344 



GEORGE W. CROSBIE 



)H OH 



H 2 0/ ? \+2H 



oh y \ OH 



A^ch 2 n( a 



L JU-H 



0''^hr X H 

 / R 

 NJ-2H HaO^ 



^Y C H ^ H -H 2 Q , f^* 



O' ^NT X H 



R 



O" ^NT H 

 R 



Fig. 5. Possible mechanisms of thymidine-5'-phosphate (II, R = 5-phosphode- 

 oxyribosyl) formation from the postulated intermediate (I, R = 5-phosphodeoxy- 

 ribosyl) in the thymidine-5'-phosphate synthetase reaction. 



mate, N 10 — CHO— FH 4 , iV 5 ,iV 10 -methenyl-FH 4 , and A^ 5 ,^V 10 -methylene- 

 FH 4 are discussed elsewhere in this volume (Chapter 35). 



The mechanism of the "methylation" reaction is not yet clear but sev- 

 eral mechanisms have been considered (Fig. 5). Scheme A (Fig. 5) envisages 

 either a reductive fission 98 of the postulated intermediate at the CH 2 — N 

 linkage or an elimination reaction to yield TMP and dihydrofolic acid 

 (FH 2 ), the reduced pyridine nucleotide requirement being in respect of 

 regeneration of FH 4 from FH2 . Evidence bearing on the latter possibility 

 has been provided by the observation by Greenberg and Humphreys 151 of 

 a stoichiometric relationship between thymine methyl group formation 

 and FH 4 concentration in the enzymic system previously described by 

 Phear and Greenberg. 149 DPNH stimulates methyl group synthesis in the 

 presence of suboptimal concentrations of FH 4 . 



Scheme B (Fig. 5) involves a hydrolysis of the postulated intermediate 

 to yield a 5-hydroxymethylpyrimidine derivative which is successively re- 

 duced to the 4,5-dihydro derivative, dehydrated and rearranged. 121, 153 It 

 is of interest to note that Cohen et a/. 153 have reported that catalytic hydro- 

 genation of 5-hydroxymethyluracil and the corresponding deoxyribonu- 

 cleoside yields thymine and thymidine, respectively, in addition to 4,5- 

 dihydro derivatives (cf. Fink 154 ). 



153 S. S. Cohen, M. Green, and H. I). Barner, Biochim. et Biophys. Ada 22, 210 (1956). 



154 R. M. Fink, R. E. Cline, and K. Fink, Federation Proc. 15, 251 (1956). 



