ENZYMES ATTACKING NUCLEIC ACIDS 601 



for the biosynthesis of nucleotides involving as its first step the biosynthesis 

 of a nucleoside and as the second step its esterification with phosphate. The 

 first experimental evidence for the occurrence of this second step in cells 

 was reported by Ostern and Terszacowec,'*^ who found that acetone-dried 

 yeast or toluene-poisoned yeast was capable of converting added adenosine 

 to AMP and ATP in the presence of fructose diphosphate or phosphogly- 

 ceric acid. 



Esterification of nucleosides with phosphate, however, is not the only, 

 and perhaps not the most important, mechanism for the biosynthesis of 

 nucleotides. Observations suggesting the cleavage of the bond between 

 purine bases and ribose at the nucleotide stage had been made by Schmidt^" 

 in 1932 and by Wajzer and Baron'*^ in 1949. Since 1952, several enzyme 

 reactions have been found in which certain purines, pyrimidines, or some of 

 their nitrogenous precursors are condensed with the 1 '-carbon atoms of 

 5'-phosphorylated ribose derivatives to nucleotides. Hydrolases acting on 

 the A^-glucoside linkage of certain nucleotides have also been described. The 

 study of these enzyme reactions which was initiated mainly in the labora- 

 tories of G. R. Greenberg,^«« of J. M. Buchanan,^" of H. M. Kalckar,i^« and 

 of A. Kornberg^^^ is rapidly becoming one of the central problems of cur- 

 rent research on nucleotide metabolism. 



History. Purine nucleosidases were discovered by Levene and Medi- 

 greceanu^ soon after the elucidation of the general structure of nucleotides. 

 The specificity of these enzymes toward purines and their inactivity toward 

 nucleosides of pyrimidines and dihydropyrimidines was recognized in early 

 investigationsby Levene. '^''•^^^ Much later, Deutsch and Laser'^'- discovered 

 in Thannhauser's laboratory the presence of specific pyrimidine nucleo- 

 sidases in bone marrow. An important advance was made when Klein' ^^ 

 found in 1935 that purine nucleosidase prepared from beef spleen lost its 

 acti\aty by dialysis but was reactivated by phosphate or arsenate. The 

 explanation for this behavior was given in 19-47 by Kalckar,'^^ who estab- 

 lished the phosphorolytic nature of phosphate-dependent nucleosidases. 

 Carter, '^^ however, furnished the first evidence for the conclusion that not 



>8^ P. Ostern and J. Terszacowec, Z. physiol. Chem. 250, 155 (1937). 

 '85 J. Wajzer and F. Baron, Bull. soc. chim. hiol. 31, 750 (1949). 



186 G. R. Greenberg, Federation Proc. 13, 745 (1954). 



187 W. J. Williams and J. M. Buchanan, J. Biol. Chem. 203, 5S3 (1953). 



188 H. M. Kalckar, Biochim. et Biophys. Acta 12, 250 (1953). 



189 A. Romberg, I. Lieberman, and E. S. Simms, J. Am. Chem. Soc. 76, 2027 (1954). 



190 P. A. Levene and I. Weber, /. Biol. Chem. 60, 707 (1924). 



191 P. A. Levene, *Yamagawa, and I. Weber, J. Biol. Chem. 60, 693 (1924). 



192 W. Deutsch and R. Laser, Z. physiol. Chem. 186, 1 (1927). 



193 W. Klein, Z. physiol. Chem. 231, 125 (1935). 



194 H. M. Kalckar, J. Biol. Chem.. 167, 461 (1947). 

 1" C. E. Carter, J. Am. Chem. Soc. 73, 1508 (1951). 



