596 THE CHEMISTR Y 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. 



C NIL NH C = Ev 



I II >co i i )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 6 H 4 N 4 2 C 6 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. 



Gfuanin in an imido-xanthin ; 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 6 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 Hasp. Rep., London, 1895 ; Luff, Goulstonian Lectures, Lect. i. 



