996 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



in the tissues and fluids of the body and in the urine. It is, like hypoxan- 

 thin, a decomposition product of nuclei. 1 It is converted into hypoxanthin 

 through the action of nitrous acid. 



(b) XANTHIN BASES. 



NH-C H 



/ II 



Xanthin, OC C NEL 



v ;>CO. This substance, like the two last 



N / 



named, is found in the tissues and the fluids of the body, and is a decomposi- 

 tion product of nuclein. Occasionally it occurs in the form of a urinary cal- 

 culus, as a stone of exceptional hardness. 



Monomethyl Xanthin, or Heteroxanthin, C 6 H 6 N 4 O 2 . This has likewise 

 been detected in the urine (see Caffei'n). 



Theobromin, Dimethyl Xanthin, or Paraxanthin. 



NCH 3 C H 



/ II 



O = C 



\ I 



N C = N 



This is the principal alkaloid in cacao (chocolate). When fed it is in part 

 excreted as monomethyl xanthin in the urine (see Caffein). Its silver com- 

 pound treated with methyl-iodide yields caffein. 



NCH 3 -C-H 



/ II 



Theophyllin, = C' C NHv 



\ /CO. This is found in tea and may be 



3 -C = N 



converted into caffein through the addition of a third methyl group. 2 



NCH 3 C H 



/ II 



Caffein, Them, Trimethyl Xanthin, O = C C NCH 3V 



\ I > CO. 



NCH 3 C=N 



This is the alkaloid of coffee, tea, guarana, and the cola nut, imparting the 

 nerve-stimulating properties to each. A cup of coffee contains 0.1 gram of 

 caffein. If caffein be fed it appears in part as methyl xanthin in the urine. a 

 That the compounds theobromin and caffein may be demethylated in the tissue 

 is an interesting commentary on the methylation of tellurium, selenium, and 

 pyridin by the tissues. 



Guanin, Imido-xanthin, C 5 H 4 N 4 ONH. This is found, like hypoxanthin, 

 adenin, and xanthin, in tissues rich in nuclei, and in the blood. 4 It is a decom- 



1 Kossel : Zeitschrift fur physiologische Chemie, 1886, Bd. 7, p. 250. 



2 Ibid., 1889, Bd. 13, p. 298. 



3 Boudzynski nnd Gottlieb : Archivfur exper. Pathologic und Pharmakologie, 1895, Bd. 36, p. 45. 



4 Kossel : Zeitschrift fur physiologische Chemie, 1884, Bd. 8, p. 404. 



