PHOSPHOLIPIDS 63 



pholipid sjm thesis as measured by the incorporation of P'^ into the phos- 

 pholipid molecules. Their effectiveness is in proportion to the quantity 

 of material administered, ^^^-^^^ and to the relative lipotropic activities of 

 the substances in question. Thus, betaine is less effective in preventing 

 fat accumulation in the liver than is choline. ^^^-^^^ In doses up to 50 mg. 

 per rat, its capacity to stimulate phospholipid synthesis is also less than 

 that of choline. ^^^ Artom and Cornatzer*^^ reported that choline, admin- 

 istered alone, exerted less stimulatory effect upon the formation of in- 

 testinal phospholipids than did choline and fat given simultaneously. 



The administered choline apparently speeds up phospholipid synthesis 

 by being available as one of the essential building stones. Stetten^*^ 

 proved that labeled choline finds its way into the phospholipid molecule, 

 while Welch and Welch'^--^^^ reported that arsenocholine is incorporated 

 into the lecithin molecule to yield arsenolecithin. 



On the other hand, Artom and Swanson^^" reported somewhat contra- 

 dictory experimental results on the stimulatory effect of choline on phos- 

 pholipid synthesis. Although this base did, to a certain extent, increase 

 the rate of synthesis of phospholipids in hver slices from rats previously 

 maintained on low-protein diets, choline actually inhibited phospholipid 

 synthesis in liver slices from rats previously on a high-protein regimen. 

 In the absence of added choline, the rate of phospholipid synthesis as 

 measured by P*^ uptake was lower in liver slices from rats which had re- 

 ceived a stock diet or one containing 25% of casein than in livers from 

 animals which had received a high-protein regimen. Since oxidation is re- 

 quired for the incorporation of inorganic phosphate into liver slices, it is 

 suggested that the depressed rate of phosphate incorporation in liver slices 

 from rats on protein-low diets is related to a decrease in cytoplasmic pro- 

 teins which reduces the proportion of oxidative enzymes. 



The increased phospholipid synthesis was observed in many cases, 

 even though the total phospholipid in the liver showed no measurable 

 change. Since the phosphohpid content of the liver is relatively uniform, 

 the stimulatory effect of choline on phospholipid sjmthesis can be observed 

 only when labeled compounds are employed. ^^^ 



Perlman and collaborators^^'^-^-^ made the interesting observation that, 

 not only did methionine stimulate phosphorylation, but cystine and cys- 

 teine brought about the same reaction. Of these three suKur-containing 



«i8 A. P. Piatt, Biochem. J., S3, 505-511 (1939). 



319 C. Artom and W. E. Cornatzer, /. Biol. Chem., 165, 393-394 (1946). 



320 C. Artom and M. A. Swan.son, J. Biol. Chern., 193, 473-480 (1951). 



3" I. Perlman, N. Stillman, and I. L. Chaikoff, /. Biol. Chem., 135, 359-364 (1940). 



