METABOLISM OF PROTEINS 597 



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. 



It has been suggested that arginin and histidin constitute the 

 raw material for the synthesis of the purin ring. Rats will grow 

 when fed with an amino-acid mixture prepared by hydrolyzing 

 caseinogen and lactalbumin, but they rapidly lose weight if arginin 

 and histidin have been removed from the mixture, and the allantoin 

 in the urine decreases. Restoration of the missing amino-acids 

 causes growth to be resumed and a rise in the allantoin excretion to 

 normal. This is true, although in lesser degree, even when one of 

 these amino-acids is restored to the mixture, as they seem to be inter- 

 convertible (Ackroyd and Hopkins). 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 Creatin and Creatinin in Protein Metabolism. 

 A glance at the tables of composition of the urine (p. 477) will show 

 that creatinin, 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. 



Creatin is a-methylguanidin-acetic acid, and creatinin is derived 

 from it by loss of the elements of water : 



/NH.J /NH.J /NH 2 /NH CO 



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



Guanidin. Methylguanidin. Acetic acid. Creatia. Creatinin. 



On heating with baryta-water creatin is decomposed, yielding urea, 

 methylglycocoll or sarcosin, and other substances. It can be prepared 

 synthetically from sarcosin and cyanamid. Thus: 



/NH 2 H.N(CHo) /NH 2 



,C=N +1 = CfNH 2 



CH a .COOH \N(CH 8 ). CH 2 .COOH 



Cyanamid. Methylglycocoll. Creatin. * 



Creatin is found in considerable amount in muscular tissue 

 (much less in children than in adults), and in traces in other 

 tissues and in blood, which also contains small amounts of 

 creatinin. 



Creatinin can be so readily obtained from creatin outside the 

 body that it is tempting to suppose that the portion of the creatinin 



