EXCRETIONS 495 



of creatin in the vertebrate has been suggested by Riesser(6), who has pre- 

 sented evidence in experiments on rabbits suggesting that both the 

 creatin content of the muscle and the creatinin elimination are increased af- 

 ter the administration of these substances. Myers and Fine(/) found that 

 the creatin content of the muscle of rats was very slightly increased (2.5 

 per cent) as a result of feeding with edestin, a protein relatively rich in 

 arginin. Bauman and Hines(fc) have perfused arginin, sarcosin, methyl- 

 guanidin, betain and cholin through dog muscle (hind leg) without 

 obtaining conclusive evidence of their being creatin formers. 



Uric Acid. Uric acid results from the cleavage and oxidation of 

 nucleoprotein, which is the chief constituent of all cell nuclei. Nucleo- 

 protein is split into protein and nucleic acid. When the nucleoprotein 

 is present in the food, this process takes place in the alimentary tract 

 under the influence of trypsin ; when the body cells are the source of the 

 nucleoprotein this transformation takes place in the tissues probably 

 through the agency of a similar enzyme. The protein fraction is digested 

 in the usual way, and the nucleic acid is further transformed, ultimately 

 yielding uric acid. Nucleic acid is a complex substance containing phos- 

 phoric acid, carbohydrate, pyrimidin and purin groups. In the molecule 

 there is a union of 4 complex radicals called nucleotids. A nucleotid is 

 a combination of phosphoric acid, a carbohydrate and a basic group which 

 may be purin (e. g., adenin or guanin) or a pyrimidin (e. g., cytosin, 

 uracil or thymin). In nucleic acids of plant origin, the carbohydrate is 

 usually a pentose (d-ribose), while a hexose is the carbohydrate found 

 in animal nucleic acids. Animal nucleic acids further differ from the 

 plant variety in having the pyrimidin, thymin, instead of uracil. 



The nucleic acid is split into its component nucleotids, which experi- 

 ence another cleavage resulting in the liberation of phosphoric acid, 

 leaving carbohydrate-purin and carbohydrate-pyrimidin combinations. The 

 latter compounds are known as nucleosids and are eventually split, liberat- 

 ing the free purin and pyrimidin bases. The purin bases, adenin and 

 guanin, are then converted respectively into hypoxanthin and xanthin, this 

 change being accomplished by the enzymes adenase and guanase. Finally 

 by means of an oxidizing enzyme, xanthin is transformed to uric acid. 

 This process is graphically represented on the following page, the en- 

 zymes being enclosed in parenthesis. 



The pyrimidins, especially cytosin, have been suggested as possible 

 purin precursors by Kossel, but no experimental evidence has been ad- 

 duced in support of this hypothesis. The fate of the pyrimidins appears to 

 be quite uncertain. Mendel and Myers found that when the three pyrim- 

 idins found in nucleic acid were administered to man or animals they 

 reappeared in the urine unchanged, and Wilson has made similar observa- 

 tions regarding the pyrimidin nucleosids. 



