U4 THE PHYSIOLOGY OF PROTEIN METABOLISM 



be considered as unwearable, like the jewelled bearing of a watch? 

 Such a conception hardly can be true, for, if the work be carried out 

 under unfavourable conditions, there is soon evidence of use in the in- 

 creased output of nitrogen in the urine. Why then, under absolutely 

 normal conditions, is there little or no evidence of use? Either (I) 

 the actual wastage of protein is small in amount ; the protein tissue 

 as it is broken down separates into two distinct portions, one of which, 

 the non-nitrogenous part, is used solely for dynamic purposes, whereas 

 the nitrogen-containing moiety is reutilized resynthetized within 

 the body ; or (2) the protein tissue is actually broken down, but an 

 equivalent amount of nitrogen is taken from the food supply to re- 

 place that wasted, with the result that there is little increase in the 

 amount of nitrogen excreted. 



Which of these two hypothesis approximates more closely to the 

 true condition is well-nigh impossible to state. The first presupposes 

 only a small requirement of protein for the body. It fits in ex- 

 tremely well with the facts observed in connexion with the output 

 of endogenous nitrogenous waste products, and in those feeding ex- 

 periments where the daily intake of protein is small. The small 

 increase in the output of nitrogen during work and the apparently 

 small endogenous exchange do apparently balance one another. 



The second hypothesis has little or no experimental evidence to 

 support it. It seems to me it would practically entail the acceptance 

 of Pfliiger's statement that the body needs are satisfied mainly by pro- 

 tein. As already pointed out, there is direct evidence that the energy 

 needs of the body are not solely supplied from a protein source during 

 work. It would also involve the acceptance of the statement that the 

 amount of nitrogen excreted represents exactly the amount of protein 

 catabolism which has taken place in the tissues. Such a belief cannot 

 now be adhered to. Evidence is steadily growing which shows that 

 all the nitrogen ingested is not necessarily converted into urea with 

 subsequent rapid excretion, but that part of it may be retained in the 

 body, either by resynthesis into fresh tissue protein, or in some simple 

 form not yet definitely understood. 



Although resynthesis certainly takes place, still every case of 

 nitrogen retention cannot be attributed to it without further evidence. 

 Thus Kovalevsky and Markevidz (228) have shown that when am- 

 monium carbonate is introduced into the blood stream of dogs it 

 rapidly disappears the ammonia content of the blood soon returns to 

 normal. The question is what is the fate of this ammonia ? Salaskin 

 (349) P ut forward the hypothesis that ammonia can be stored, or fixed, 



