432 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



(d) Two pyrimidine bases, namely, cytosine (see p. 421), and 



thymine (methyl-dioxypyrimidine). 



The purine bases are specially interesting because of their close 

 relationship to uric acid, and we shall have to deal with them again 

 in our description of that substance. They are all derivatives of an 

 atomic complex, named purine by Fischer, and their relationship to 

 each other is best seen by their f ormulse : 



Purine C 5 H 4 N 4 



fHypoxanthine (monoxy-purine) C 5 H 4 N 4 



! - 



Purine baseB Xanthine (dioxy-purine) C H N 4 0, 



'1 Adenine (amino-purine) C 5 H 3 N 4 . 

 [Guanine (amino-oxy-purine) C 5 H 3 N 4 . NH 2 



Uric Acid (trioxy-purine) C 5 H 4 N 4 3 



The two bases obtained from nucleic acid are the two which con- 

 tain the NH 2 group. If xanthine and hypoxanthine are obtained, 

 they are the secondary effects of oxidative and de-amidising enzymes. 



(2) G-uanylic acid. This is a simpler form of nucleic acid found 

 in certain organs (pancreas, liver, etc.), mixed with the nucleic acid 

 proper. It yields on decomposition only three substances, namely: 



(a) Phosphoric acid. 



(6) A carbohydrate (probably a pentose). 



(c) Guanine, but no adenine. 



From his work on the nucleic acid of yeast, Levene finds that it is composed of 

 complexes consisting of phosphoric acid, carbohydrate (ribose), and a base. These 

 are termed nucleotides. Guanylic acid, described above, is a mono-nucleotide, but 

 the majority of nucleic acids are poly-nucleotides. When these are broken down by 

 chemical reagents, the first change is the removal of the phosphoric acid, leaving 

 intact the combinations of base and carbohydrate ; these latter combinations are 

 called nucleosides ; thus 



Adenine + ribose = adenosine. 

 Guanine + ribose = guanosine. 



These nucleosides may be further split into base and ribose; or they may be 

 de-amidised (i.e., the amino-group is removed) and nucleosides obtained in which 

 hypoxanthine and xanthine are united with the ribose, and these in their turn may 

 be split into base and ribose. 



The same cleavages are accomplished in the body by the action of tissue- 

 enzymes contained in varying degrees in the different organs and tissues. As 

 these enzymes are specific, the number which may come into successive play in the 

 decompositions which occur in the body is very large. These enzymes are 

 spoken of under the general term nucleases. 



Protein-hydrolysis. 



When protein material is subjected to hydrolysis, as it is when 

 heated with mineral acid, or superheated steam, or to the action of 

 such enzymes as pepsin or trypsin in the alimentary canal, it is 

 finally resolved into the numerous amino-acids of which it is built. 

 But before this ultimate stage is reached, it is split into substances of 

 progressively diminishing molecular size, which still retain many of 



