G. ROBERT GREENBERG 



A rather interesting example which has not yet been re- 

 solved may be cited from the IMP synthesis problem. In 

 studying the de novo synthesis of IMP in pigeon liver extracts it 

 was found that AMP was not being formed at all nor did it 

 appear to be an intermediate as determined by the lack of ap- 

 pearance of a C^^ purine precursor in carrier AMP. Yet 

 AMP synthesis does occur in these extracts. 5-Amino-4-imida- 

 zolecarboxamide-5'-phosphoriboside (IRMP) is converted to 

 IMP by the reactions: 



HCi^OOH + ATP + tetrahydrofolate > 



Nio-CI^HO tetrahydrofolate + ADP -{- Pi 



IRMP + Nio-CI^HO tetrahydrofolate > 



Ci4_IMP + tetrahydrofolate 



Under these circumstances it was found that not only IMP but 

 also AMP was labeled.* This situation might be explained by a 

 reference to the following scheme : 



carboxamidine compound > AMP 



?r ^ E. 



+ "NH2"X 



precursors > IRMP 



IMP 



It may be considered that the K^ of IRMP with E2 is much lower 

 than it is with Ei and therefore at the low level of IRMP en- 

 countered during the steady-state synthesis of IMP not enough 

 IRMP would be present to be carried over route Ei and E3. 

 Thus whereas the actual rate of Ej when saturated with sub- 

 strate might be greater than that of E2, such a condition may 

 not be satisfied except by the artificial addition of the inter- 



* Dr. C. E. Carter has obtained evidence for synthesis of AMP (3) by a 

 pathway other than through IMP and apparently from the carboxamide 

 compound. Private communication. 



546 



