586 METABOLISM, NUTRITION AND DIETETICS 



more than one ferment concerned in its transformation. It would 

 be well, therefore, not to speak of uricolysis as if it were synonymous 

 with the well-ascertained process by which allantoin is formed 

 from uric acid, and not to identify all enzymes which may take 

 part in uricolysis with uricoxydase. 



It is worthy of remark in this connection, as a further illustration 

 of the differences which may exist in the purin metabolism in 

 different kinds of animals, that in man and the anthropoid apes the 

 quantity of purin bases in the urine is small in proportion to the 

 quantity of uric acid. In the pig, which is included among the 

 animals that form allantoin from uric acid, the purin bases exceed 

 the uric acid in amount, whereas in the dog, which likewise excretes 

 allantoin, the purin bases exist in very small amount compared 

 with the uric acid. 



In concluding our consideration of the metabolism of the nucleic 

 acids, the question may be raised whether it is related to the metabo- 

 lism of the other substances carbo-hydrates, fats, and proteins 

 in such a way that derivatives of nucleic acid can contribute to the 

 formation of any of these, or derivatives of carbo-hydrates, fats, or 

 proteins contribute to the formation of any of the components of 

 nucleic acid. It has been already mentioned that the phosphoric 

 acid can aid in the synthesis of phosphatides, and that the carbo- 

 hydrate groups probably take their place in the ordinary carbo- 

 hydrate metabolism. There is no evidence that the purin bases can 

 take part or can yield products capable of taking part in the forma- 

 tion of any of the other substances. The purin metabolism, so far 

 as is known, moves in a closed circuit. Of the fate of the pyrimidin 

 bases nothing is surely known. Without doubt nucleic acid can be 

 formed in the body when none is contained in the food. More than 

 one source of the phosphoric acid and the carbo-hydrate are known 

 and have been already pointed out, but how and from what materials 

 the purin and pyrimidin bases are formed cannot yet be stated. 

 Recently the synthesis of nucleosides has been accomplished in the 

 laboratory (Fischer). It only needs the introduction of phosphoric 

 acid in the appropriate way into the molecule to give nucleic acid. 



The Significance of Kreatin and Kreatinin in Protein Metabolism. 

 A glance at the tables of composition of the urine (p. 471) will show 

 that kreatinin, as regards the quantity excreted, is a much more 

 important product of nitrogenous metabolism than uric acid, stand- 

 ing, indeed, with the ammonia compounds, next in order to urea; 

 but our information as to its source and significance is very scanty. 



Kreatin is a-methylguanidin-acetic acid, and kreatinm is derived 

 from it by loss of the elements of water : 



/NH 2 /NH 2 /NH 2 /NH CO 



C^NH q=NH CH 3 .COOH cf NH C=NH 



\NH 2 \NH-CH 3 a \N(CH 3 ) .CH 2 .COOH \N(CH 3 ) .CH 2 



Guanidin. Methylguanidin. Acetic acid. Kreatin. Kreatinin. 



