116 SCIENCE. PROGRESS 
as leucylglycylglycine 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, ze. 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, eg. 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 
