NUCLEIC ACIDS 137 



( Ascoli, 1900) are chemically referable to hypothetical pyrimidine. Cyto- 

 sine is 6-amino-2-oxypyrimidine and uracil is 2-6-dioxy-pyrimidine. 



Uracil 

 C 4 H 4 N 2 O 2 



The two substances are corresponding oxy- and ammo-compounds, so that 

 one may pass into the other by deaminization 



Hf\ r\ TT "v ^ 

 2 w l^JjLoJtN 2^- 



Cytosine Uracil 



in fact, cytosine can easily be converted into uracil, and will be so con- 

 verted in a laboratory manipulation of the material unless precautions are 

 taken against it. The relation of the two substances to each other sug- 

 gests the possible metabolic conversion of one of the compounds into the 

 other by the deaminiziug ferments of the tissues. This is, of course, pos- 

 sible, but the transformation has not been shown either by an organism 

 or by a tissue extract. In fact, very little is known about the metabolism 

 of the pyrimidine derivatives, so that of the six fundamental decomposi- 

 tion products of yeast nucleic acid, physiological interest is directed almost 

 exclusively to the purine derivatives. 



The Purine Derivatives. By hydrolysis of yeast nucleic acid with 

 dilute mineral acid, it is possible to obtain only the two amino-purines, 

 guanine and adenine ; but in studying the metabolism of these two, it is 

 necessary to consider three other purine derivatives, viz., . hypoxanthine, 

 xanthine and uric acid. The chemical relation of these five substances to 

 one another is shown in the following arrangement, in which the purine 

 ring is represented by the letter P. 



[In this article, purine formulas are used to which the physician may 

 not be accustomed and a word of explanation may not be superfluous. 

 There are two tautomeric formulas for purine derivatives (enol formulas 

 and ketol formulas) which are not chemically distinguishable from each 

 other. One of these formulas is almost universally (but arbitrarily) used 

 by chemists and physiologists. The other formula has been adopted in the 

 following pages for its exceeding convenience in dealing with the prob- 

 lems under consideration.] 



