290 



PETER REICHARD 



Counts recovered 



20 40 



Time, minutes 



Fig. 5. Incorporation of formate-C'' into hypoxanthine (HX), inosine, and in- 

 osinic acid (IMP-5) in pigeon liver homogenates at different times (Greenberg^') . 



TABLE VII 



Radioactive Formate as Precursor of Inosinic Acid, Inosine, and 



Hypoxanthine in Pigeon Liver Homogenate" 



Substance 



Specific activity, counts//iM. 



Inosinic acid 



Inosine 



Hypoxanthine 



18,600 

 9,900 

 4,730 



the activity was located in the nucleotide (Table VII) while the nucleoside 

 occupied an intermediate position between inosinic acid and hypoxanthine. 

 Thus inosine could not have been a precursor of inosinic acid but it might 

 well have been an intermediate in the conversion of the nucleotide to hypo- 

 xanthine. On the basis of these experiments Greenberg" postulated the 

 following reactions for the synthesis of hypoxanthine: 



COo + 3 NH3 + glycine + 2 "formate" + ribose-1 -phosphate —>■ inosine-5- 



, , ^ -H3PO4 . . +H3PO4 , ,, . , ., 1 u u ^ 



phosphate ^inosine ^hypoxanthine + nbose-1-priosphate. 



The reversibility of the last two reactions was demonstrated by incuba- 

 tion of a pigeon liver homogenate with labeled hypoxanthine. Isolation of 

 the inosine and inosinic acid formed and analysis of their C^"* content demon- 

 strated the conversion of hypoxanthine to these substances. Inosine con- 

 tained almost twice as much C^^ as did the nucleotide and was therefore 

 considered to be an intermediate in the synthesis of inosinic acid from 

 hypoxanthine. 



