766 demonstrate that the change occurs before these compounds 

 pass into the lacteals; these workers isolated triglycerides from the chyle 

 after the feeding of monoglycerides. It has already been shown that, 

 when soaps, fatty acids, and ethyl esters of the fatty acids are fed, they are 

 converted to triglycerides (see page 154), inasmuch as no free fatty acids 

 are detected in the chyle or in the depot fats. The intestinal cell would 

 seem to be the most logical site for this transformation. 



766 A. Argyris and O. Frank, Z. Biol, [2], 41 {59), 143-164 (1912). 



767 J. Mul'ler and H. Murschhauser, Biochem. Z., 78, 63-96 (1917). 



768 R. Reiser and M. C. Williams, J. Biol. Chem., 202, 815-819 (1953). 



769 R. E. Asnis and A. F. Brodie, /. Biol. Chem., 203, 153-159 (1953). 



