D. RITTENBERG AND D. SHEMIN 



excreted. The other nitrogen which is excreted must have arisen from 

 other sources, probably the tissue proteins (unpubhshed experiments). 



The phenylalanine-tyrosine conversion was studied in rats by 

 feeding them a normal diet to which was added a small amount of 

 labeled phenylalanine (23) prepared by replacing the normal hydrogen 

 atoms of the benzene ring with deuterium atoms. From the tissues 

 of rats which had been fed this labeled phenylalanine, tyrosine was 

 isolated containing a high concentration of deuterium. Here is a 

 direct proof for the conversion. This transformation of phenylalanine 

 to tyrosine took place, not on a diet deficient in tyrosine and calculated 

 to force its synthesis, but on a diet containing an adequate amount. 

 The conversion of phenylalanine to tyrosine is independent of the 

 dietary composition. Such a reaction could have been detected by 

 no previously known experimental technique. 



The method just described is of course of the broadest generality, 

 but there are some cases in which its experimental application is 

 difficult. Recourse must then be taken to more indirect methods. 

 The feeding to animals of benzoic acid results in the excretion of 

 hippuric acid, benzoylglycine. In experiments in which glycine 

 labeled with heavy nitrogen was fed to rats, it was found that one- third 

 of the glycine used for conjugation with the benzoic acid was of dietary 

 origin and two-thirds was supplied by the proteins of the tissues (27). 

 The feeding of benzoic acid thus results in the excretion of a sample 

 of glycine of the tissues. Conversions of other compounds to glycine 

 may easily be tested by feeding the labeled test substance, together 

 with benzoic acid. The feeding of these compounds which are con- 

 verted to glycine in vivo will result in the excretion of hippuric acid 

 containing a high concentration of N^l Serine gives such a positive 

 result, whereas leucine, alanine, and ethanolamine give negative 

 results (39). The fact that ethanolamine is not converted to glycine 

 indicates that the mechanism for the conversion of serine to glycine 

 does not involve decarboxylation to ethanolamine and subsequent 

 oxidation of the alcohol, but suggests that the /3-carbon atom of serine 

 is split off to yield glycine directly. The proof of the conversion of 

 serine to glycine is not as direct as that of phenylalanine to tyrosine, 

 for only the amino group of serine was labeled and not the carbon 

 chain, it is conceivable that only the amino group was transferred. 

 In control experiments it has been demonstrated that the feeding of 



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