222 



KoNRAD Block 



The Origin of the Side Chains of Phenylalanine and 

 Tyrosine 



In the experiments with labelled acetate the isotope con- 

 centrations were low in the entire phenylalanine and tyrosine 

 molecules, and therefore more direct products of glucose 

 metabolism must be postulated as carbon sources also 

 for the alanyl side chains of the aromatic amino-acids. In 



Table V 



Isotope Concentrations in Side Chains of Aromatic Amino-Acids and- 



IN Serine, Alanine and Glycine 



particular, pyruvate and alanine, which become labelled 

 under these conditions, cannot be major precursors. On the 

 other hand, carbon from [1-^*C] glucose is incorporated into 

 the alanyl side chains, and, as shown in Table V, is located 

 primarily in the ^ positions. The specific activity found at 

 the ^ positions is about half of that at C^ in ^*C glucose, a value 

 in good agreement with the concept that a three-carbon 

 intermediate of glycolysis had furnished the skeleton for the 

 side chains. 



