Phenylalanine and Tyrosine Synthesis 



217 



acid will be lower to an extent which is determined by the 

 relative rates of the cycle reactions and of CO2 fixation. Some 

 labelled pyruvate will be formed by decarboxylation of 

 oxaloacetate, but its quantity is probably small compared to 

 that of non-isotopic pyruvate originating directly from 

 glucose. The isotope concentrations in alanine will therefore 

 be reduced still further. These relationships are illustrated 

 in Fig. 1. 



PYRUVI^C ACID 

 °CH. 



GLUCOSE = 



ALANINE 



°CH, 

 ol ^ 

 =^ HCNH, 



COOH 



ASPARTIC ACID 

 tOCH 



"«-KETOGLUTARICAClD GLUTAMIC ACID 



,CH,COOH 



CH-COOH 



I ^ 

 CH, 



I ^ 

 HCNH, 



I ^ 

 COOH 



HO-C-COOH 



^CHo 



*COOH 

 CITRIC ACID 



COOH 



*COOH 

 ACETIC ACID 



Carbon atoms derived from glucose marked ° 

 Carbon atoms derived from acetate marked 



Fig. 1. Probable pathway of incorporation of acetic acid into 



benzenoid amino acids. 



The non-participation of acetate, pyruvate and inter- 

 mediates of the citric acid cycle in the synthesis of phenylala- 

 nine and tyrosine limits the possible carbon sources for these 

 amino-acids to glucose itself, or to intermediates of glycolysis. 

 For the identification of the precursors, [1-^^C] glucose seemed 

 a suitable and relatively accessible substrate. The yeast 

 growth experiment carried out with this labelled glucose was 

 in all respects identical with earlier ones, except that the 

 acetic acid in the medium was labelled by ^^C only (Table 

 III). In this experiment the isotope distribution in pheny- 

 lalanine and tyrosine was of primary interest. 



