PURINES 171 



Three are known : — 



NH— CO N— C— NH2 NH— CO 



II II II 



CO C— CH3 CO CH CO CH 



II I II I II 



NH— CH NH— CH NH— CH 



Thymine. Cytosiue. Uracil. 



Of these, thymine and cytosine occur in animal tissues 

 as components of nuclear material. Little is known of 

 their metabohsm. They can be synthesised in the body; 

 they do not appear in the urine. 



Nucleic Acid 



Nuclear tissue consists of nucleo -protein — a protein 

 conjugated with nucleic acid. 



Nucleic acid as it occurs in animals is composed of four 

 molecules of phosphoric acid, four of a hexose derivative 

 and one molecule each of adenine, guanine, thymine 

 and cytosine. These are beheved to be combined in the 

 following way : — 



Phosphoric acid- -Hexose — Guanine 



• Phosphoric acid — Hexose Thymine 



Phosphoric acid — Hexose Cytosine 



Phosphoric acid— Hexose — Adenine 



The combination, hexose + nitrogenous base, is termed 

 a nucleoside, and the combination, phosphoric acid + hexose 

 + nitrogenous base, a mononucleotide. Nucleic acid is 

 therefore called a tetranucleotide. 



Briefly, the problem before us is to correlate the purines 

 taken in with the food, the amino-purines of nucleic acid, 

 the hypoxanthine of muscle, and the purines excreted in 

 the urine. 



