116 



A. I. RINGER 



the highway of protein metabolism through which most of the amino acids 

 have to travel while on their catabolic path. 



Protein Metabolism 



The studies of the metabolism of proteins date back to the days of 

 Bischoff and \ r oit, in the middle of the last century, when it was recog- 

 nized that the nitrogen excreted in the urine was derived from the catabol- 

 ized proteins. Twenty-four hours are usually considered the unit of time 

 for a protein metabolism experiment. Analysis is made of all the ingested 

 food and of all the excreta. By determining the amount of nitrogen 

 and multiplying that figure by 6.25, the protein factor is obtained. If 

 the amount of nitrogen in the excreta, urine and feces, is equal to the 

 amount of nitrogen in the food, we speak of the individual as being in 

 a state of nitrogenous equilibrium. If there is less nitrogen excreted in 

 the urine and feces than was ingested, the individual has stored some 

 of the ingested nitrogen in the body. We therefore speak of his being in 

 positive nitrogen balance. If, on the other hand, more nitrogen is ex- 

 creted in the urine and feces than was ingested in the food, the individual 

 must have lost nitrogen from his body, and we speak of that as his being 

 in a negative nitrogen balance. 



If an animal or human individual is allowed to fast for a long period of 

 time, we find that nitrogen is excreted in the urine throughout the entire 

 period of the fast up to the moment of death. This shows that protein 

 destruction goes on in the body irrespective of any protein ingestion in 

 the food. The amount of nitrogen excreted in the urine gradually di- 

 minishes in amount, in all probability due to the gradual depletion in the 

 mass of the body proteins. Thus in the experiments by E. and O. Freund 

 (1901) on Succi they obtained the following results: 



TABLE IV 



