292 



PETER REICHARD 





CH + CoF 



*^'^ ^Mbose-PO« 

 Inosinic acid 



Formyl-CoF + HC^OOH — 





,CH + Formyl CoF 



H2N' 



^N-ribose-POi 



4-Amino-5-imidazolecarboxainide 

 ribotide 



■♦ Formyr-CoF + HCOOH 



,N 



II CH + FormyP-CoF 



H.N'^ N-ribose-P04 



.N 



\ 



CH + CoF 



HN 



xs N-ribose-P04 



4-Amino-5-imidazolecarboxaniide ribotide Inosinic acid-2-C'* 



Fig. 6. "Enzymic exchange" of formate with position 2 of inosinic acid.*' CoF 

 indicates 5,6,7,8-tetrahydrofolic acid; formyl CoF indicates N-5-formyl 5,6,7,8- 

 tetrahydrofolic acid; X designates radioactive carbon. (See also fig. 8 in Chapter 24.) 



The following reaction sequence for the synthesis of hypoxanthine sum- 

 marizes the present experimental evidence: 



glycine + CO3 + 3 "NH3" 



+ ribose 



glycine intermediate ,„„„*■ 

 ° •' + H3PO4 



,.„.,., + "HCOOH" . ., ., ,. , + "HCOOH 



glycme mtermediate ribotide »■ carboxamide ribotide 



citrovorum 

 factor 



inosinic acid 



hypoxanthine 



Many of the reactions are still very obscure, especially those involving 

 the hypothetical "glycine intermediate." This intermediate probably differs 

 from the carboxamide by more than the absence of the carbon in position 8 

 of the purine ringJ^^ The main purpose of the scheme is to show that ribo- 

 tide formation takes place before formation of the carboxamide structure. 



3. Interconversion of Purines 



Although the synthetic reactions leading to the formation of inosinic 

 acid are known to some extent, no link between this substance and the 

 purine bases of nucleic acids has been demonstrated. One cannot conclude 

 a priori that the synthesis of inosinic acid, which in the pigeon eventually 



''"' The present evidence points to glycine-N-amido-5'-phosphoribotide as being the 

 first ribotide compound formed (cf. G. R. Greenberg, Federation Proc. 13, 745 

 (1954). 



