346 GEORGE W. CROSBIE 



dine). In view of the speculations regarding the role of cytosine derivatives 

 as "1-C" acceptor molecules in thymine ring biosynthesis, the evaluation 

 of the quantitative significance of the — >UMP —» dUMP — * TMP pathway 

 will be awaited with interest. The mechanism of the reduction at postition 

 2' of the ribonucleotide is not yet understood. Brown et al. lb9 have tenta- 

 tively speculated on the role of a pyrimidine-O 2 ,2'-cyclonucleoside structure 

 in deoxypentose synthesis although stereochemical factors would argue 

 against the participation of analogous derivatives in purine deoxynucleo- 

 tide synthesis. It should be noted that Reichard 159a could find no evidence 

 for the utilization of 2 ,2'-cyclouridine-2-C 14 for polynucleotide pyrimidine 

 synthesis in a chick embryo mince system which incorporates uridine-2-C 14 

 into polynucleotide pyrimidines and deoxyuridine-2-C 14 into DNA-thymine. 

 Acetone powder extracts of the embryonic cell system possessed, however, 

 no significant kinase activity toward the cyclonucleoside and the evaluation 

 of the role of such cyclic structures in deoxypentose formation must there- 

 fore await appropriate experimentation with 2 ,2'-cyclouridine-5'-phos- 

 phate. 



c. Biosynthesis from Preformed Thymine Derivatives 



Free thymine, like uracil and cytosine, has generally been considered not 

 to be utilized 39 " 41 for mammalian DNA synthesis although Reichard 96 in 

 confirmation of earlier work of Holmes et al. no has observed a small but 

 significant incorporation of thymine-2-C 14 into regenerating rat liver DNA. 

 Thymidine, like other pyrimidine deoxyribonucleosides is extensively uti- 

 lized for DNA-thymine formation in a number of avian 161 ' 162 and mamma- 

 lian 96 - 151163 tissues. The incorporation of thymidine into the DNA of rat 

 liver homogenates has been reported. 164 



Isotope competition experiments have revealed that both thymine and 

 thymidine are not utilized for E. coli DNA synthesis. 165 The relatively in- 

 sensitive nature of the technique would, however, obscure the small but 

 significant incorporation of thymine and thymidine into E. coli DNA re- 

 ported by Graham and Siminovitch 166 and Kozloff, 167 respectively. It is of 



159 D. M. Brown, D. B. Parihar, C. B. Reese, and A. R. Todd, J. Chem. Soc. p. 3035 

 (1958). 



159a P. Reichard, J. Biol. Chem. 234, 2719 (1959). 



160 W. L. Holmes, W. H. Prusoff, and A. D. Welch, J. Biol. Chem. 209, 503 (1954). 



161 M. Friedkin and H. Wood, J. Biol. Chem. 220, 639 (1956). 



162 M. Friedkin, D. Tilson, and D. Roberts, J. Biol. Chem. 220, 627 (1956). 



163 P. Reichard and B. Estborn, J. Biol. Chem. 188, 839 (1951). 



164 F. J. Bollum and V. R. Potter, J. Am. Chem. Soc. 79, 3603 (1957). 



165 L. Siminovitch and A. F. Graham, Can. J. Microbiol. 1, 721 (1955). 



166 A. F. Graham and L. Siminovitch, in "Canadian Cancer Conference" (R. W. Begg. 

 ed.), Vol. I, p. 358. Academic Press, New York, 1955. 



167 L. M. Kozloff, Cold Spring Harbor Symposia Quant. Biol. 18, 1209 (1953). 



