848 PHYSIOLOGY OF DIGESTION AND SECRETION. 



nucleic acid of the tissues or the nucleic acid of the food.* Nucleic 

 acid exists in the body in more or less definite combination with 

 protein to form the so-called nucleoproteins which are supposed to 

 occur chiefly in the nuclei of the cells. The nucleic acid itself on 

 hydrolysis yields several nitrogenous bases, guanin, adenin, cyto- 

 sin, and thjanin, in addition to phosphoric acid and a carbohydrate. 

 Guanin and adenin are purin bases, while the cytosin and thymin 

 have a different structure and are known as pyrimidin bases. 

 Within the molecule of nucleic acid these constituents are grouped, 

 so that each nitrogenous base is united to a carbohydrate and a 

 phosphoric-acid complex. Such a grouping is designated as a 

 nucleotide, and nucleic acid is frequently spoken of as a tetranucleo- 

 tide whose structure may be represented diagrammatically as 

 follows : 



Adenin-carbohydrate-phosphoric acid. 



Guanin-carbohydrate-phosphoric acid. 



Cytosin-carbohydrate-phosphoric acid. 



Thymin-carbohydrate-phosphoric acid. 



The breaking down of the nucleic acid involves the action of a 

 number of different enzymes and the story is complicated and not 

 wholly agreed upon by the different workers. It is supposed that 

 the nucleic acid is liberated from its combination with protein by 

 the action of the known proteolytic enzymes, such as pepsin or 

 trypsin or the similar autolytic enzymes in the tissues. For the 

 destruction of the nucleic acid itself there is a series of special 

 enzymes whose action as described by Jones is essentially as follows : 



1. The tetranucleotide is split into two dinucleotides (purin and 

 pyrimidin) by the enzyme tetranuclease. The further history of 

 the pyrimidin nucleotides is uncertain. 



2. The purin nucleotides are split by two enzymes, phospho- 

 nuclease and purin-nuclease. The former splits off phosphoric 

 acid and leaves the purin bases in combination with the carbo- 

 hydrate, giving two compounds known as nucleosides, namely, 

 adenosin and guanosin. The purin-nuclease splits off adenin and 

 guanin in free form . 



3. The four substances, adenin, guanin, adenosin, and guanosin, 

 are acted upon by corresponding deaminizing enzymes, adenase, 

 guanase, adenosin-deaminase, and guanosin-deaminase, and con- 

 verted to oxypurins. For the adenin and guanin the reaction is as 

 follows : 



CsHsNs + H2O = C5H4N4O + NH3 



Adenin. Hypoxanthin. 



CsHsNsO + H2O = C5H4N4O2 + NH3 



Guanin. Xanthin. 



* For a full account and the literature consult Jones, "Nucleic Acids^ 

 Their Chemical Properties and Physiological Conduct," 2d ed., 1920. 



