654 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



(c') The Importance of Vitamin B 12 in Choline Synthesis: Drill and Mc- 

 Cormick 678 were the first to demonstrate that vitamin Bi 2 exerts a lipotropic 

 effect, which is not attributable to its choline content, when injected into 

 rats on a high-fat diet. The choline requirement necessary to maintain a 

 normal liver fat level was shown by Strength et al. m to be markedly reduced 

 by vitamin Br> and folacin. The effectiveness of vitamin Bi 2 in preventing 

 fatty livers in rats has also been demonstrated by Bennett and associates. 680 

 This vitamin functions in a similar manner in dogs. 681 



The mechanism by which vitamin Bi 2 acts on fatty livers is not entirely 

 understood. One of the suggested methods by which the vitamin functions 

 is by its effect on transmethylation. Oginsky 682 reported that the livers of 

 vitamin Bi 2 -deficient rats exhibited a lower capacity for synthesizing methi- 

 onine from homocysteine plus choline or betaine than was observed in the 

 case of control animals. Bennett et al. m likewise state that vitamin B J2 is 

 concerned with the synthesis of methionine, but not with its demethylation. 



Another suggestion as to the mechanism of action of vitamin B i2 is that it 

 aids in the synthesis of the active methyl group. Arnstein and Neuberger 

 believe that vitamin B i2 is essential for the synthesis of methyl from 

 glycine, 683 as well as from unknown precursors. 684 Vitamin B i2 may be 

 active in the synthesis of labile methyl not only from glycine but also from 

 serine, 685 although Stekol et al. m now believe that it is concerned only with 

 glycine. 



As a result of in vitro studies, Dubnoff 687 concluded that vitamin B 12 is 

 concerned with the reduction of homocystine to homocysteine, which is the 

 direct methyl acceptor in the synthesis of methionine. In subsequent 

 studies with a mutant of Escherichia coli (Escherich's intestinal bacillus) 

 which required vitamin B J2 or methionine, it was found that the bacteria 

 can grow and synthesize methionine in the absence of vitamin B X2 if homo- 

 cysteine or certain reducing agents which can act on homocystine are 

 present. 688 It is believed that these data implicate vitamin B J2 in main- 



678 V. A. Drill and H. M. McCormick, Proc. Soc. Exptl. Biol. Med., 72, 388-390 (1949). 



679 D. R. Strength, E. A. Schaefer, and W. D. Salmon, J. Nutrition, 4-5, 329-343 ( 1951 ). 



680 M. A. Bennett, J. Joralemon, and P. L. Halpern, J. Biol. Chem., 193, 285-291 

 (1951). 



681 M. M. Burns and J. M. McKibbin, J. Nutrition, U, 487-499 (1951). 



682 E. L. Oginsky, Arch. Biochem., 26, 327-329 (1950). 



683 H. R. V. Arnstein and A. Neuberger, Biochem. J., 48, ii-iii (1951). 



684 H. R. V. Arnstein and A. Neuberger, Biochem. J., 50, xxxviii (1952). 



685 J. A. Stekol and K. Weiss, J. Biol. Chem., 186, 343-350 (1950). 



686 J. A. Stekol, S. Weiss, and K. Weiss, Arch. Biochem. Biophys., 86, 5-10 (1952). 



687 J. W. Dubnoff, Arch. Biochem., 27, 466-467 (1950). 



« 88 J. W. Dubnoff, Arch. Biochem. Biophys., 37, 37-45 (1952). 



