IV. BIOCHEMICAL SYSTEMS 29 



SO far as either is added to the diet as a synthetic product. It is not known 

 definitely if any one molecule of methionine can serve the dual function of 

 supplying both methyl for transmethylation and sulfur for cysteine forma- 

 tion. "» 



Borsook and Dubnoff"'* first demonstrated the in vitro synthesis of methi- 

 onine from homocysteine and choline by rat liver slices, homogenates, and 

 lyophilized preparations. Betaine was superior to choline as a methyl donor, 

 and homocysteine to homocysteinethiolactone or homocystine as a methyl 

 acceptor. The reaction was independent of oxygen and of inhibitors of 

 cellular oxidative reactions. These workers"^ subsequently compared the 

 methyl-donating capacities of choline, betaine, dimethylthetin, and propio- 

 /3-dimethylthetin in rat liver slice preparations under anaerobic conditions 

 and in the presence of added homocysteine. Both sulfur derivatives were 

 more effective than betaine, and dimethylthetin was ten to twenty times as 

 effective. An enzyme was isolated in a partially purified state which trans- 

 ferred one methyl from the thetins. The enzyme was present in the liver 

 and kidney of rat, guinea pig, and hog but was absent from muscle, 

 pancreas, and spleen. 



It appears quite reasonable to believe that methylation of homocysteine 

 may involve the direct transfer of methyl from the betaine formed by the 

 oxidation of choline. However, there is no certainty that homocysteine is 

 methylated directly by formate-to-methyl synthesis, although this is a 

 plausible conclusion. Using inhibition and dilution studies in which a labeled 

 methionine methyl appeared after incubation of guinea pig liver slices with 

 homocysteine and C^^-formate, Berg concluded that neither choline nor 

 betaine methyl was an intermediate.^ ^^ In subsequent studies with a cell- 

 free extract of pigeon liver the chromatographic separation of the incubation 

 mixture yielded evidence for an unidentified product formed from C'*- 

 labeled formate and homocysteine.^" Reference has already been made to 

 active methionine studied by Cantoni.*^ It is again emphasized that such 

 a compound as adenosylmethionine may represent the methyl donor struc- 

 ture of methionine^ ^^' ''^ without necessarily being the molecule in which 

 methyl first appears as a result of formate-to-methyl synthesis. Methionine 

 sulfoxide has been reported to be the first intermediate in transmethyl- 

 ation. ^^ 



Stetten fed N'^-labeled aminoethanol to rats and determined the N^^ in 



'" D. J. Mulford and W. H. Griffith, J. Nutrition 23, 91 (1942). 



"■' H. Borsook and J. W. Dubnoff, ./. Biol. Chnn. 169, 247 (1947). 



"5 J. W. Duhnoff and H. Borsook, ./. Biol. Chcm. 176, 789 (1948). 



"6 P. Berg, ./. Biol. Chem. 190, 31 (1951). 



'" P. Berg, Federation Proc. 11, 186 (1952). 



'18 G. L. Cantoni, /. Biol. Chem. 189, 745 (1951). 



1'^ G. L. Cantoni, Federation Proc. 11, 330 (1952). 



