102 PHYSIOLOGY 



separating the cells by means of the centrifuge, the clear fluid is decanted off and 

 acidified with acetic acid. A precipitate is produced consisting of ' tissue fibrinogen.' 

 This substance is soluble in excess of acid and is easily soluble in alkalies. All the 

 tissue fibrinogens are highly unstable bodies and undergo changes in the mere act of 

 precipitation and re-solution. When injected into the blood they cause intravascular 

 clotting. On digestion with gastric juice they yield a precipitate of nuclein, and this 

 precipitate contains a large proportion of the lecithin present in the original substance. 

 In the nucleoproteins nucleic acid is combined with proteins in two degrees, a large 

 portion of the protein being separable by gastric digestion, while the remainder needs 

 stronger reagents for its dissociation. The relation of the two portions of the nucleo- 

 protein may be represented therefore by the following schema : 



Nucleo -protein 

 Protein Nuclein 



Protein Nucleic acid 



(generally histone 

 or protamine) 



By various means, all of which involve hydrolysis, the nucleic acid may 

 be broken up into its proximate constituents. These differ according to 

 the source of the nucleic acid. Whatever the source, the disintegration 

 products belong to closely allied groups of substances. These may be 

 grouped as follows : 



(1) Phosphoric Acid. The proportion of phosphorus varies within but 

 narrow limits in the different nucleic acids, the average being about 10 per 

 cent. It is probable that the phosphoric acid represents, so to speak, the 

 combining medium for the groups contained in the nucleic acid molecule, as 

 is the case with the various groups which make up the lecithin molecule. 



(2) The Purine Bases. Among the products of disintegration of nucleic 

 acid we find constantly one of the bases adenine, C 5 H 5 N 5 , and guanine 

 (C 5 H 5 N 5 0). These substances, with the products of their oxidation, xan- 

 thine, C 5 H 4 N 4 2 , hypoxanthine, C 5 H 4 N 4 0, have long been known to be 

 cU>sely allied to uric acid, C 5 H 4 N 4 3 , but their true relationships have only 

 been thoroughly known since the researches of Fischer on this group. 

 According to Fischer they can be all regarded as derivatives of the body 

 purine, iN 



2 HC 5 C NH 7 



II II }OH' 



3N_ 4 C N 9 * 



Each group in this purine ring is generally designated with a number indicated 

 in the structural formula, in order that it may be possible to represent the 

 position of any substituted groups in its derivatives. Uric acid itself is 

 2-6-8-trioxypurine with the following formula : 



HN CO 



I I 

 OC C NH V 



I II >co 



HN C NH/ 



