NITROGENOUS COMPOUNDS 127 



made by Loewi. This investigator allowed protein to 

 digest until it no longer gave a reaction with the biuret 

 test an indication that products possessed of a protein 

 nature had all been reduced to a lower stage. Upon 

 feeding this mixture of amino acids together with 

 fat and carbohydrate to a dog, Loewi demonstrated 

 that nitrogen in the form contained in his digestion 

 mixture was not alone capable of maintaining the life 

 of the animal, but furthermore kept it in a state of 

 nitrogenous equilibrium, a retention of nitrogen to- 

 gether with an increase in weight being observed. 

 The experiments of Loewi were quickly followed by 

 those of Henderson and Dean, who were the first to 

 employ digestion products formed through the action 

 of acids rather than by ferments. They found nitro- 

 gen retention but were uncertain whether it signified 

 protein synthesis. As a result of many succeeding 

 investigations it soon became clear that the power of 

 the digestion products to replace body tissue depended 

 upon the manner in which such products were formed. 

 To put it differently, digestion products formed from 

 protein by the agency of enzymes were fully capable 

 of supplying to the body its necessary quota of nitro- 

 gen whereas those products obtained from proteins 

 through the action of acids could not take over so 

 completely this function but were regarded as of 

 value inasmuch as they could be looked upon as being 

 "protein sparers." In this connection it may be well 

 to cite some experiments of Abderhalden and Rona 

 with mice. To these mice were fed different prepara- 



