808 PETER REICHARD 



quence is represented below: 



ribose-5'-phosphate + ATP :;:^ 5'-phosphoribosylpyrophosphate + AMP 

 PRPP + orotic acid :;=^ orotidine-5'-phosphate + pyrophosphate 



orotidine-5'-phosphate -^ UMP + CO2 



PRPP seems to occupy a central role in nucleotide formation, since this 

 compound together with adenine, guanine, and hypoxanthine in the pres- 

 ence of the proper enzyme fraction forms the corresponding 5'-nucleotides. 



Saffran and Scarano^"*^ had earlier postulated ribose-1 ,5-diphosphate as 

 an intermediate in the formation of AMP from adenine, ribose-5'-phos- 

 phate, and ATP in pigeon hver. It is likely, however, that these authors 

 were studying the same enzymes as Romberg's group and that also in their 

 case PRPP rather than ribose-1 ,5-diphosphate was the actual intermediate. 



'« M. Saffran and E. Scarano, Nature 172, 949 (1953). 



