IV. BIOCHEMICAL SYSTEMS 43 



ethanol. Stetten-"^ fed N^^-labeled serine to rats and found the N^^ in tissue 

 phospholipids and proteins, in cystine, and in aminoethanol. Levene and 

 Tarver^o^ observed a rapid uptake of C^'^-jS-carbon-labeled serine in tissue 

 proteins and the appearance of the C^* in aminoethanol in rats fed the 

 labeled serine. Similiar evidence has been presented bj^ other workers.^^ ■ ** 

 94, 210 jn ^j^g experiments of Weissbach et al.,'^^° rats fed serine, doubly labeled 

 by C'* in the ,8-carbon and by N^^ converted the amino acid into choline- 

 containing C^* in the alcoholic carbon and N^^ This would be expected by 

 the decarboxylation of serine to aminoethanol and by the methylation of 

 the latter. 



In so far as the carbon chain of choline is concerned, the evidence is 

 strong for a cycle as described in Fig. 1. In theory at least, the nitrogen- 

 carbon combination of choline might remain in the cycle indefinitely, but 

 with each complete series of reactions a new second carbon would enter the 

 cycle, entering as the /S-carbon and leaving as carbon dioxide from the car- 

 boxyl carbon of serine. As far as is kno^\^l the glycine-serine reaction is the 

 only reversible reaction of consequence among those concerned with the 

 carbon chain of choline (compounds 1 to 10, Fig.l). 



6. Oxidation of Methyl Carbons 



The rapid appearance of radioactive carbon dioxide in the expired air 

 of rats after the feeding of C^'^-methyl-labeled methionine clearly demon- 

 strated the oxidizability of the labile methjd group.^^^ A similar result fol- 

 lowed the intraperitoneal injection of C^''-labeled choline, betaine, 

 dimethylthetin, and dimethylpropiothetin.212-214 jn each of the latter experi- 

 ments the highest level of radioactivity of carbon dioxide was found in the 

 first 4-hour period. Less choline was oxidized, but this may only have been 

 a reflection of its normal use in lecithins and in other metabolites. Reference 

 has already been made to the appearance of the labeled j8-carbon of serine 

 after administration of methyl-labeled choline or methionine. For this rea- 

 son, it is reasonable to assume, as indicated in Fig. 1, that formate is an 

 intermediate in the catabolism of a labile methyl and that the formate car- 



208 D. Stetten, Jr., /. Biol. Chem. 144, 501 (1942). 



2»9 M. Levine and H. Tarver, /. Biol. Chem. 184, 427 (1950). 



^lo A. Weissbach, D. Elwyn, and D. B. Sprinson, /. Am. Chem. Soc. 72, 3316 (1950). 



2" C. G. Mackenzie, J. P. Chandler, E. B. Keller, J. R. Rachele, N. Cross, D. B. 



Melville, and V. du Vigneaud, J. Biol. Chem. 169, 757 (1947). 

 2'2 C. G. Mackenzie, J. P. Chandler, E. B. Keller, J. R. Rachele, N. Cross, and V. 



du Vigneaud, /. Biol. Chem. 180, 99 (1949). 

 2'3 C. G. Mackenzie, J. R. Rachele, N. Cross, J. P. Chandler, and V. du Vigneaud, 



J. Biol. Chem. 183, 617 (1950). 

 ^'^ M. F. Ferger and V. du Vigneaud, J. Biol. Chem. 185, 53 (1950). 



