652 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



ethanolamine moiety by decarboxylation; on the other hand, D-serine does 

 not act as a precursor of choline. 661-663 The conversion of L-serine to eth- 

 anolamine is illustrated in (3) . 



HOCH 2 CH(NH 2 )COOH > HOCH 2 CH 2 NH 2 + C0 2 (3) 



Serine Ethanolamine 



Barrenscheen and Papadopoulou 664 called attention to an entirely new 

 and unsuspected factor in choline synthesis. Thus, it was shown that 

 carbohydrate metabolism plays a role in this change. It was found that a 

 very limited synthesis of choline obtains when the liver glycogen is at a 

 low level. 



From this discussion, the reason is now clear why casein serves as a bet- 

 ter source of the lipotropic factor than does a like amount of methionine. 

 Not only is methionine required for the synthesis of choline, but also one or 

 two additional amino acids are involved, i.e. glycine and/or serine, which 

 are likewise supplied by casein; one would therefore expect the most effi- 

 cient synthesis of choline to obtain when all the necessary components are 

 simultaneously present. 



(&') The Role of the Citrovorum Factor in Choline Synthesis: Dubnoff 665 

 first demonstrated that the presence of choline oxidase is essential for the 

 transfer of choline methyl to homocj^steine. Thus, the transfer of this 

 group does not occur unless the alcohol group of choline has been oxidized ; 

 this oxidation requires the intermediation of the enzyme, choline oxidase. 

 Dubnoff believes that the extent of utilization of choline methyl is regu- 

 lated by this enzyme system. Muntz 666 confirmed the hypothesis that cho- 

 line oxidase is necessary for transmethylation. 



Folic acid has been shown to play an important part in the functioning of 

 choline oxidase. Dinning, Keith, and Day 667 suggest that this vitamin 

 functions as a constituent of the prosthetic group of this enzyme. It was 

 likewise observed by Dinning et al. m that the choline oxidase activity of 

 monkey liver and of the bone marrow of chickens was decreased when 

 aminopterin was injected. The authors explain this effect as due to dis- 

 placement of folic acid in the choline oxidase molecule by aminopterin. 



661 H. R. V. Arnstein, Biochem. J., 47, xviii-xix (1950). 



662 H. R. V. Arnstein, Biochem. J., 48, 27-32 (1951). 



663 S. Jansson and W. A. Mosher, J. Am. Chew. Soc, 72, 3316 (1950). 



664 H. K. Barrenscheen and D. Papadopoulou, Z. physiol. Chem., 284, 236-242 (1949). 



665 J. W. Dubnoff, Arch. Biochem., 24, 251-262 (1949). 



666 J. A. Muntz, J. Biol. Chem., 182, 489-499 (1950). 



667 J. S. Dinning, C. K. Keith, and P. L. Day, Arch. Biochem., 24, 463-464 (1949). 



668 J. S. Dinning, C. K. Keith, P. L. Davis, and P. L. Day, Arch. Biochem., 27, 89-93 

 (1950). 



