ISOLATION AND ESTIMATION OF TYROSINE 21 



other amino acids, all of which were tested except tryptophan, oxy- 

 tryptophan and oxyproline, nor to the formation of phenols during 

 hydrolysis. They therefore concluded that, in the absence of any 

 amino acid at present unknown, these values more correctly represent 

 the amounts of tyrosine in proteins than the gravimetric values. 



Abderhalden and^uchs [1913] subsequently maintained that the 

 values for tyrosine by the gravimetric method were more exact than 

 those by the colorimetric method. They showed that tyrosine added 

 to gelatin could be almost entirely Recovered from the mixture of the 

 products of hydrolysis, and that a reaction was given by oxytryptophan 

 and by tryptophan. These reactions are not of a similar colour in- 

 tensity as the reaction with tyrosine. 



Folin and Denis [1913] conceded that their figures may be a little 

 too high, but do not consider that the presence of tryptophan, though 

 it may react after boiling with acids, will cause the great differences 

 observed especially as the higher results are of a similar magnitude with 

 caseinogen and with zein. 



Abderhalden [1913] stated that oxyproline gave a blue colour re- 

 action with the reagent, but seeing that gelatin with 3 per cent, of 

 oxyproline only shows a trace of colour and that other proteins prob- 

 ably contain still smaller amounts of oxyproline, the error due to 

 oxyproline, together with that due to oxytryptophan and tryptophan, 

 is not sufficient to account for the differences. We must therefore 

 conclude that the data by the colorimetric method more nearly repre- 

 sent the actual amount of tyrosine in proteins than the data by the 

 gravimetric method. 



