NUCLEIN OR PURINE METABOLISM . 819 



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 

 HG G-NH 



^CH 

 N G N^ 



Purine 



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

 to purine itself has been given on p. 100. 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 



to 



C NIL 



\CO 



HN 



Uric acid = 2-6-8-trioxypurine 



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

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



/N 

 half a purine nucleus, consisting of a C<T chain joined to a 3-carbon chain. 



N 



Three pyrimidine bases have been isolated from the decomposition products 

 of nuclein, namely, thymine, cytosine, and uracil. 



After separation of the purine and pyrimidine bases and phosphoric acid, 

 a substance is left over which gives the reactions of a carbohydrate. 



This carbohydrate differs in different nucleic acids. In plant nucleic acid, as well 

 as in guanylic acid from the pancreas and inosinic acid from muscle, the carbohydrate 

 is a pentose, d-ribose. Most nucleic acids of animal origin yield Isevulinic acid on 

 hydrolysis and must therefore contain a hexose. 



FORMATION OF NUCLEINS IN THE BODY 



In the case of the proteins we saw reason to believe that in the higher 

 animals at any rate, there was no power of converting one amino-acid into 

 another (with the exception of the lowest members of the series, namely, 

 glycine and alanine), and that on this account the food had to contain 

 ipresentatives of every amino-acid (or perhaps of the corresponding oxy- 





