IV NUCLEOTIDE SYNTHESIS IO5 



with the "one carbon compound" to produce 5-methyl deoxycytidine (or a 

 derivative thereof) and the latter compound may then be deaminated to thy- 

 midine. The presence of deoxycytidine increases the conversion of formate to 

 DNA-5-methylcytosine in lymphatic tissues and tumors (Kit, 1957; Fig. 46). 



The deamination of cytosine compounds to uracil compounds has already been 

 discussed. Three further points concerning DNA-thymine synthesis should be 

 mentioned. First, a hydroxymethyl derivative of thymine may well be the primary 

 product of the reaction of deoxyuridine with the hydroxymethyl derivative of 

 tetrahydrofolic acid. Hydroxymethyl cytosine is a component of the DNA of E. 

 coli phage. Secondly, it is possible that phosphorylated, or dihydroderivatives of 

 deoxyuridine rather than the free nucleosides are the physiological acceptor 

 compounds of formate. Thirdly, there is the question of the origin of deoxycytidine 

 and deoxyuridine. These compounds possibly are derived from cytidine and uridine 

 Labelled cytidine and (at high concentrations) labelled uridine are precursors of 

 DNA cytosine and thymidine in the rat (Reichard, 1955; Grossman and Visser, 

 1954; Hammarsten et al., 1950). The presence of non-labelled uridine and cytidine 

 stimulate the conversion of formate- '"'C to DNA-thymine by Ehrlich tumor cells 

 suspensions (PrusofT and Lajtha, 1956) while the presence of cytidine greatly 

 increases the conversion of formate-^'^C or serine-3-^'*C to DNA-thymine and 

 increases somewhat the labelling of thymidine of normal lymphatic tissue and 

 tumor cell suspensions (Kit, 1957). 



The conversion of cytidine to the deoxycytidine and thymidine of DNA occurs 

 in the rat without the cleavage of the pyrimidine-sugar linkage {KoW et al., 1956a; 

 Rose and Schweigert, 1953). There is some cleavage of the pyrimidine-sugar 

 linkage of uridine, however, during its transformation to DNA-thymine (Roll 

 et al., 1956a). In E. coli B cells, which can utilize free cytosine or uracil for growth, 

 cytidine and uridine are not obligatory intermediates in nucleic acid synthesis 

 (Rose and Schweigert, 1953; Bolton, 1954). 



It is to be noted that orotic acid is well utilized for the synthesis of DNA-cytosine 

 and thymine in vivo and in vitro by normal tissues and tumors (Harrington and 

 Lavik, 1955; Hurlbert and Potter, 1952; Lagerkvist and Reichard, 1954; Weed 

 and Wilson, 1954). Thymidine is in rapid equilibrium with the proximal DNA- 

 thymine precursors. Thymidine-2-^'^G is readily incorporated into DNA thymine 

 but not into other nucleic acid pyrimidines (Reichard, 1955). The utilization of 

 labelled thymidine for DNA-thymine synthesis has been demonstrated in bacteria 

 (Downing and Schweigert, 1956) plant tissues, protozoa, and animal tissues in 

 vivo and in vitro (Friedkin et al., 1956; Friedkin and Wood, 1956). The presence 

 of non-labelled thymidine inhibits the incorporation of 2-^'^C-uracil deoxyriboside 

 (Friedkin and Roberts, 1956) formate-^'*C, or serine-^'*C into the DNA thymine of 

 bone marrow cells and tumors (Prusoff ^fa/., 1956; Kit, 1957). 



Several enzyme fractions have been partially purified from E. coli B cells which 

 convert ^"^C-thymidine to thymidylate, thymidine triphosphate, and a polyde- 

 oxyribonucleotide having the properties of DNA. The polynucleotide is acid 

 insoluble, destroyed by deoxyribonuclease, alkali stable, and resistant to ribo- 

 nuclease. The conversion of 2-^'*C thymidine to the polynucleotide is reduced 50% 

 by the addition of non-labelled thymidylate, which in turn becomes radioactive. 



Literature p. 124 



