160 II. DIGESTION AND ABSORPTION OF FATS 



ever, if phosphorylcholine is the residual portion of lecithin set free to serve 

 as the matrix for synthesis of a new phospholipid molecule, the hypothesis 

 has little support. Riley 767 found that administered phosphorylcholine 

 was readily converted to inorganic phosphate. It should not be inferred, 

 therefore, that the phosphoric acid -f- nitrogen base complex enters and 

 leaves the phospholipid molecule as a stable unit. 



(a) Phospholipids as Obligatory Components in Fat Synthesis. Although 

 there is no question that the synthesis of phospholipids is an important func- 

 tion of the intestinal mucosa, it is still debatable whether the newly syn- 

 thesized phospholipids represent obligatory intermediates in the formation 

 of the triglyceride molecule from fatty acids and glycerol (or from mono- or 

 diglyeerides), or whether the phospholipids are to be considered to be 

 terminal products which are necessary in the transport and further me- 

 tabolism of fat. 



There have been several reports which suggest that phosphorylation is a 

 prerequisite in the synthesis of triglycerides. 554 - 702 - 758 Reiser, 759 using mono- 

 and triglycerides in which both the fatty acids and glycerol were tagged 

 with C 14 , interprets his experiments as indicating that fats are converted to 

 monoglycerides, from which phospholipids are formed in the intestinal mu- 

 cosa; the phospholipids so formed are then converted to triglycerides. 

 However, Frazer 645 states that, irrespective of whether or not resynthesis 

 occurs in the intestinal cell, the concept that phosphorylation activates it is 

 open to question. For example, the reversible nature of lipolysis has been 

 demonstrated in vitro, and neither change requires an intermediate phos- 

 phorylation. Moreover, Frazer 645 questions the validity of proof of the 

 importance of phosphorylation in fat absorption based upon the interfer- 

 ence in the utilization of this foodstuff caused by monoiodoacetate or 

 phlorhizin. 701 The doses of the inhibiting agents were believed to be ex- 

 cessively large, and were reported to produce extensive destruction of 

 intestinal mucosa. 760-762 



Moreover, although phospholipid synthesis does occur in the intestinal 

 mucosa, recent experiments have indicated that it does not proceed at a 

 sufficient rate to account for the synthesis of all the fat passing through the 

 intestinal cells. Although the phospholipid content of intestinal and tho- 



757 R. F. Riley, J. Biol. Chem., 153, 535-549 (1944). 



758 R. G. Sinclair, Oil and Soap, 16, 70-74 (1938). 



759 R. Reiser, Federation Proc, 12, 257 (1953). 



760 Simon and White, personal communication to K. A. KlinghofTer, /. Biol. Chem., 

 126, 201-205 (1938), p. 204. 



761 K. A. Klinghoffer, /. Biol. Chem., 126, 201-205 (1938). 



762 R. Ohnell and R. Hober, J. Cellular Comp. Physiol, IS, 161-174 (1939). 



