FUNCTION 



Two other substances which are able to replace the " replaceable 

 choline " ^^ are dimethylthetin (IV) and dime thy l-/8-propio the tin 

 ry\22a 



(CH3)2S . CH2 . COO- (CH3)2S . CH2 . CH2 . COO- 

 IV V 



The latter was isolated from algae, and may be present in pineapple. 



Transmethylation Reactions 



It is evident, therefore, that in the animal body, a number of 

 substances exist that are capable of donating a methyl group to 

 another substance, which serves as acceptor. These reactions are 

 now referred to collectively as transmethylation reactions and are 

 recognised to be of considerable importance in metabolism. The 

 formation of choline and creatine from methionine, of methionine 

 from choline and of choline from betaine are examples of transmethyla- 

 tion reactions, but other reactions are in competition with them for 

 the available supplies of labile methyl groups. One of these is the 

 methylation of nicotinamide to give N^-methylnicotinamide (page 254), 

 and the lipotropic effect of choline was reduced by the addition of 

 nicotinic acid to the diet,^^. 24 because some of the additional methyl 

 groups supplied by the choline were utilised in methylating nicotin- 

 amide. Other substances conceivably formed by methylation in vivo 

 are adrenaline, ergothioneine, anserine and dimethyl sulphone. 



Function of Choline 



Choline is therefore essential for the functioning of the animal 

 organism, and its partial replacement by methionine or betaine is 

 simply due to the fact that these substances are able to promote the 

 formation of choline from aminoethanol or from mono- or dimethyl- 

 aminoethanol (page 599). They are not completely biologically 

 equivalent because they are responsible for the methylation of other 

 substances, and choline has other functions in addition to its methylat- 

 ing action. 



The lipotropic action of choline is believed to be due to its incor- 

 poration into phospholipid molecules, which facilitate the transport 

 and metabolism of fatty acids.^^ It seems probable, however, that 

 choline acts by stimulating the formation and utilisation of fats in 

 the liver rather than by increasing fat transport in the plasma, as it 

 increased the turnover of liver lecithin, but not that of the plasma 

 phosphatides.^^ Aminoethanol, methylaminoethanol and dimethyl- 

 aminoethanol had a similar action, but diethanolamine reduced 

 phosphorylation because it was incorporated into phospholipids that 



601 



