KIDNEY AND SKIN AS EXCRETORY ORGANS. 857 



HN CO 



HC C NH 



Jlj. ^ ^>CIL. Addition of two atoms of oxygen gives xan- 



HN CO 



thin, C 5 H 4 N 4 2 : CO C NH 



jj'jj |l N \CH. And addition of three atoms 



HN CO 



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



HN C NH 



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

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

 pound, adenin (C 5 H 5 N 5 ), is obtained, and the substitution of an 

 NH 2 group in hypoxanthin gives the compound guanin (C 5 H 5 N 5 0). 

 Moreover, caffein, the active principle of coffee and tea, and theo- 

 bromin, the active principle of cocoa, are respectively trimethyl 

 and dimethyl compounds of xanthin. We have to distinguish, 

 therefore, three classes of purin compounds, namely, the oxypurins, 

 comprising monoxypurin or hypoxanthin, dioxypurin or xanthin, 

 and trioxypurin or uric acid; the aminopurins, comprising adenin or 

 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 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 



* See Burian and Schur, " Archiv f . die gesammte Physiologic," 94, 273, 1903. 



