BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 331 



phate cannot be an intermediate in this reaction, because it is inhibitory. 

 Many nucleotides are obtained from the requisite nucleosides by prolonged 

 incubation; and much inorganic phosphate is formed by concomitant 

 splitting of the phenylphosphate. The phosphorylation seems to occur in 

 position 5 of the sugar; the reaction products were identified by chroma- 

 tography and could be dephosphorylated by 5-nucleotidase.'^ 



It appears that one is dealing here wath a model system; the artificial 

 phosphate donor, phenyl phosphate, under natural conditions may be re- 

 placed by a biological phosphoric acid ester, perhaps a nucleotide such as 

 adenosine or inosine phosphate. In recent years several similar phosphate 

 transfer mechanisms by hydrolytic enzymes have been observed.'"" The 

 analogy of this phosphate transfer to the transglycosidation described by 

 McNutt*° is obvious. Important results may be expected from the explora- 

 tion of this field. 



IV. Bios)rnthesis by Reaction of Pentose or Pentosephosphate with 



Incomplete Pyrimidine and Purine 



Systems 



The first suggestion that pentose or pentose phosphate may combine with 

 a nitrogenous precursor of nucleic acid bases came from the studies of 

 Loring and Pierce'"' and Mitchell and co- workers. '"^''"^ These investiga- 

 tors used various mutants of Neurospora crassa; some of them were found to 

 utilize the free pyrimidines very poorly in comparison with the correspod- 

 ing nucleosides, an observation which has its counterpart in tissue and whole 

 animal studies which hkewise proved inert toward the pyrimidines with the 

 exception of orotic acid.^^ This suggested that the pyrimidine bases are not 

 intermediates in nucleoside synthesis as exemplified by Fig. 6. 



Experiments to verify this hypothesis have so far remained suggestive 

 rather than conclusive. For lack of synthetic aliphatic ribosides, numerous 

 surmised precursors of orotic acid have been tested with various organ- 

 isms'"^-'"* (see also Chapter 23). If any of them were the primary reactant 

 with the pentose, it should surpass orotic acid in its growth-promoting 

 abihty or in the rate of incorporation into nucleic acids. Such precursors 

 have not been found.'"* -'"^''"^^ 



The question remains whether ring closure to orotic acid, or formation of 



100 R. K. Morton, Nature 172, 65 (1953). 



101 H. S. Loring and J. G. Pierce, J. Biol. Chem. 153, 61 (1944). 



102 H. K. Mitchell and M. B. Houlahan, Federation Proc. 6, 506 (1947). 



103 H. K. Mitchell, M. B. Houlahan, and J. F. Nye, J. Biol. Chem. 172, 525 (1948). 



lot L. D. Wright, K. A. Valentik, D. S. Spicer, J. W. Huff, and H. R. Skeggs, Proc. 



Soc. Expll. Biol. Med. 75, 293 (1950). 

 106 U. Lagerkvist, P. Reichard, and G. Ehrensvard, Acta Chem. Scand. 5, 1212 (1951). 

 106a L. D. Wright, C. S. Miller, and C. A. Driscoll, Proc. Soc. Exptl. Biol. Med. 86, 



215 (1954). 



