CHAPTER LXIX 

 THE METABOLISM OF PROTEIN (Cont'd) 



THE END PRODUCTS OF PROTEIN METABOLISM 



Introductory. So far we have approached the problem of protein 

 metabolism by studying the behavior of the absorbed products of pro- 

 tein breakdown, and we have seen that these become gradually assimilated 

 by the tissues and used by them in their metabolic processes. "We have 

 been unable, however, to offer any facts regarding the exact chemical 

 changes which each amino acid undergoes during this process of tissue 

 metabolism. At first sight it might appear an easy matter to collect 

 such information by direct examination of the tissues themselves, either 

 by searching in them for amino derivatives which might be derived from 

 absorbed amino acids, or by studying the changes which occur when 

 the amino acids are subjected to the action of the isolated tissue en- 

 zymes that must be responsible for the change. Such methods of in- 

 vestigation are, however, fraught with technical difficulties so great that 

 very little can be learned from them, and for the present at least we 

 must be content to piece our information together from facts derived 

 by less direct methods. Such a method is offered by investigating 

 the behavior of the end products of protein metabolism. 



The main end product is urea along with traces of its precursor am- 

 monia, but these are not the only ones, for some amino acids after being 

 incorporated with the tissue proteins break down into products that 

 are no longer members of the amino-acid series, although they may be 

 closely related to certain amino ocids. Such substances are creatine and 

 its anhydrid creatinine. A part of the amino acids during their pres- 

 ence in a free state in the blood may also be excreted unchanged by 

 the kidney. Our list so far therefore includes urea, ammonia, creatine, 

 creatinine, and amino nitrogen, of which the last is usually included in 

 metabolism investigations in the fraction designated undetermined 

 nitrogen. 



Another group of closely related substances coming, not from the 

 general protein metabolism of the tissues, but from the metabolism 

 which is peculiar to the nuclei, consists of the so-called purine bodies. 

 Furthermore, so as to serve as a check on results obtained by examining 

 these nitrogenous metabolites, it is important to observe the manner of 



613 



