BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 329 



The phosphorylation of adenosine by a yeast enzyme in presence of a 

 suitable phosphate donor was confirmed by Sable.^® He also reported that 

 D-ribose is phosphorylated while deoxyribose and other pentoses were 

 inert in this yeast system. Ribokinase and adenosine phosphokinase are 

 not identical, and these enzymes could not be detected in tissues. The 

 action of yeast and tissue adenosine phosphokinase was studied in more 

 detail by Caputto,^^ and by Kornberg and Pricer^' who established the 

 equation 



Adenosine + ATP ;=± adenosine-5'-phosphate + ADP (20) 



The pH optimum is approximately 6, and Mg+^ or Mn++ is required. There 

 is specificity for adenosine with the exception of 2-aminoadenosine (2,6- 

 diamino-O-jS-D-furanosylpurine), which proved reactive but inferior to 

 adenosine. 2-Oxyadenosine (crotonoside), deoxyadenosine, inosine, guano- 

 sine, uridine, cytidine, and several synthetic nucleosides proved inert. 



The failure of various tissue enzymes to phosphorylate nucleosides is 

 disappointing.^^-*^-'" Phosphorylated pentoses are secured in the cell by 

 other routes, if a direct phosphorylation is not feasible (see Chapter 22). In 

 view of the failure to achieve experimentally the enzymic phosphorylation 

 of nucleosides, one may be inclined to assume combination of phosphory- 

 lated pentoses with bases or base fragments. Experiments along this line 

 are described below, but they are far from satisfactory. 



c. Nucleotide- N-rihosidase 



The early work with nucleolytic enzymes seemed to indicate that the 

 phosphoric acid group in the nucleotides has to be removed before the 

 glycosidic hnkage can be resolved by enzymes. ^-^^ This concept has been 

 challenged by Ishikawa and Komita.^^ Some experiments of G. Schmidt^^ 

 seemed to indicate that guanylic acid, when dearainated, is split simul- 

 taneously to yield xanthine and ribose phosphate; the formation of the 

 latter was inferred from the observation that no inorganic phosphate ap- 

 peared during the incubation. Small amounts of what seemed to be ribose 

 phosphate were isolated, but the phosphate content was found to be 20 % 

 below the theoretical value. Moreover, the behavior of the product during 

 acid hydrolysis resembled that of ribose-5-phosphate rather than ribose-3- 

 phosphate. The Japanese investigators^^-^' made similar observations with 



86 H. Z. Sable, Proc. Soc. Exptl. Biol. Med. 75, 215 (1950). 

 " R. Caputto, /. Biol. Chem. 189, 801 (1951). 



88 G. E. Youngburg, Arch. Biochem. 4, 137 (1944). 



89 F. Schlenk and M. J. Waldvogel, Arch. Biochem. 9, 455 (1946), 12, 181 (1947). 



90 A. Canzanelli, R. Guild, and D. Rapport, Am. J. Physiol. 162, 168 (1950). 

 " P. A. Levene and A. Dmochowski, J. Biol. Chem. 93, 563 (1931). 



92 H. Ishikawa and Y. Komita, J. Biochem. (Japan) 23, 351 (1936). 

 " Y. Komita, J. Biochem. {Japan) 25. 405 (1937); 23, 191 (1938). 



