24 



CHOLINE 



3. Nature of Methyl Donors in Transmethylation 



The process of transmethylation involves demethylation of a methyl 

 donor and methylation of a methyl acceptor. Donors of intact methyl 

 groups appear to be limited to methylated quaternary nitrogen and sulfo- 



Acetylclioliiie 



Lecithins 



Spliingoniyclins 



(2) Betaine 

 . aldeliyde 



(10) Dimethyl- 

 aminoethanol 



+M or F? 



(9) IMonomethyl 

 aininoethanol 



+ MorF? 



Cephalins 



(4) Dimethjiglycinc 



(5) Mononietliylglycine 



Arginine 



(21) Cuanidoacetic acid 



+M 



Cystine 



Cysteine 



(20) Niacinamide 

 and otlier acceptors 



(22) Creatine 



Fig. 1. Schematic representation of pathways of transmethylation, of formate- 

 to-methyl sj^nthesis, and of methyl-to-formate oxidation. Letters M and F repre- 

 sent labile methyl and active formate, respectively. Directions of reactions are 

 indicated, but reacting compounds are not shown with the exception of the reaction 

 of homocysteine with meth3'l to yield methionine and with serine to j'ield cj^stathi- 

 onine. A distinction is indicated between the methyls of methionine and of active 

 methionine. The use of formate in the sj'nthesis of thetin, shown by the dash line, 

 is hjrpothetical. 



nium compounds. It is reasonable to relate lability of a methyl to its 

 attachment to a nitrogen or sulfur atom which has acquired an additional 

 covalent bond and positive charge. The main nitrogen compounds in this 



