IV. BIOCHEMICAL SYSTEMS 33 



of ATP. On the other hand, methionine must also be converted to a form 

 having ternary sulfur in order to serve as a methyl donor. For its formation 

 ATP is necessary, and the reaction consists in the addition of adenosine 

 to methionine with the splitting out of 3 moles of H3P04.*^ Active methio- 

 nine, or S-adenosylmethionine, is believed to be the donor of methyl to 

 guanidoacetic acid, to niacinamide, and, probably, to aminoethanol. Ac- 

 cording to the hypothesis presented earlier, homocysteine is remethylated 

 with methyl from betaine, the oxidation product of choline, or with methyl 

 from thetin. 



Little is know^i of the mechanisms and of the energetics of formate-to- 

 methyl synthesis. Because of the original concept of labile methyl as an 

 indispensable dietary factor, emphasis has been largely on the proof of a 

 metabolic origin of methyl groups. The evidence for this is satisfactory from 

 a qualitative viewpoint. However, whether the intermediate is formate, 

 formaldehyde, a phosphorylated derivative of a one-carbon molecule, or, 

 possibly, folinic acid, is unknown. Of utmost importance is the identi- 

 fication of the formate acceptor, for instance, in the reaction that yields 

 monomethylaminoethanol. Does reduction of the carbon occur before or 

 after its addition to the amino nitrogen atom? If before, is thetin the formate 

 acceptor? Answers to these questions will mark a significant milestone in 

 the progress of intermediary metabolism. 



5. Choline Oxidase and the Choline-Betaine-Glycine Relationship 



a. Choline Oxidase 



The demonstrated transfer of labeled methyl in vivo from choline to homo- 

 cysteine gives no clue to the intermediate steps between the original donor 

 and the final acceptor. Several lines of investigation have supported the 

 hypothesis that the methyl of choline becomes a labile methyl only after 

 the oxidation of the alcohol, choline, to the acid, betaine. The presence of 

 an oxidizing enzyme, a choline oxidase, in tissues was first suggested by 

 Bernheim and Bernheim,^'*'* who observed an increase in oxygen uptake when 

 acetylcholine was added to liver and kidney extracts of rat and cat. Guinea 

 pig liver was found to be inactive in this respect. The fact that choline was 

 oxidized by tissues was confirmed by TrowelV"*^ using liver slices. Mann 

 and QuasteP^^ isolated the oxidation product of choline as the reineckate 

 and identified it as betaine aldehyde. Some evidence was obtained that 

 betaine aldehyde was also oxidized by rat liver slices and rat liver or kidney 

 extracts, though at a much slower rate than choline. It was suggested that 

 the oxidation product in this case was betaine. 



»" F. Bernheim and M. L. C. Bernheim, .4m. J. Physiol. 104, 438 (1933); 121, 55 (1938) 



'" O A. Trowell, J. Physiol. (London) 85, 356 (1935). 



'"« P. J. G. Mann and J. H. Quastel, Biochem. J. 31, 869 (1937). 



