KIDNEY AND SKIN AS EXCRETORY ORGANS. 757 



HN CO 



of oxygen gives uric acid, C 5 H 4 N 4 O 3 : CO C NH , which 



II >CO 

 HN C NH 



from this standpoint might be named trioxypurin. If one of the H 

 atoms in the purin is substituted by an amido-group, NH 2 , the com- 

 pound, adenin (C 5 H 5 N 5 ), is obtained, and the further addition of oxy- 

 gen gives guanin (C 5 H 5 N 5 O). Moreover, caffein, the active principle 

 of coffee and tea, and theobromin, the active principle of cocoa, are re- 

 spectively trimethyl and dimethyl compounds of purin. Uric acid, 

 xanthin, hypoxanthin, and in smaller amounts other members of this 

 group are found constantly in the urine. 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 ma- 

 terial. A portion of the amount daily secreted comes, however, 

 from a metabolism of the proteid 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, probably of the liv- 

 ing tissues, that goes on independently, in great measure, of the 

 mere oxidation of food. The view generally adopted at present is 

 that first proposed by Horbaczewsky,f namely, that the purin 

 bodies are the end-product, so far as the nitrogen is concerned, of 

 the physiological oxidation of the nuclein (see appendix) found in 

 the nuclei of the cells, and especially perhaps of the nuclein of the 

 leucocytes. On this view the purin bodies give a measure of the 

 extent of metabolism in the cell-nuclei. The actual amount of these 

 substances found in the urine does not in all probability represent 

 the entire amount formed in the body. It is found that when 

 xanthin, hypoxanthin, uric acid, or materials such as liver or thymus 

 rich in purin bodies are fed, only about half of the material is ex- 

 creted as such in the urine, and it seems probable that the same 

 fate occurs to a part of the endogenous purin material normally 

 formed in the body. Among birds and reptiles uric acid represents 

 the chief nitrogenous excretion of the urine, taking physiologically 

 the place held by urea in the mammalia. In the birds it has been 

 shown that the uric acid is formed in the liver. Extirpation of the 

 kidneys in these animals leads to an accumulation of uric acid in 



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

 1903. 



f See Minkowski, "Archiv f. exper. Pathol. u. Pharmakol.," 41, 375. 



