FATE OF AMINO-ACIDS 



165 



in two doses separated by an interval of about three weeks, im- 

 mediately upon administration of the second dose the animal 

 becomes collapsed and dies. 



The Subsequent History of the Amino-Acids 



Before considering the significance of the above facts it 

 is necessary to trace the metabohsm of nitrogen compounds 

 from the other end — that is, from their ehmination. Much 

 Ught is thrown by a study of the effects upon nitrogen 

 ehmination on variations in the amount of protein absorbed. 

 This is shown in the accompanying table. 



(Folin). 



It will be seen that while creatinine is almost unaffected 

 by diet, urea undergoes a very considerable variation, the 

 other urinary constituents occupying an intermediate posi- 

 tion. These observations led Fohn to distinguish two 

 forms of nitrogen metabohsm. In one form the amino- 

 acids not required for tissue-building are split into ammonia 

 and a nitrogen-free residue. The ammonia is converted 

 into urea, and the non-nitrogenous part is burnt up hke 

 a carbohydrate or fat. This Folin termed " exogenous 

 metabolism." In the other form the amino-acids are 

 taken up by the tissues and incorporated into the 

 structure of the cell. Now since the cell is constantly 



