35. BIOSYNTHESIS OF PURINE NUCLEOTIDES 313 



A^-formyl derivative [Eq. (5)]. The reaction is reversible. Upon purifica- 

 ,V 6 ,An o -Anhydro-FH 4 + H 2 <=i WO-formyl-FH, + H+ (5) 



tion of glycinamide ribonucleotide transformylase the cyclohydrolase is 

 lost. It is then possible to demonstrate 46 that the actual donor of the formyl 

 group to glycinamide ribonucleotide is the iV 5 ,A no -anhydroformyl-FH4 as 

 shown in Eq. (4). 



The next step in purine nucleotide biosynthesis 47 • 48 involves the reaction 

 of formylglycinamide ribonucleotide with glutamine and ATP to yield 



Formylglycinamide ribonucleotide + glutamine + ATP + H2O — * 



formylglyeinamidine ribonucleotide + ADP -f- orthophosphate (6) 



+ glutamic acid 



formylglyeinamidine ribonucleotide according to Eq. (6). Studies with 

 highly purified preparations of the enzyme which catalyze Eq. (6) suggest 

 that this reaction proceeds without the formation of free intermediates. 

 Incubation of P 32 -labeled orthophosphate or C 14 -labeled glutamic acid with 

 the enzymic system does not lead to the synthesis of radioactive ATP or 

 glutamine. It is therefore believed that, like the previous glutamine reac- 

 tion of this series, Eq. (6) is also in effect irreversible. 



This enzyme is readily inhibited by low concentrations of the antitumor 

 agents, azaserine and 6-diazo-5-oxonorleucine (Fig. 6) 49 (see also Chapter 

 39). They compete with the natural substrate of the reaction, glutamine, 

 for the enzyme site. The degree of inhibition is thus a function of the ratio 

 of glutamine to inhibitor. With the chicken liver enzyme there is half in- 

 hibition of the reaction when the ratio of concentration of glutamine to 

 azaserine is 18 or when the ratio of concentration of glutamine to 6-diazo- 

 5-oxonorleucine is 727. Mole-for-mole, 6-diazo-5-oxonorleucine is therefore 



NH 2 — CO— CH 2 — CH 2 — CHNH 2 — COOH 

 Glutamine 

 - + 



N=N=CH— CO— O— CH 2 — CHNH 2 — COOH 

 Azaserine 

 - + 



N=N=CH— CO— CH 2 — CH 2 — CHNH 2 — COOH 



6-Diazo-5-oxonorleucine 



Fig. 6. Structures of glutamine, azaserine, and 6-diazo-5-oxonorleucine. 



46 S. C. Hartman and J. M. Buchanan, 4th Intern. Congr. Biochem., Vienna, 13, 97 

 (1958). 



47 B. Levenberg and J. M. Buchanan, J. Biol. Chem. 224, 1019 (1957). 



48 I. Melnick and J. M. Buchanan, J. Biol. Chem. 226, 157 (1957). 



19 B. Levenberg, I. Melnick, and J. M. Buchanan, J. Biol. Chem. 225, 163 (1957). 



