KIDNEY AND SKIN AS EXCRETORY ORGANS. 851 



aminopurin and guanin or aminohypoxanthin, and the methyl- 

 purins, comprising caffein or trimethyl xanthin (C 8 H 10 N 4 O 2 or C 5 H- 

 (CH 3 ) 3 N 4 O 2 ) and theobromin or dimethyl xanthin (C 7 H 8 N 4 O 2 or 

 C 5 H 2 (CH 3 ) 2 N 4 O 2 ). Uric acid, xanthin, and hypoxanthin are found 

 constantly in the urine and in the feces small amounts of xanthin, 

 hypoxanthin, adenin, and guanin may also occur. It has been 

 pointed out * that these substances come partly from purin bodies 

 taken as food. If materials containing the purin bodies, such as 

 meat, are fed, these bodies are excreted in part in the urine. It is 

 proposed to designate the uric ; acid, etc., that has this origin as the 

 exogenous purin material. A portion of the amount daily secreted 

 comes, however, from a metabolism of the protein material of the 

 body, and this portion may be distinguished as the endogenous purin 

 bodies. This latter amount is found to be practically constant, 

 0.15 to 0.20 gm. per day for any one individual, and the amount is 

 not affected by changes in the quantity or character of the food, 

 but varies within certain limits with the manner of life. Evidently 

 the endogenous purin nitrogen represents a special metabolism, 

 propably of the living tissues, that goes on independently, in great 

 measure, of the mere oxidation of food. Evidence has accu- 

 mulated which shows that the purin bodies of the urine repre- 

 sent the end-products of the metabolism of nucleic acid, either the 

 nucleic acid of the tissues or the nucleic acid of the food.f Nucleic 

 acid exists in the body in more or less definite combination with 

 protein to form the so-called nucleoproteins which are supposed to 

 occur chiefly in the nuclei of the cells. The nucleic acid itself on 

 hydrolysis yields several nitrogenous bases, guanin, adenin, cyto- 

 sin, and thymin, in addition to phosphoric acid and a carbohydrate. 

 Guanin and adenin are purin bases, while the cytosin and thymin 

 have a different structure and are known as pyrimidin bases. 

 Within the molecule of nucleic acid these constituents are grouped, 

 so that each nitrogenous base is united to a carbohydrate and a 

 phosphoric-acid complex. Such a grouping is designated as a 

 nucleotide, and nucleic acid is frequently spoken of as a tetranucleo- 

 tide whose structure may be represented diagrammatically as 

 follows: 



Adenin-carbohydrate-phosphoric acid . 



Guanin-carbohy drate-phosphoric acid . 



Cy tosin-carbohydrate-phosphoric acid . 



Thymin-carbohydrate-phosphoric acid . 



The breaking down of the nucleic acid involves the action of a 

 number of different enzymes and the story is complicated and not 



* See Burian and Schur, "Archiv f. die gesammte Physiologic," 94, 273, 

 1903. 



f For a full account and the literature consult Jones, "Nucleic Acids 

 Their Chemical Properties and Physiological Conduct," 1914. 



