BIOCHEMISTRY OF THE SEXUAL ORGANS 309 



observers.! The nature of _ the substances which supply the material 

 for the formation of the purine bodies remained unknown until 

 Ackroyd and Hopkins ^ found that in the urine of rats the amount of 

 allantoin, which is derived from uric acid, was greatly diminished 

 when the diet of these animals was so constituted as to be free from 

 the diamine acids histidine and arginine. They conclude, therefore, 

 that these constituents of the protein molecule represent specifically 

 the raw material for the synthesis of the purine bodies. 



The structure of nucleic acid is almost completely established. 

 As has been stated above, animal nucleic acid contains phosphoric 

 acid, a hexose and four nitrogenous ring compounds. These are 

 arranged in such a way in the nucleic acid molecule that a carbo- 

 hydrate group links a phosphoric acid group with either a purine or 

 a pyrimidine group. Such a compound is called a nucleotide. As an 

 example, guanine-nucleotide may be given thus: phosphoric acid — 

 hexose — guanine. In nucleic acid four such mononucleotides are 

 bound together, so that the structure of animal nucleic acid may be 

 represented thus : — 



Phosphoric acid — hexose — guanine. 



\ 



Phosphoric acid — hexose — thymine. 



Phosphoric acid — hexose — cytosine. 



Phosphoric acid — hexose— -adenine. 



In other words, the "skeleton" of the nucleic acid molecule is 

 formed by four molecules of phosphoric acid combined with four 

 molecules of a sugar :^ a tetraglyco-phosphorio acid, similar to the 

 glycerophosphoric acid which forms the " skeleton " of phosphorised 

 fats. In the nucleic acid molecule this glycophosphoric acid is 

 combined with four different nitrogenous substances, of which two are 

 pyrimidine derivatives and two are purine derivatiA^es. 



1 Burian and Schur, " Uber Nukleinbildung im Saugethierorganismus," 

 Zeitsch. f.physiol. Chem., vol. xxiii., 1897. Osborne and Mendel, " Beobachtungen 

 iiber Wachstum bei Fiitterungsversuchen mit isolierten Nahrungssubstanzen," 

 Zeitsch. f. physiol. Chem., vol. Ixxx., 1912. Benedict, " Uric Acid in its Relation 

 to Metabolism," Jow. Lab. and Clin. Medicine, vol. ii., 1916. 



^ Ackroyd and Hopkins, "Feeding Experiments with Deficiencies in the 

 Amino-Acid Supply : Histidine and Arginine as possible Precursors of Purines," 

 Biochem. Jour., vol. x., 1916. 



^ The sodium-salt of a tetraphosphoric acid can be prepared by fusing 

 together the sodium metaphosphate and pyrophosphate (Kraut and Uelsmann, 

 Liehi^s Annalen, vol. cxviii., 1861). The organic derivatives of this base have 

 not yet been studied. 



