THE X AN THIN BA SES. 5 9 7 
German writers l under the term, "alloxuric substances," a name meant 
to show their relatioE on the one hand to alloxan,- and on the other to 
urea ; the bases themselves may lie designated the "alloxuric bases." 
The amount of the xanthin bases in the mine has been generally 
understated until lately; they amount collectively to something like 
one-tenth of the urie acid present; that is to say, an average of - l to 0*07 
grm. of the combined liases is excreted per diem (Camerer, Salkowski). 
Of xanthin itself some 0*02 to 003 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- 
tungsticor phpsphomolybdic 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. Chem., Strassburg, 1896, Bd. xx. S. 176. 
2 AUoxani CO CO f is an oxidation product of uric acid. The xanthin bases and uric 
(TSH—CO} 
i-( CO COr " 
1 I 
Inh— coJ 
acid will all be seen to contain the urea residue and the three-carbon chain, which together 
comprise the so-called " alloxan ring." 
