The biosynthesis of pentoses and their incorporation into mononucleotides 



and C (2 ) of glycogen. These findings, therefore, are by no means in disagreement 

 with the reaction catalysed by the transketolase. Similar experiments performed with 

 E. coli suggest that the oxidative decarboxylation of 6-phosphogluconic acid is the 

 primary pathway for pentose formation in this organism (Cohen, 1951; Sowden 

 etal, 1954). 



We have now seen by which possible mechanism ribose may be formed in living 

 organisms. But by which reactions are the ribose phosphates linked to the purines 

 and pyrimidines to form the nucleotides, the building blocks of the nucleic acids? 

 About eight years ago Kalckar (1947) demonstrated the enzymatic synthesis o 



Table II 

 Relative 14 C distribution in ribose and glycogen of chicks fed 14 CH 2 NH 2 COOH 



The figures in parentheses indicate the level of specific activity (counts per minute per millimole of carbon) 

 at which the determination was actually made for the carbon assigned a value of 100 (From I. A. Bernstein 

 (!953) J- B iol. Chetn. 205, 317). 



nucleosides with the nucleoside phosphorylase system. The reaction is the well- 

 known reversible phosphorolytic cleavage of nucleosides to free base and ribose- 1- 

 phosphate. The equilibrium of the reaction: 



Hypoxanthine riboside -f- orthophosphate ^ hypoxanthine + ribose- 1 -phosphate 



is in favour of the synthesis of the riboside. High enzyme activity is present in both 

 mammalian organisms and in micro-organisms. The enzyme is active towards a 

 number of different purine nucleosides and also towards purine deoxynucleosides 

 (Friedkin, 1953). Nicotinamide riboside is attacked by an enzyme which is probably 

 identical with the enzyme of Kalckar (Rowen and Kornberg, 1951). Pyrimidine 

 nucleosides, however, can be split by an apparently different phosphorylase (Lampen, 

 1952). Thus, with these reactions we can account for the formation of the linkage 

 between ribose and several of the nitrogen bases of the nucleic acids. The nucleosides 

 formed in this way might then be phosphorylated to nucleotides by a kinase reaction, 

 i.e. with ATP as phosphate donor. Such reactions have been demonstrated by 

 Kornberg and Pricer (1951). They found that both adenine riboside and 2-amino 

 adenine riboside can be phosphorylated to the corresponding 5 / -nucleotides. The 

 enzyme is, however, strictly specific with regard to adenosine and 2-amino adenosine, 

 and other nucleoside kinases have not been found yet. 



71 



