IV. BIOCHEMICAL SYSTEMS 27 



radioactivity in the methyl groups of choline or of methionine isolated from 

 tissues of rats after the administration of €'■* precursors indicates that an 

 appreciable transfer of carbon from methanol/^' ^'^' ^* formaldehyde,^^' ^^' ^° 

 formic acid,*^"^^- ^^' ^'- acetone,^^ and serine^- ■ ^'^ occurs. Formate is not the 

 only precursor of the /3-carbon of serine, according to Kruh0ffer.^^ In his 

 studies rat liver homogenates transferred glycine but not formate to serine, 

 although the conversion of formate to serine did occur with liver slices. 

 Although no evidence has been presented for the transmethylation of the 

 intact methyl group of methanol,*'^ the administration of sodium deuterio- 

 C^^-formate subcutaneously in the rat was followed by the isolation of tissue 

 choline containing C'Mabeled methyl with no detectable loss of deuterium. ^^ 

 This finding suggests that in the formate-to-methyl synthesis one hydrogen, 

 at least, remains attached to carbon in the intermediate forms. The in- 

 corporation of C^"* into the /3-carbon of serine occurs more rapidly from form- 

 aldehyde than from formate or methanol,^" but this is not necessarily evi- 

 dence that formaldehyde is the precursor of the methyl group. Jonsson and 

 Mosher^* found the C^"* label in both the carbon chain and in the methyl 

 of liver choline after the administration of ;S-C^^-labeled serine in the rat. 

 Reid and Landefeld^^ and Toporek et al.^^' ^^ have presented evidence 

 that histidine is an important dietary source of the carbon of methyl groups 

 of choline and creatine. L-Histidine-2-C^^ was fed to rats, and the labeled 

 carbon was found not only in tissue choline and creatine methyls but also 

 in the aminoethanol portion of choline. In the work of Toporek et al.,^^ the 

 transfer to choline was enhanced in choline deficiency both in the intact 

 animal and in isolated perfused cirrhotic livers of rats with chronic choline 

 deficiency. Interestingly, these authors have calculated the histidine con- 

 tent of diets used by Griffith and Mulford^"^ and by Rose et al}°^ and have 

 concluded that this amino acid played an important role in their experi- 



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'' S. Jonsson and W. A. Mosher, J. Am. Chem. Soc. 72, 3316 (1950). 

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»6 C. Ressler, J. R. Rachele, and V. du Vigneaud, J. Biol. Chem. 197, 1 (1952). 

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