BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 321 



above and verified the transglycosidation mechanism. Exchange of adenine 

 bound in adenine deoxyriboside with free adenine-8-C''' was also observed. 



There is no indication as yet that enzymes of this type function in nucleic 

 acid synthesis of higher animals. A study of the analogous transfer of the 

 ribosyl group in bacteria and other organisms is needed ;^'^ a complicating 

 factor is the apparent lack -of suitable test organisms requiring ribonucleo- 

 sides for growth, and the presence of powerful ribonucleosidases in bacterial 

 preparations of the type described above. These difficulties could be over- 

 come by using induced mutants requiring ribose nucleosides for growth. 



Some observations by Kritskii" may be interpreted as transribosidation; 

 this author, however, assumes a special mechanism in which phosphoric 

 acid ion, bound to the enzyme protein, comes into play. 



8. Interconversion of Nucleosides and Nucleotides by 

 Reactions other than Transglycosidation 



Experimental data and observations suggest that the nucleosides and 

 nucleotides in most cells are not assembled individually; rather, the pro- 

 duction may be restricted to one compound each of the pyrimidine and 

 purine type. Others are produced from these as needed by oxidation and 

 amination. The changes occurring are confined to the nitrogenous bases; 

 depending on the type of cells or tissue, they may take place at the level of 

 the free bases, the nucleosides, or at the nucleotide stage. The first alterna- 

 tive has been treated in Chapter 23. Here, the interconversion of the nucleo- 

 sides and the nucleotides will be discussed. The following exemplifying 

 equations are formulated with the nucleosides; in some instances they occur 

 with the nucleotides. The reactions to be considered are: 



Inosine ^ Adenosine (14) 



Inosine ;=i Xanthosine ;=^ Guanosine (15) 



Orotidine ^ Uridine (16) 



The same conversions may occur with the corresponding deoxyribose nu- 

 cleosides. In addition, some of the reactions appear to be restricted to the 

 deoxyribonucleosides, as follows: 



Deoxycytidine ;=i 5-Methyldeoxycytidine (17) 



Deoxyuridine :;=^ Thymidine (18) 



The type of cells determines which of these reactions occur and whether 

 they or transglycosidations are the preferred routes of synthesis. Not all of 



"» J. L. Ott and C. H. Werkman, Arch. Biochem. and Biophys. 48, 483 (1954). 



« G. A. Kritskis Doklady Akad. Nauk S.S.S.R. 70, 667 (1950), cf. Chem. Abstr. 44, 



7367c (1950); Doklady Akad. Nauk S.S.S.R. 82, 289 (1950), cf. Chem. Abstr. 46, 



7600f (1952). 



