NUCLEOPROTEINS, NUCLEIC ACIDS, RELATED SUBSTANCES 



N=C— OH HN— 0=0 



II II 



HO-C C-NH 0=C C-NH 



155 



C— OH 

 // 



\ 



C=0 



/ 



N— C-N HN— C-NH 



Uric acid 

 (hydroxy form) (carbonyl form) 



The naturally occurring purines may be referred to the synthetic base, 

 purine. The various atoms in the rings are numbered to denote the 

 position in the structure. Thus adenine is designated 6-aminopurine, and 

 guanine is 2-amino-6-hydroxypurine. In solution the hydroxy compounds 

 exist in two tautomeric forms, as is shown in the formulas for uric acid. 



Adenine and guanine are constituents of native nucleic acids, and the 

 other three compounds are products derived from the first two as a result 

 of metabolism. Hypoxanthine is formed in the body by deamination of 

 adenine, and on oxidation this product forms xanthine, which may also 

 originate from deamination of guanine. Oxidation of xanthine gives 

 uric acid, which is the end product of purine metabolism in man. 



Three other purines occur in our common beverages. Caffeine (1,3,7- 

 trimethylxanthine) is found in the coffee bean to the extent of about 

 1 per cent and in tea leaves to about 2 per cent. Theobromine (3,7-di- 

 methylxanthinc) occurs in the cocoa bean (about 2 per cent) and theo- 

 phylline (l,3-dimeth3''lxanthine) is found is small quantities in tea leaves. 

 Caffeine, removed from the coffee bean in the making of decaffeinized 

 coffee, is used in the manufacture of cola drinks. However, this supply 

 is not sufficient for the purpose, and much of the caffeine used in soft 

 drinks is made synthetically. 



The pyrimidines have the following structural formulas: 



(1) N=CH(6) N=C— NHj N=C— NH2 



II II II 



(2)HC CH(5) HO— C CH HO— C C— CHj 



II II II II II II 



(3) N— CH(4) N— CH N— CH 



Pyrimidine Cytosine 5-Methylcytosine 



(synthetic base) 



N=C— OH N=C— OH 



II II 



HO— C CH HO— C C— CH3 



II II II II 



N-CH N-CH 



Uracil Thymine 



The pyrimidine ring forms a part of the purine structure and is numbered 

 in the same way. Thus cytosine is 2-hydroxy-6-aminopyrimidine, uracil 

 is 2,6-hydroxydipyrimidine, and thymine is 2,6-dihydroxy-5-methyl- 

 pyrimidine. 5-Methylcytosine, reported many years ago as occurring 



