Phenylalanine and Tyrosine Synthesis 223 



Two other amino-acids isolated from yeast proteins, serine 

 and glycine, also do not appear to be closely related meta- 

 bolically to acetate and intermediates of the citric acid cycle. 

 Serine and glycine contained relatively low isotope concentra- 

 tions when labelled acetate was the precursor (Table V), 

 and therefore the major source of carbon for these two amino- 

 acids must be confined to the same group of compounds which 

 were considered for the side chains of the aromatic amino- 

 acids. Moreover, in the experiments with [1-^^C] glucose the 

 isotope distribution and the level of ^*C in the ^ positions are 

 similar for serine and for the side chains of phenylalanine and 

 tyrosine, pointing to a com.mon precursor. 



The very low concentrations of isotopic carbon in glycine, 

 both in the experiments with labelled acetate and with [1-^*C] 

 glucose, agree well with what is currently known about the 

 metabolic relationship of serine and glycine. The breakdown 

 of serine to glycine by removal of the ^ carbon atom, as 

 demonstrated by Shemin (1946) will, under the conditions of 

 our experiments with labelled acetate, involve a precursor 

 which already contains very little isotope in the carboxyl and 

 a positions. When glycine is formed from serine which had 

 been derived from [1-^^C] glucose, the highly labelled ^ carbon 

 will be eliminated and a product with a low ^*C concentration 

 will result. 



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