92 INTERMEDIARY METABOLISM AND GROWTH I 



195 1). Carbon dioxide is the precursor of carbon 6 of the purine ring; the car- 

 boxyl- and a-carbons of glycine give rise to carbons 4 and 5 respectively, while 

 formate and the ^-carbon of serine are the precursors of carbons 2 and 8 

 (Buchanan, 1951; Elwyn and Sprinson, 1954) (Fig. 43). By comparing the in- 

 corporation of glycine- 1 -^"^C and the ^^N of ^^N-labelled compounds into the 

 hypoxanthine synthesized by extracts of pigeon liver, 

 organic substrates which are the precursors of nitrogen 

 atoms of the purine ring base have also been deter- 

 mined. One mole of glycine nitrogen, two moles of 

 the amide nitrogen of glutamine, and one mole of ho- 

 aspartic acid nitrogen are incorporated for every mole 

 of glycine- 1 -^■^C (Levenberg et al., 1956; Sonne et al., p- .„ Uj.j^, ^^-^^ 



1956). The nitrogen of glycine is the precursor of 



purine atom number 7, the amide nitrogen of glutamine gives rise to nitrogen 

 9 and 3, while aspartic acid is the precursor of nitrogen i of the purine ring. 



The incorporation of labelled glycine, formate, or serine into the nucleic acid 

 purines or the purines of acid sokible nucleotides (Elwyn and Sprinson, 1954) 

 has been readily demonstrated in vivo (Edmonds and LePage, 1955; Bennett et al., 

 1955, 1956; Harrington and Lavik, 1955; Drochmans et al., 1952) and in vitro 

 (Mannell and Rossiter, 1955; Totter, 1954; Abrams and Goldinger, 1951) in 

 normal tissues and tumors (LePage, 1953) and yeast cells (Abrams, 1952). 



As a result of experiments with pigeon liver extracts, it became apparent that 

 ribose phosphate derivatives are intermediates in the synthesis of purine nucleo- 

 tides (Greenberg, 1954; Korn et al., 1955; Levenberg and Buchanan, 1956). 

 The under-mentioned reaction : the formation of 5-phosphoribose pyrophosphate 

 may be taken as the starting point of purine nucleotide synthesis (Kornberg et al., 

 1955a; Goldthwait et al., 1955). 



Ribose-5-phosphate + ATP — > AMP + 5-phosphoribosepyrophosphate 



The enzyme catalyzing this reaction has been partially purified from pigeon liver 

 and has also been demonstrated in mammalian liver and in microorganisms. The 

 succeeding steps are outlined in Fig. 44. In the presence of labelled glycine, PRPP, 

 ATP, and glutamine, liver enzymes catalyze the formation of GAR (Goldthwait 

 et al., 1955; Goldthwait, 1956). Azaserine is a competitive antagonist of glutamine 

 in this reaction. Presumably, PRA is an intermediate in GAR synthesis although 

 this substance has not as yet been isolated from the reaction mixtures. It is known, 

 however, that this intermediate can be readily degraded to ribose-5-phosphate. 

 Chemically synthesized PRA can replace glutamine and PRPP in GAR synthesis. 

 However, ATP is still needed, presumably, for the activation of glycine. In the 

 presence of a pigeon liver extract which contains all of the enzymes needed for the 

 ncorporation of labelled glycine into inosinic acid, azaserine inhibits inosinic acid 

 synthesis and at the same time, labelled GAR and FGAR accumulated (Hartman 

 et al., 1955, 1956). The formation of GAR and FGAR in the absence of azaserine 

 has also been demonstrated (Goldthwait et al., 1956). The FGAR also is labelled 

 when formate-'^'C is used as substrate, but in this case, the GAR in unlabelled. 

 Either GAR-^'^C or FGAR-''*C can be converted bv the liver enzvmes to inosinic- 



