METABOLISM OF PROTEINS 



593 



pyrimidin bases (uracil, C 4 H 4 N 2 O 2 ; cytosin, C 4 H 3 N 2 O.NH a ; thymin 

 C 4 H 3 N a O a .CH 3 ); phosphoric acid and a carbo-hydrate group. 



Some of the nucleic acids contain all these components; they are 

 sometimes spoken of as the true nucleic acids. In others certain of the 

 components are absent, and to these nucleic acids the name nucleotids 

 has been applied. The purin bases are always present. The carbo- 

 hydrate group varies in different nucleic acids, being in some a hexose 

 (p. 537), in others a pentose (p. 487). The pentose d-ribose is especially 

 often met with. It is probable that the nucleotids are merely simpler 

 decomposition products of the true nucleic acids. Thus, inosinic acid, 

 a nucleotid first isolated from meat extract, yields phosphoric acid, 

 <2-ribose, and the purin base hypo xan thin. The nucleotid guanylic acid 

 found in the pancreas yields phosphoric acid, d-ribose, and the purin 

 base guanin. There is evidence that nucleic acids may be built up out 

 of a number of nucleotid groups, and for this reason they have been 

 termed polynucleotids (Levene). The purin bases have a very close 

 chemical relationship to uric acid, which, like them, is characterized by 

 the possession of a group called the purin nucleus. For convenience 

 of reference the atoms composing the purin nucleus are numbered, 

 and the purin bodies are named with reference to the position of the 

 carbon atom or atoms at which oxygen or the amino-group (NH 2 ) is 

 introduced. Purin consists of the nucleus with H atoms introduced at 

 the points shown in the constitutional formula. Adenin is a 6-a.mino- 

 purin i.e., purin in which NH 2 replaces the H attached to C(j. Guanin 

 is 2-amino-6-oxypurin, NH 2 being united with C@) and oxygen with Cj) 

 in purin. Uric acid is 2, 6, 8-trioxypurin i.e., purin in which oxygen 

 is united to the carbon atoms 2, 6, and 8. Hypoxanthin is 6-0 xy purin, 

 oxygen being introduced at the position of C( 6 ) in purin. By removal 

 of the amino-group from, adenin hypo xan thin is formed. Xan thin is 

 2, 6, dioxypurin, oxygen being introduced at C( 2 > and C( 6 ) in the purin 

 molecule. Xan thin can be derived from guanin in the same way as 

 hypoxanthin from adenin. 



C( 6 ) - N(T) 



N( 3 ) C, 4) N( 9 ) 



N=CH 

 HC C NH 



;CH 



N C N 



Purin nucleus. 



Purin. 



N =C.NH 2 



I I 

 HC C NH 



> H 



N C N 



Adenin. 



NH 2 .C 



NH CO 



L 



NH 



NH CO 



Jo c!-: 



N=C.OH 



NH CO 



NH HC C NH 



o 



NH 





CH 



CO 



N C N 



Guanin. 



NH C NH 



Uric acid. 



> 



CH 



N C N 



Hypoxanthin. 



NH C N 



X anthin. 



)CH 



Besides the purin bases combined in the nuclein substances, purin bases 

 and uric acid are widely spread in the tissues in the free state, although 

 in very small amounts. 



A portion of the intake of purin bodies is therefore ready formed, 

 especially in the animal constituents of the food, and does not require 

 the decomposition of nucleic acid for its liberation. The nuclei of 

 vegetable cells contain nucleo-proteins, and accordingly can contribute 

 to the purin intake. The most interesting contribution of vegetable 



38 



