IV. BIOCHEMICAL SYSTEMS 429 



In addition, formate can give rise to the methyl carbon of choline, and the 

 side chain of choline may he derived from serine so that by the administra- 

 tion of a-taggod glycine to a rat it is possible to obtain from the tissues 

 choline with all its carbon atoms labeled. Formaldehyde has been reported 

 to be even more effective than formate in giving ri.se to the /3-carbon of 

 serine.'^ The carbon atoms in the purine ring in the 2 and 8 positions, in 

 the methyl group of methionine, and in the 5-methyl group of thymine 

 have all been shown to be capable of being derived from formate carl)on. 

 The experimental work in this field has recently been summarized in tabular 

 form by Shive^^ (Table I). Vitamin B12 and folic acid have been implicated 

 as coenzjane-like factors in many of these chemical reactions. 



Gl\'cine can give rise to formate in biological systems, and formate can 

 provide the carbon atom for the methyl groups of choline, methionine, 

 and thymine, for the 2 and 8 positions of the purine ring, and for the 

 /3-carbon of serine. It therefore follows that any enzyme system which aids 

 in the production of formate from glycine or with the sul)sequent utilization 

 of formate will be implicated in the biological systhesis of those compounds 

 into which the carbon atom of formate is incorporated. Vitamin B12 and 

 p-aminobenzoic acid have been shown by Shive^^ to be involved in E. coli 



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