596 THE CHEMISTRY OF THE URINE. 
conditions of leukaemia, and this may be said to be the only -well- 
established fact as to the effect of disease on uric acid excretion. In 
gout, although the urate deposits form so prominent a factor, the question 
of the amount excreted in the urine is still unsettled. In this country, 
at any rate, many cases occur in which, as originally observed by Sir A. 
Garrod, the excretion during the chronic condition is greatly diminished, 
whereas, in relation to the acute attack, increased elimination may 
occur. 1 Pyrexia alone does not produce any marked increase ; but 
in certain specific fevers with a definite crisis, a large temporary 
increase may occur, depending, according to Horbaczewski, upon the 
associated leucocytosis. 
Uric acid is one of the commonest constituents of urinary calculi. 
(e) The xanthin bases. — Several members of this chemical group 
are found in urine, in variable but always small amount. Xanthin 
itself was discovered by Marcet in 1819 as a constituent of a urinary 
calculus, and its presence in urine was first demonstrated by Strecker in 
1857. In addition to xanthin, the following members of the group 
may be present — heteroxanthin, paraxanthin, hypoxanthin (sarkin), 
guanin, adenin, and carnin. 
All these substances are closely related to each other and to uric 
acid ; and the chemical group to which they belong also contains 
certain important vegetable bases. 
The relation of xanthin to uric acid is best understood by a com- 
parison of the structural formulae; our knowledge of the constitution of 
the base being due to E. Fischer. 
XII— C— NIL NH— C = N x 
I II >0 | | >co 
CO C— NH/ CO C— NH/ 
II II 
NH— CO NH— CH 
(uric acid) (xanthin) 
Xanthin contains one atom less oxygen than uric acid, while hypo- 
xanthin contains one less than xanthin. 
C 5 H 4 N 4 3 C 5 H 4 N 4 2 C 5 H 4 N 4 
(uric acid) (xanthin) (hypoxanthin) 
In the laboratory means have not been found to pass from one of 
these three compounds to another by oxidation or reduction ; but in the 
body the steps involving oxidation can certainly occur. 
Heteroxanthin and paraxanthin are homologues of xanthin, the 
former being its methyl- and the latter its dimethyl-derivative ; 
paraxanthin is therefore an isomer of the vegetable bases, theobromin 
and theophyllin. 
Guanin in an imido-xantbin : that is to say, it is xanthin with an 
oyygen atom replaced by an NH group ; and adenin bears the same 
relation to hypoxanthin. 
C 5 H 4 N 4 0.0 C 5 H 4 N 4 O.NH C 5 H 4 N 4 .0 C 5 H 4 N 4 .NH 
(xanthin) (guanin) (hypoxanthin) (adenin) 
Uric acid and the xanthin bases are grouped together by recent 
1 Cf. Fawcett, Guy's Hosp. Rep., London, 1895 ; Luff, Goulstonian Lectures, Lect. i. 
