u6 SCIENCE PROGRESS 



as leucylglycylgfycine is acted upon, though not leucylglycine. 

 None of these polypeptides was hydrolysed by pepsin. In all 

 probability the chain is not sufficiently long, and it would be 

 interesting to know the result of the action of pepsin upon the 

 recently prepared octapeptide and dodekapeptide. 



The results undoubtedly show which combinations are most 

 likely to occur in the protein, and they are of great importance 

 for their synthesis, as only those polypeptides containing 

 optically active amino acids combined together in the way that 

 trypsin acts upon them need be prepared. 



In the earlier experiments, an extract of dried pancreas was 

 employed as the hydrolytic agent, but in the later experiments 

 pure pancreatic juice, obtained from a pancreatic fistula by 

 Pawlow, was used. There was a striking difference in their 

 behaviour to certain dipeptides — namely, leucylalanine, leucyl- 

 glycine, leucylleucine. These were not hydrolysed by the pure 

 juice, but by the extract of dried pancreas, which would contain 

 the autolytic enzyme as well as the enzyme trypsin, they were 

 hydrolysed. 



These differences of enzymes to polypeptides may show 

 differences in the pancreatic juice of various animals; it is well 

 known that some animals are able to digest substances left 

 untouched by others, and further, that though not hydrolysed 

 by trypsin, they may be attacked and converted into amino 

 acids by the enzymes of other organs such as the liver, spleen, 

 etc., and thus made assimilable by the animal organism. 



Investigations as to whether the various polypeptides are 

 hydrolysed by the organism have been made by Abderhalden and 

 several co-workers, by injecting them and ascertaining whether 

 they were excreted as such or as their constituent amino acids 

 in the urine. These products were not found in the urine, and 

 it is concluded that they are utilised in the same way by the 

 organism as the proteins themselves are in metabolism, i.e. they 

 are split up and their nitrogen excreted as urea. In fact, not 

 only were the optically active compounds utilised, but also the 

 racemic compounds. Slight differences were noticed between 

 dogs and rabbits which did not entirely hydrolyse the racemic 

 products, e.g. dl-leucylglycine. The anhydride of leucylglycine 

 and glycocoll were found in the urine when somewhat large 

 quantities were injected. 



The synthetical polypeptides have also been employed by 



