Pfizer Handbook of Microbial Metabolites 



552 



The "active formaldehyde" form of the coenzyme is in- 

 termediate in the interconversion of glycine and serine: 



"Active 

 Formaldehyde" 



CH2— COOH ^ 



± HOCH2— CH— COOH 



NH2 



NH2 



Glycine 



The large literature on this subject has been reviewed.^ 



The "active formaldehyde" form may also be considered 

 to be a methyl group donor, although much remains to be 

 learned about the mechanisms of these donations. In the 

 biosynthesis of thymine from uracil, serine, formaldehyde 

 or formate are more effective precursors of the introduced 

 methyl group than is methionine, and this precursor effect 

 is inhibited by foUc acid antagonists.® Actually, the ac- 

 ceptor is probably not uracil, but deoxyuridine or deoxy- 

 uridylic acid: 



OH 



2'-Deoxyuridine 

 5'-phosphate 



OH 



2'-Deoxy-5-methyiol- 

 uridine-5'-phosphate 



Thymidine- 

 5'-phosphate 



The occurrence of 5-hydroxymethylcytosine in some spe- 

 cies has been cited as suggestive of formation of a hy- 

 droxymethyl intermediate in this way, at least in the 

 cytosine series."- ^^ On the other hand it has been re- 

 ported that in Lactobacillus leicJimamiii there is a vitamin 

 B12 requirement for the conversion of formic acid to the 

 thymine methyl group, and that the route does not involve 

 either methionine or a hydroxymethyl group. ^"^ 



" Seymour S. Cohen and Lawrence L. Weed, J. Biol. Chem. 209 

 789 (1954). 



15 Maurice Green and Seymour S. Cohen, ibid. 225 387 (1957). 



1*^ James S. Dinning, Barbara K. Allen, Ruth Young and Paul L. 

 Day, ibid. 233 674 (1958). 



