668 METABOLISM 



occurs in combination with other substances in the nuclear material. The second oxi- 

 dation product is xantliine and its amino derivative, guanine. They occur in the same 

 places as hypoxanthine and adenine. The highest oxidation product of all is the well- 

 known urinary constituent, uric acid, which may therefore be chemically designated as 

 trioxypurine. In addition to the purines of animal origin, there are also certain ones of 

 vegetable origin the methyl purines, which exist as the alkaloids of tea and coffee 

 namely, caffeine, theobromine, and theine. 



To understand the chemical structure of this group of substances, it is perhaps 

 simplest to start with that of uric acid. This consists essentially of two urea molecules 

 linked together by a central chain of three carbon atoms, as will be evident from the 

 accompanying structural formula: 



HN-CO 

 OC C - NH 



CO 



I II / 



HN - C - NH 



(urea) (urea) 

 (central chain) 



This structure can be shown by methods both of decomposition and of synthesis. 

 When uric acid is decomposed by oxidizing it with nitric acid, it yields urea and a 

 residue called alloxan; or it can be synthetized from urea and trichlorlactamide, a 

 derivative of lactic acid, which it will be remembered contains three carbon atoms. 

 The changes involved in this synthesis will be made clear by examination of the ac- 

 companying structural formula, in which the manner of production of the by-products 

 of the reaction (NH , H^O and HC1) are shown by dotted lines: 



\ \ / 



\ C- ! Cl H NH (urea) 



\ / '- 



(urea) NH. ! H Cl ! 



(trichlorlactamide) 



By milder oxidation by means of potassium permanganate in the cold, uric acid be- 

 comes quantitatively converted to allantoine: 



C.H 4 N 4 O 3 f H 2 O + O C 4 H 6 N 4 O 3 + CO 2 

 (uric acid) (allantoine) 



The importance of this transformation lies in the fact that in most animals, man and 

 the higher apes being exceptions, uric acid is thus decomposed in the animal body. 

 The structural formulas for the other purine bodies in relationship with those of 

 purine and uric acid are given below. 



