108 THE AMINO ACIDS 



in this way. The question has been tested experiment- 

 ally in an indirect manner. If protein with a certain 

 amino acid lacking is fed to animals it is reasonable 

 to assume that if nitrogenous equilibrium can be main- 

 tained a synthesis of the missing amino acid must have 

 occurred. From experiments planned to test this 

 hypothesis it has been shown that no amino acids with 

 the exception of glycocoll are ordinarily formed by 

 synthesis. For glycocoll the evidence is strongly indic- 

 ative of synthesis. As a rule in the body there is 

 about 5 per cent of glycocoll nitrogen in every 100 

 grams of protein nitrogen. It is well known that 

 benzoic acid ingested is united with glycocoll to form 

 hippuric acid in other words, benzoic acid feeding 

 robs the body of glycocoll. If benzoic acid is fed in 

 sufficient quantities to exhaust the possible content of 

 glycocoll preformed in the tissues, the continued forma- 

 tion of hippuric acid must be provided for by glycocoll 

 newly formed or synthesized. Hippuric acid does con- 

 tinue to be formed under these circumstances and 

 hence glycocoll must be synthesized. It is possible, of 

 course, that glycocoll may be formed from the trans- 

 formation of some other amino acid, as by cleavage of 

 a long change amino acid. Another evidence in favor 

 of the synthesis of glycocoll is the following milk 

 proteins are very poor in glycocoll, yet suckling ani- 

 mals are capable in a short time of building up in their 

 bodies proteins which contain far more of this amino 

 acid than can be accounted for by the ingestion of 

 glycocoll yielded by the milk. 



