THE METABOLISM OF PROTEIN 615 



The most striking results of the above investigation are that creatinine 

 remains unchanged during starvation, but that urea becomes relatively 

 increased. The former must be derived from metabolic processes going 

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

 them, whereas the latter must depend, if not entirely, yet very largely, 

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

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

 exogenous metabolism. 



Other metabolites namely, ammonia, uric acid and the undetermined 

 nitrogen, as well as the ethereal sulphates must represent processes 

 of metabolism that are partly exogenous and partly endogenous. 



Having made ourselves acquainted with the general nature of the 

 changes that 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 each of the metabolites separately and 

 see 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 metabolites 

 together. During the intermediary metabolism of the majority of the 

 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 propor- 

 tion of the ammonia may combine with other acid radicles, such as 

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

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

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

 urine. The process of urea formation may therefore be considered as 

 having the function of preventing the accumulation of ammonium car- 

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

 is converted into an innocuous substance a detoxication process, in 

 other words. 



Kegarding the nature of the chemical process involved in this trans- 

 formation of ammonium carbonate into urea, reference to the formulas 

 below will show that the ammonium carbonate that is formed by the 

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

 becomes ammonium carbamate, which by repetition of the process be- 

 comes transformed into urea : 



