36. BIOSYNTHESIS OF PYRIMIDINE NUCLEOTIDES 341 



deoxyribose is derived from ribose or a precursor thereof. Comparable con- 

 elusions have been reached by Bagatell et al. ul using acetate-1-C 14 as a car- 

 bon source. 



The exact role of the Racker aldolase enzyme 142 in deoxypentose synthesis 

 is not yet clear. Lanning and Cohen 143 have shown that bacteriophage in- 

 fection alters the labeling of deoxypentose from glucose- 1-C 14 in a manner 

 tentatively considered to indicate the operation of the Racker aldolase 

 rather than the normal pathway which in E. coli is predominantly from 

 phosphogluconate via ribose phosphate. 144 



The role of vitamin B12 in nucleic acid synthesis is still obscure. In an at- 

 tempt to clarify the role of B12 in deoxynucleoside synthesis, Downing and 

 Schweigert 145 have studied the incorporation of thymidine-G-C 14 into L. 

 leichmanii, an organism whose B12 requirement can be replaced by deoxy- 

 nucleosides. In the absence of added B i2 the deoxyribose of the labeled sub- 

 strate was utilized for DNA-pentose formation without dilution. In the pres- 

 ence of B i2 , however, considerable dilution occurred. The results strongly 

 suggest a role of B i2 in deoxynucleoside synthesis, although participation in 

 the Racker aldolase reaction or in the synthesis of the iV-glycoside linkage 

 does not appear to occur. In contrast to these results Wagle et a/. 146 have 

 shown using Bi 2 -deficient piglets and chicks that the vitamin has no in- 

 fluence either on the incorporation of formate-C 14 , formaldehyde-C 14 , gly- 

 cine-2-C 14 , serine-3-C 14 , or methionine-methyl-C 14 into polynucleotide bases 

 or on the utilization of glucose-C 14 for polynucleotide pentose formation. 



Dinning et al. m have reported that replacement of the B i2 requirement of 

 L. leichmanii by deoxycytidine considerably reduces the utilization of for- 

 mate-C 14 for DNA-thymine synthesis. No influence of B r2 on the utilization 

 of glycine-2-C 14 , serine-3-C 14 , or methionine-methyl-C 14 was observed. The 

 authors have proposed a role for B J2 in the reduction of formate during 

 methyl group synthesis. An involvement of B i2 in "1-C" metabolism has 

 also been suggested recently by Reichard 147a on the basis of a study of the 

 incorporation of cytidine-G-C 14 , deoxyuridine-G-C 14 , and thymidine-G-C 14 

 into the pyrimidine nucleoside residues of the polynucleotides of B ^-de- 

 ficient chick embryo minces. In this system, the incorporation of cytidine 



141 F. K. Bagatell, E. M. Wright, and H. Z. Sable, Federation Proc. 17, 184 (1958). 



142 E. Racker, J. Biol. Chem. 196, 347 (1952). 



143 M. C. Lanning and S. S. Cohen, /. Biol. Chem. 216, 413 (1955). 



144 M. C. Lanning and S. S. Cohen, J. Biol. Chem. 207, 193 (1954). 



145 M. Downing and B. S. Schweigert, J. Biol. Chem. 220, 521 (1956). 



146 S. R. Wagle, D. A. Vaughan, S. P. Mis try, and B. Connor Johnson, J. Biol. Chem. 

 230, 917 (1958). 



147 J. S. Dinning, B. K. Allen, R. S. Young, and P. L. Day, J. Biol. Chem. 233, 674 

 (1958). 



147a A. Bolinder and P. Reichard, J. Biol. Chem. 234, 2723 (1959). 



