626 G. SCHMIDT 



ably, these substances might be intermediaries in the biosynthesis of nu- 

 cleic acids although no direct evidence in favor of such a hypothesis exists 

 as yet. 



Several enzymes catalyzing the formation of nucleoside polyphosphates 

 are known. The pathways involved include transphosphorylation between 

 nucleoside polyphosphates as well as reactions in which inorganic phosphate 

 participates in the enzymatic synthesis. 



(1) Diphosphokinase reactions: Berg and Joklik found that enzyme frac- 

 tions of muscle and of bakers' and brewers' yeast mediate the reversible 

 transphosphorylation between adenosine triphosphate and hypoxanthosine 

 diphosphate with formation of adenosine diphosphate and hypoxanthosine 

 triphosphate.^*^ 



(2) Reaction of the myokinase'^^^ type (Transphosphorylations between 

 nucleoside triphosphates and nucleoside-5'-monophosphates) . According to 

 Lieberman, Romberg, and SimmSj^^" cell free yeast preparations catalyze 

 a transphosphorylation between adenosine triphosphate or uridine triphos- 

 phate, on the one hand, and 5'-uridylate on the other hand, to the diphos- 

 phates (uridine diphosphate is inert in this system). 



(3) Formation of guanosine triphosphate or hypoxanthosine triphosphate 

 with participation of inorganic phosphate. Sanadi et al.^^^ found that the 

 enzymic synthesis of adenosine triphosphate from adenosine diphosphate, 

 inorganic phosphate and succinyl coenzyme A requires the intermediary 

 formation of either guanosine- or inosine triphosphate from the respective 

 diphosphates. Guanosine-, as well as hypoxanthosine triphosphate, in turn 

 can act as phosphate donors in reversible enzymic phosphorylations of 

 other mono- or dinucleotides. 



In some cases^*^'^^" it has been demonstrated that the enzymes cata- 

 lyzing transphosphorylations between nucleoside triphosphates and nu- 

 cleoside diphosphates are different from those catalyzing transphos- 

 phorylations between nucleoside tri- and nucleoside monophosphates. The 

 discussion of such reactions in the three groups of this chapter, however, 

 does not imply a general validity of this classification with regard to enzyme 

 specificit> . 



*88 p. Berg and W. K. Joklik, Nature 172, 1008 (1953). 



289 S. P. Colowick and H. M. Kalckar, J. Biol. Chern. 148, 117 (1943). 



«9o I. Lieberman, A. Kornberg, and E. S. Simms, J. Am. Chern. Soc. 76, 3608 (1954). 



"1 R. Sanadi, D. M. Gibson, and P. Ayengar, Biochim. et Biophys. Acta 14, 434 (1954). 



