96 



INTERMEDIARY METABOLISM AND GROWTH 



PRPP reactions. Two mechanisms are known for the utihzation of free purines. 

 The first, involving the reaction of the purine base with PRPP, has been studied 

 in Hver and yeast extracts (Kornberg et al., 1955b). Nucleotides are the prodvicts 

 of these reactions : 



Adenine liver AMP 



Guanine ^ PRPP ^ PP + GMP 



Hypoxanthine yeast IMP 



Nucleoside phosphorylase. A second mechanism for purine utilization is mediated 

 by nucleoside phosphorylase enzymes. Purine nucleosides are formed as a result 

 of these reactions (Korn and Buchanan, 1956) (Table 7). 



TABLE 7 



NUCLEOSIDE PHOSPHORYLASE 



Substrate 



Product 



Guanine 



Hypoxanthine 



Xanthine + ribose-i -phosphate 



Adenine (beef liver) 



AICA 



liver, E. coli 



Guanosine 

 Inosine 

 Xanthosine 

 Adenosine 

 AICA riboside 



H,PO. 



Hypoxanthine- 

 Hypoxanthine deoxyriboside 



liver, E. coli, thymus Guanine- 

 Guanine + deoxyribose- 1 -phosphate ^ » deoxyriboside 



Xanthine- 

 Xanthine deoxyriboside 



A nucleoside phosphorylase, specific for inosine and guanosine, occurs in ery- 

 throcytes and in brain tissue (Huennekens etal., 1956). Hypoxanthine, guanine or 

 xanthine, but apparently not adenine, may also react with deoxyribose- 1 -phos- 

 phate in the presence of nucleoside phosphorylase, in which case deoxyribonu- 

 cleosides are formed (Friedkin and Kalckar, 1950; Hoff-Jorgenson et al., 1950; 

 Manson and Lampen, 1951, 1953; Friedkin, 1953; Lampen, 1952; Kalckar, 

 1955)- 



(b) Mutase enzymes 



The ribose- 1 -phosphate which is required for the synthesis of the nucleosides, 

 is formed from ribose-5-phosphate in a reaction catalyzed by phosphoribose- 

 mutase. Ribose diphosphate is a cofactor in the reaction (Reissig, 1956; Klenow 

 and Emberland, 1955; Guarino and Sable, 1956; Klenow, 1953). 



ribose diphosphate 



a) Ribose-5-phosphate " r ribose- 1 -phosphate 



Mg- 



deoxyribose diphosphate (?) 



b) Deoxyribose-5-phosphate ^ ' deoxyribose- 1 -phosphate 



