304 JOHN M. BUCHANAN 



I. Introduction 



In this chapter it is proposed to review the major lines of evidence upon 

 which the current formulation of the reactions of purine synthesis de novo 

 is based. This evidence has come from isotopic experiments with the intact 

 animal and with extracts and homogenates of tissues and finally from ex- 

 periments with isolated enzyme systems. The use of microbial mutants 

 and of systems blocked with metabolic poisons has been of great value. 

 From a technical point of view the modern armamentarium of methods for 

 the analysis, isolation, and purification of new compounds and enzymes 

 has provided the means of solving complex problems of biosynthesis which 

 would not have readily been amenable to study a decade or more ago. 

 Possibly the greatest contributing factor to the solution of the problem of 

 purine nucleotide synthesis has come from the fact that avian liver repre- 

 sents a source from which all the enzymes of purine synthesis can be ob- 

 tained in soluble form and in relatively high activity. 



II. Precursors of the Purines 



The study of the synthesis of the purine nucleotides in isolated enzyme 

 systems was preceded by an investigation of the metabolic precursors of 

 the purines (Volume II, Chapter 23). Schoenheimer and his colleagues had 

 shown that excretory purines and purines of nucleic acids and nucleotides 

 are readily formed from the nitrogen of administered ammonium salts. 4 ' 5 

 In contrast, purine or pyrimidine bases, with the exception of adenine, 6 

 were not major sources of tissue purines in the rat. Likewise, urea, 3 histi- 

 dine, 7 and arginine 8 are not precursors as such of the purine ring. These 

 experiments served to indicate that purines as well as pyrimidines are 

 formed from simple metabolic units rather than from preformed dietary 

 materials of more complex structure. 



The precursors of the carbon atoms of uric acid were determined by feed- 

 ing certain compounds labeled with C 13 followed by measuring the C 13 con- 

 centration of the individual carbon atoms of the excreted uric acid. 9 " 12 The 

 individual carbon atoms of the purine ring have their origin in the precur- 



4 F. W. Barnes, Jr., and R. Schoenheimer, J. Biol. Chem. 151, 123 (1943). 



5 A. A. Plentl and R. Schoenheimer, J. Biol. Chem. 153, 203 (1944). 



6 G. B. Brown, P. M. Roll, A. A. Plentl, and L. F. Cavalieri, J. Biol. Chem. 172, 

 469 (1948). 



7 C. Tesar and D. Rittenberg, J. Biol. Chem. 170, 35 (1947). 



8 K. Bloch, J. Biol. Chem. 165, 477 (1947). 



9 J. C. Sonne, J. M. Buchanan, and A. M. Delluva, J. Biol. Chem. 166, 395 (1946). 

 lu J. M. Buchanan and J. C. Sonne, J. Biol. Chem. 166, 781 (1946). 



11 J. C. Sonne, J. M. Buchanan, and A. M. Delluva, J. Biol. Chem. 173, 69 (1948). 



12 J. M. Buchanan, J. C. Sonne, and A. M. Delluva, J. Biol. Chem. 173, 81 (1948). 



