100 THE CLEAVAGE PRODUCTS OF THE NUCLEOPROTEWS. 



N=C.NH 2 HN - CO 



OC CH OC CH C 4 H 4 N 2 O 2 



I II I II Uracil. 

 HN - CH HN - CH 



Cytosin. Uracil. 



Thymin. Thymin has been obtained from thymo-nucleinic acid, 

 spermato-nucleinic acid, the nucleinic acids of the liver and the 

 spleen, etc. It is generally regarded as a decomposition-product 

 of the thyminic acid complex, but it has not yet been definitely 

 established that a thymin radicle is present in all nucleinic acids. 

 It is supposedly absent in guanylic acid. The amount which can 

 be obtained is variable ; some nucleinic acids apparently furnish 

 more cytosin, others more thymin. 



Like uracil, thymin is a pyrimidin derivative. It is 5 methyl,- 

 2.6 dioxypyrimidin and thus isomeric with 5, methyl uracil. 



HN - CO HN - CO 



II II 



OC CH OC C - CH 3 C 5 H 6 N 2 2 



II Thymin. 



HN - CH 



Thymin 

 (methyl- uracil) . 



As a pyrimidin derivative thymin is closely related to the purin 

 bases and uric acid (see the following pages). Isodialuric acid for 

 example, which is closely related to uracil, can be readily condensed 

 with urea to uric acid, as shown in the equation : 



C 4 H 4 N 2 4 + CO(NH 2 ) 2 - C 5 H 4 N 4 3 + 2H 2 O 

 Dialuric acid. Urea. Uric acid. 



The Purin Derivatives of the Nucleinic Acids. 



The common purin derivatives of the nucleinic acids are xanthin, 

 hypoxanthin (sarcin), guanin, and adenin. Derivatives of these in 

 turn are heteroxanthin, paraxanthin, theophyllin, theobromin, caf- 

 fein, epiguanin (episarcin), and earnin. Of these, paraxanthin, 

 heteroxanthin, and epiguanin (episarcin) have thus far only been 

 found in the urine. Theophyllin, theobromin, and caffein only 

 occur in the vegetable world, while the remaining members of the 

 group are common constituents of both animal and vegetable cells. 

 Collectively they are termed purin, xanthin, or alloxuric bases. 

 They are all closely related to each other and to uric acid, which, as 

 we shall see later, is one of the most important end-products of 

 nitrogenous metabolism in the animal world. They are all deriva- 

 tives of E. Fischer's purin, and in this manner closely related to 

 the pyrimidin derivatives just considered in the foregoing section. 

 Adenin, guanin, xanthin, and hypoxanthin are direct derivatives 

 of the nucleinic acids and possibly of the cytosin complex (see 

 above). 



