PROTEIN METABOLISM 



473 



phosphate. After this process the nucleosides seem to be absorbed 

 and transported to the liver, spleen, bone marrow, and kidneys, where 

 a portion may be further split by nucleosidases to purines, pyrim- 

 idines, and ribose or deoxyribose. The pentoses thus become available 

 to the systems of carbohydrate metabolism. The nucleases, nucleoti- 

 dases, phosphatases, and nucleosidases concerned are poorly charac- 

 terized but are probably mixtures of enzymes, each individually 

 specific but together capable of hydrolyzing the naturally occurring 

 materials. 



The purines and pyrimidines not used for synthetic purposes un- 



allantoin 



Figure 20-4. Tlie calaliolism of adenine and guanine. 



