104 



SCIENCE PROGRESS 



remainder is only obtained after the removal of the glutamic acid 

 as hydrochloride. 



The ester method has not led to the discovery of a large 

 number of new amino acids ; to this category proline and 

 oxyproline belong. The possibility that proline is a secondary 

 product formed by the action of acid on another amino acid 

 is negatived by the fact that it results when the proteins are 

 hydrolysed by alkali. The great use of the ester method has 

 been to separate the amino acids, and to determine their great 

 variety in all the proteins; formerly, only those existing in 

 large amounts, such as tyrosine, leucine, aspartic acid, were 

 isolated. The ester method also gives larger amounts. Alanine, 

 serine, and phenylalanine, which was only once isolated by 

 Schulze from plant proteins, were seldom found, but now their 

 universal presence in all proteins is definitely established. 

 Phenylalanine, in its distribution, is the chief aromatic com- 

 pound, since tyrosine does not occur in all proteins, and it 

 often exceeds in amount that of tyrosine when both exist in the 

 same protein. 



These results can be seen at a glance by referring to the 

 following table, which also shows the various amounts of 

 individual amino acids in percentages, as they result on the 

 hydrolysis of proteins : 



Not only has Fischer hydrolysed proteins by acids and by 

 alkali, but also by the enzymes pepsin and trypsin. The latter 

 gives essentially the same products as acids, but its action is 



