598 THE CHEMISTRY OF THE URINE. 



procedure is our most convenient method for following the variations in the 

 excretion of the xanthin group. 1 



Tests. Xanthin and its two homologues, and also carnin, but not the 

 other bases, give Weidel's reaction. This is almost identical with the 

 murexide test described for uric acid, but chlorine water is used instead 

 of nitric acid. The bases resist oxidation with nitric acid much more 

 fully than does uric acid, but, in the presence of a small quantity of a 

 chloride, xanthin will give the ordinary murexide reaction. Very 

 characteristic of xanthin and hypoxanthin are the crystalline precipi- 

 tates which they yield with silver nitrate in the presence of nitric 

 acid. 



Variations in the amount of the urinary xanthin bases closely follow 

 those of uric acid, and for the most part depend upon the same 

 influences. The bases, however, are apt to vary even more widely. 

 According to Camerer 2 they are greatly increased by certain forms of 

 vegetable food ; thus, in one experiment, on a flesh diet the nitrogen 

 present as these bases was only 01 per cent, of the total nitrogen ; 

 while, when green vegetables formed the chief ingredient of the food, it 

 was 0'6 per cent. They are increased by diet rich in nucleins, 3 and 

 pathologically their amount is greatly raised in some forms of leukaemia. 



(f) Creatinin. This base is chemically distinct from the alloxuric 

 compounds, in that its molecule contains neither the alloxan ring nor 

 the urea residues which are characteristic of these. Nevertheless, on 

 hydrolysis, it easily yields urea and an amido-acid (methylglycine). It 

 is the anhydride of creatin, which is itself methylglycocyamin. 



X , /NH 



C(NH)< / C(NH)< 



\N(CH 3 )-CH 2 -COOH \N(CH 3 )-CH 2 -CO 



(creatin) (creatinin) 



Whether creatin itself is ever a urinary constituent is somewhat 

 uncertain. It has been stated to occur when the urine is excreted in 

 alkaline condition, 4 but the quantity is, in any case, very small. So 

 easily are the two substances converted the one into the other, that care 

 is requisite in the isolation of either. When creatin stands in acid solu- 

 tion, it tends to change into its anhydride, while creatinin in alkaline 

 solution suffers the inverse change. 



G. S. Johnson 5 has found, however, that urinary creatinin is not 

 identical, but isomeric, with that obtained artificially from creatin (e.g. 

 by the action of acids), and distinct from the creatinin found in small 

 quantity in muscles. 



The creatinin of urine was first isolated by Liebig. It is present on 

 an average to the extent of about I/O grm. in the excretion of twenty- 

 four hours, when a mixed diet is taken. 



Properties. In its compounds creatinin exhibits well-marked basic 

 tendencies, and it can liberate ammonia from ammonium salts on boil- 

 ing ; but, according to Salkowski, solutions of the pure substance react 



1 For details see Kriiger and Wulff, loc. cit. supra. 



2 Ztschr.f. Biol., Miinchen, 1891, Bd. xxviii. S. 72. 



3 Weintraud, loc. cit. 



4 Hoffemaiin, Virchaw's Archiv, 1869, Bd. xlviii. S. 358. 



5 Proc. Roy. Soc. London, 1892, vol. 1. p. 287. 



