NUCLEIN OR PURINE METABOLISM 775 



are derivatives of a base purine. They contain a central chain of three 

 carbon atoms to which is attached on each side a urea group, so that they 

 may be regarded as diureides. Purine itself has the formula 



N=CH 



I I 



HC C NH 



II II \CH 



N C NX 



Purine 



The relation of the purine bases obtained from disintegration of nucleic acid 

 to purine itself has been given on p. 103. From these formulae we see that 

 adenine and hypoxanthine are related to one another, adenine being 

 6-aminopurine, while hypoxanthine is 6-oxypurine. In the same way 

 guanine and xanthine are related, guanine being 2-amino-6-oxypurine, 

 while xanthine is 2-6-dioxypurine. The investigation of the relationships of 

 these bases was of interest to physiologists since it brought to light the close 

 relation which they have to uric acid, a substance which has been known as 

 a constituent of urine and urinary calculi for a long time, having been 

 discovered in 1776 by Scheele. Uric acid is 2-6-8-trioxypurine and has 

 the formula 



HN CO 



CO C NH X 



I II >0 



HN C NH/ 



Uric acid = 2-6-8-trioxypurine 



In uric acid the two urea groups are attached to a central 3-carbon 

 chain, and it is interesting to note that one of the first syntheses of uric 

 acid, namely, that by Horbaczewski, was accomplished by melting together 

 trichlorlactamide and urea in a sealed tube. 



Belonging to the same group of purines are the two bases which form the essential 

 constituents of tea, coffee, and cocoa. In tea and coffee is found caffeine, which is 

 l-3-7-trimethyl-2-6-dioxypurine, and in cocoa occurs the closely allied theobromine, 

 which is 3-7-dimethyl-2-6-dioxypurine. Caffeine is thus methyltheobromine. 



CH 3 .N CO HN CO 



I /CH 3 | CH 3 

 CO C N< CO C N< 



II >H | || \CH 



CH 3 .N C N' CH 3 N 



Caffeine = 1-3-7-trimethyl- Theobromine = 3-7-dimethyl- 



2-6-dioxypurine 2-6-dioxypurine 



The pyrimidine bases which are also obtained from' the hydrolysis of 

 nucleic acid are derived from a pyrimidine nucleus which is, so to speak, 



half a purine nucleus, consisting of a &Cr chain joined to a 3-carbon 



