4 6 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



In the earlier experiments by Fischer and Bergell it was found that 

 leucyl-alanine was hydrolysed by an extract of pancreas ; it was not 

 however hydrolysed by pure pancreatic juice. Such extracts probably 

 contain other enzymes, more especially the autolytic enzyme, which 

 produce the hydrolysis; the later work of Abderhalden and his co- 

 workers upon the action of enzymes from various organs also show that 

 polypeptides not hydrolysed by pure tfypsin are attacked by these 

 enzymes (see table, p. 48). 



II. The Action of Pepsin. 



Amino acids have been described by various authors as occurring 

 together with the proteoses and peptones in a pepsin digest of proteins. 

 One might have expected that pepsin would act upon certain of the 

 synthetical compounds, especially those most easily hydrolysed by trypsin. 

 Pure pepsin, prepared by Pawlow, had however no action upon glycyl-1- 

 tyrosine, leucyl-alanine, leucyl-leucine, dialanyl-cystine, leucyl-glycine, 

 and one must conclude that the chain of amino acids is not yet suffici- 

 ently long to allow of attack by pepsin. The amino acids obtained by 

 the digestion of proteins probably arise by the action of other enzymes 

 contained in the enzyme solution employed. Another explanation may 

 be that pepsin acts upon other combinations of amino acids than those 

 which are hydrolysed by trypsin. 



///. The Action of Other Enzymes. 



Not only are the synthetical polypeptides hydrolysed by the enzyme 

 of the pancreas, but they are also hydrolysed by the enzymes occurring in 

 the animal body. 



It was found by Abderhalden and Bergell, in 1903, that glycyl-glycine 

 when subcutaneously introduced into a rabbit was converted into glycine 

 which appeared in the urine, whereas glycine if administered in a 

 similar way was completely burnt up and was not excreted. Abder- 

 halden and Rona subsequently showed that glycyl-1-tyrosine was burnt 

 up in the organism of the dog when injected into the system, and Abder- 

 halden and Samuely observed that this was also the case when cystine, 

 dialanyl-cystine and dileucyl-cystine were subcutaneously introduced. 

 Abderhalden continued these investigations with Teruuchi, with the 

 result that the organism of the dog was found to be able to completely 

 utilise glycyl-glycine, alanyl-alanine and diglycyl-glycine as well as the 

 diketopiperazines, glycine anhydride and alanine anhydride, when they 

 were given by the mouth, just as the animal can utilise proteins or 

 amino acids, the nitrogen contained in these substances being eliminated 

 as urea. To this series of polypeptides capable of being utilised by the 



