LIPID STORAGE UNDER ABNORMAL CONDITIONS 651 



transfers the methyl groups necessary for transforming homocysteine into 

 methionine, as shown in (l). 652 



HSCH 2 CH 2 CH(NH 2 )COOH +CH> > CH 3 SCH 2 CH 2 CH(NH 2 )COOH ( 1 ) 



Homocysteine Choline Methionine 



It was demonstrated by Simmonds, du Vigneaud, and co-workers 657 that 

 this change actually takes place. The deuteriomethionine isolated after 

 the feeding of homocystine and deuteriocholine contained the labeled 

 methyl group, thus proving that it had been transferred from the choline 

 molecule to that of homocysteine. 



On the other hand, the lipotropic action of methionine is concerned with 

 the reverse action, namely the synthesis of choline from precursors such as 

 aminoethyl alcohol and methionine, the latter serving as a donor of methyl 

 groups. That methionine can act in this capacity has been proved by du 

 Vigneaud and his group, 658 ' 659 who fed methionine labeled with deuterium in 

 the methyl group; the choline subsequently isolated from the tissue phos- 

 pholipids contained sufficient deuterium in the methyl groups to suggest 

 that they had originated from methionine. 



(a') The Importance of Ethanolamine and of Serine in Choline Synthe- 

 sis: Ethanolamine is the structure from which choline is derived by the 

 process of methylation, as has been adequately proved by Stetten. 660 When 

 ethanolamine or choline containing N 15 was fed to rats, phosphatides con- 

 taining choline rich in the nitrogen isotope resulted. It was also shown that 

 ethanolamine was readily formed from glycine ; the latter amino acid orig- 

 inates from betaine. The series of reactions shown in (2) occur. 



Betaine > glycine > ethanolamine > choline 



/ 

 methyl ► methyl 



Schematic diagram to illustrate the synthesis of 



choline from betaine, glycine, or ethanolamine 660 (2) 



Serine serves as another source of the ethanolamine molecule necessary 

 as the precursor of choline. Thus, L-serine is readily converted to the 



667 S. Simmonds, M. Cohn, J. P. Chandler, and V. du Vigneaud, J. Biol. Chem., 149, 

 519-525(1943). 



668 V. du Vigneaud, J. P. Chandler, M. Cohn, and G. B. Brown, J. Biol. Chem., 134, 

 787-788(1940). 



659 V. du Vigneaud, M. Cohn, J. P. Chandler, J. R. Schenck, and S. Simmonds, /. Biol. 

 Chem., 140, 625-641 (1941). 



660 De W. Stetten, Jr., J. Biol.Chem., 140, 143-152 (1941). 



