THE METABOLISM OF PROTEIN 615 



The most striking results of the above investigation are thai creatinine 



remains unchanged during starvation, but thai urea l>» mes relatively 



increased. The former must be derived from metabolic pro >ing 



on in the tissues independently of the supply of foodstuff carried to 

 them, whereas the Latter must depend, it' nol entirely, ; ely. 



on the protein content of the food. Creatinine may therefore be called 

 an cud product of endogenous metabolism, and urea an end producl 

 i xogenous metabolism. 



Other metabolites uamely, ammonia, uric acid and the undetermined 

 nitrogen, as well as the ethereal sulphates- must represent procee 

 of metabolism that arc partly exogenous and partly endogenous. 



Having made ourselves acquainted with the general natur< the 



changes thai occur in the nitrogenous metabolites when protein metab- 

 olism is stimulated by the taking of food or depressed by starvation, 

 we may now proceed to take up cadi of the metabolites separately and 

 what other information can be obtained regarding their source and 

 origin in the animal body. 



UREA AND AMMONIA 



For various reasons it is important to consider these two metabolil 

 together. During the intermediary metabolism of the majority of I 

 amino acids, the amino group becomes broken off as ammonia, which 

 immediately combines with the available acids to form neutral ammonium 

 salts. The most available acid for this purpose is carbonic acid; there- 

 fore ammonium carbonate is formed in large amounts. A small prop 

 t i < >n of the ammonia may combine with other acid radicles, roch 

 chlorine, to form ammonium chloride. The fate of these two types 

 salt is very different. The ammonium carbonate becomes quickly trans- 

 formed into urea, whereas the ammonium chloride is excreted in the 

 urine. The proc •' urea formation may therefore bi ridered as 



having the function of preventing the accumulation of ammonium c 

 bonate in the animal body. It is the means by which a harmful substance 

 is converted into an innocuous substance a detoxication pi - . in 

 other words. 



Regarding the natun of tin chemical process involve. 1 in this trans 



formation of ammonium carbonate into urea, referei rmulas 



below will show that the ammonium carbonate thai - rmed by I 

 union of carbonic acid with ammonia, by losing one molecule of water 

 ! noes ammonium carbamate, which by repetition ^>( the pi 



comes transformed into un 



