THE XANTHIN BASES. 597 



German writers l under the term, " alloxuric substances," a name meant 

 to show their relation on the one hand to alloxan, 2 and on the other to 

 urea ; the bases themselves may be designated the " alloxuric bases." 



The amount of the xanthin bases in the urine has been generally 

 understated until lately ; they amount collectively to something like 

 one-tenth of the uric acid present ; that is to say, an average of 0*1 to 0'07 

 grm. of the combined bases is excreted per diem (Camerer, Salkowski). 

 Of xanthin itself some O02 to O03 grm. is found, upon a mixed 

 diet. 



General properties. That the xanthin compounds, unlike uric acid, 

 are basic in character, is probably due to the fact that the CO group is 

 absent from the central carbon chain. (Cf. graphic formulae above.) 



Their basicity is, however, very feeble, and many of their compounds 

 with acids are decomposed by water just as is the " sulphate " of uric 

 acid ; while, on the other hand, they are all capable of forming metallic 

 derivatives and compounds with other bases. They contrast sharply 

 with uric acid in their easy solubility in mineral acids. In ammonia 

 they are also soluble (with the exception of guanin). Xanthin itself 

 dissolves to a very slight extent in water, but the other bases are more 

 soluble. 



They are precipitated from urine (1) By the addition of phospho- 

 tungstic or phosphomolybdic acids in acid solution ; (2) by silver nitrate 

 in ammoniacal solution ; and (3) by copper salts ; especially in the 

 presence of thiosulphates. When precipitated by any of these methods, 

 they are accompanied out of solution by uric acid (vide infra). 



Isolation and estimation. It is beyond the scope of this article to describe 

 in detail the separation of the xanthin bases individually. Very large quanti- 

 ties of urine (100 litres and upwards) are required for the purpose. If the 

 precipitate obtained by adding ammonia, and afterwards ammoniacal nitrate 

 of silver solution, be decomposed by sulphuretted hydrogen, and the filtrate 

 from the silver sulphide acidified with hydrochloric acid, concentrated, and 

 allowed to stand, the uric acid crystallises out. This being filtered off, the 

 liquid is again made alkaline with ammonia and the bases again precipitated 

 with silver nitrate. The varying solubility of the silver compounds so 

 obtained in nitric acid permits of a preliminary fractionation of the bases ; 

 and, when liberated from combination with silver, their diverse solubilities in 

 water and other media yield methods for their final separation from each 

 other. 



If urine (100 c.c.) be heated to boiling, and precipitated with a mixed 

 solution of copper sulphate and sodium thiosulphate, some chloride of barium 

 being afterwards added, a precipitate is obtained which contains all the uric 

 acid and xanthin bases, but no other nitrogenous constituent (Kriiger and 

 Wulff). By estimating the nitrogen in this precipitate by means of Kjeldahl's 

 process, we obtain a measure of what may be called the " alloxuric nitrogen," 

 an important urinary constant. If a separate estimation of the uric acid be 

 made, the nitrogen proper to this may be deducted from the "alloxuric 

 nitrogen," and we obtain a value for the "nitrogen of the bases." Such a 



1 Kriiger and Wulff, Ztschr. f. physiol. Ohem., Strassburg, 1896, Bd. xx. S. 176. 



rNH-co^ 



2 Alloxan-! CO CO r is an oxidation product of uric acid. The xanthin bases and uric 



INH-CO>! 



acid will all be seen to contain the urea residue and the three-carbon chain, which together 

 comprise the so-called " alloxan ring." 



