672 THE PRECURSORS OF UREA 



are excreted in the urine as urea when estimated by Bunsen's 

 method. Salkowski, using his own modification of Bunsen's 

 method, has recently shown that aspartic acid introduced into 

 the stomach of a rabbit is excreted chiefly as urea and not as 

 the corresponding amide, asparagin. But some of the aspartic 

 acid absorbed was excreted not as urea but probably unaltered. 

 Although we can be certain that mono-amino acid absorbed from 

 the alimentary canal is excreted mainly as urea, we are com- 

 pletely in the dark as to how much of the urea has this origin. 



We know but little about the production of mono-amino 

 acids in the tissues. They occur abundantly during the autolysis 

 of organs, and their formation has been ascribed to the action 

 of proteolytic tissue ferments, resembling trypsin. Vernon has 

 lately described tissue ferments which resemble erepsin in their 

 action i.e. they are incapable of attacking most native proteids, 

 but are able to hydrolyse albumoses or peptones into mono- 

 amino acids, ammonia and diamino-acids. He found them 

 most plentiful in the kidney, intestinal mucous membrane, 

 pancreas, spleen, and liver ; but they were present in the heart, 

 muscle, brain, and all other tissues investigated. We are bound, 

 therefore, to believe that the formation of mono-amino acids 

 represents an important step in the disintegration of tissue 

 proteid, and consequently in the ultimate production of urea 

 from proteid. 



We have some evidence how a mono-amino acid is further 

 dealt with in the body in the process of its conversion into 

 urea. We have already dealt with the chemical possibilities 

 exemplified in the views of Hofmeister, Schmiedeberg, and 

 Drechsel. Jacoby found that a water extract of fresh liver 

 tissue mixed with a mono-amino acid converted it into ammonia. 

 Lang greatly extended these observations. He pounded up 

 various organs with salt solution, toluol, and the substance to 

 be investigated. He showed that besides the liver many other 

 organs, such as the kidneys, pancreas, wall of the alimentary 

 canal, &c., were capable of converting the NH 2 group of glycin 

 into ammonia. He further showed that the same was true for 

 many other amido substances, such as tyrosin, asparagin, glutamin, 

 &c. These experiments suggest that the power of tissues to 

 convert the NH 2 group of a mono-amino acid into ammonia is 



