18 



The purine and also the pyrimidine derivatives can be changed 

 one into the other by chemical means. In a parallel manner much 

 the same results can be obtained through the biochemical changes 

 brought about by bacteria and enzymes. By chemical agents Kos- 

 sel and Steudel ( 1903) transformed cytosine into uracil. In like 

 manner Fischer (1882) changed guanine into xanthine, and Kossel 

 (1886) changed adenine into hypoxanthine. It was shown by Schit- 

 tenhelm and Schroter (1904) that putrefactive bacteria, especially 

 those of the coli group, were able to convert guanine and adenine 

 into xanthine and hypoxanthine and that bacteria also have the 

 ability of breaking down nucleic acid itself. This change of nucleic 

 acid is also accomplished by certain enzymes, the nucleases (cf. 

 Jones 1914 and Euler 1912). 



It will be clear from the above that the decomposition of 

 nucleic acid may take place in many steps and that the intermediate 

 as w'ell as the final products may be transformed one into the other. 

 This may be accomplished in the soil through the agency of micro- 

 organisms or enzymes. Schreiner and Lathrop (1912) working 

 with steam heated soils found that from the heated samples less 

 nucleic acid was obtained than from the unheated samples. On the 

 other hand the decomposition products of nucleic acid were present 

 in larger amounts in the heated soil, indicating that hydrolysis of 

 nucleic acid has been accomplished in this manner. That nucleic 

 acid decomposition does take place in the soil is evidenced by 

 the isolation of certain of its decomposition products, e. g., cytosine, 

 xanthine, hypoxanthine (Schreiner and Shorey 1910 a, 1910 c), 

 adenine (Shorey 1913), and guanine (Lathrop 1912, Schreiner and 

 Lathrop 1912). 



3. Lecithin decomposition. A similar instance of a single 

 substance decomposing into several substances is that of the 

 lecithins. They are closely related to the fats in constitution and 

 are possible primary constituents of all plant and animal cells. 

 Lecithins are esters of glycerol with two molecules of higher fatty 

 acids (palmitic, stearic, oleic acids or other unidentified saturated 

 or unsaturated acids) with a molecule of phosphoric acid, which 

 is at the same time combined with the base choline. Mathews 

 (1915) states that in some cases choline can be replaced by neurine. 

 There are a number of different lecithins which are characterized 

 by the nature of the organic acid radicals present. The hydrolysis 

 may be indicated as follows : 



L,ecithin 



Acids Bases Glyceryl-phosphoric acid 



I I 



Palmitic Choline Glycerol 



Stearic or Phosphoric acid 



Oleic Neurine 



or 



Other higher 

 fatty acids. 



