IV. inOClIKMICAL SYSTEMS 441 



to be almost completely Iroo from folic acid.*"' '" Later it was reported by 

 Bennett'^ that both xitamin B12 and folic acid were needed to produce 

 growth in rats on such a diet. The growth was slow, 0.8 to 1.0 g. daily. 

 The addition of succinylsulfathiazole had evidently prevented the rats from 

 obtaining a supply of folic acid from their intestinal flora. When vitamin 

 Bi2 and folic acid were added in the experiments by Bennett, it may be 

 conjectured that certain of the dietary amino acids, perhaps glycine and 

 serine, were able to pro'/ide the necessary precursors for the methylation 

 of homocystine. It is important to note that methionine could luring about 

 the resumption of growth even when vitamin B12 and folic acid were not 

 added."^ 



The findings of Bennett were confirmed by Stekol and Weiss,"* who 

 included folic acid in their basal diet and emphasized the role of vitamin 

 B]2, whereas the experiments of Bennett were carried out under conditions 

 of folic acid deficiency which drew attention to the well-know^n role of the 

 intestinal bacteria in supplying folic acid. The basal diet used by Stekol 

 and Weiss contained dextrin, sucrose, fat, amino acids, minerals, and vita- 

 mins. Rats 30 days or older were able to grow on such diets in the absence 

 of methionine and choline if vitamin B12 and homocystine (or homocysteine) 

 were added. If -v'itamin B12 was omitted, they did not grow with homo- 

 cystine, but they grew at a suboptimal rate if methionine was added, or if 

 homocystine and choline were added. A relation between vitamin B12 and 

 the formation of methionine was indicated by the oljservations of Ogin- 

 sky,"^ who found that liver homogenates from ^^tamin Bi2-deficient rats 

 showed a lowered ability to form methionine as compared with homogenates 

 from animals recei\dng vitamin B12 when homocystine was added with 

 choline or betaine to the homogenates. In another report,"^ homogenates 

 of livers of rats deficient in folic acid or B12 or both were studied. It was 

 reported that B12 deficiency was associated with a lowered capacit}^ for the 

 synthesis of methionine from either choline or betaine and homocysteine, 

 confirming the oljservations by Oginsky,"^ whereas folic acid-deficient 

 homogenates showed a reduced ability to synthesize cysteine from homo- 

 cysteine and serine, presumably via cystathionine. 



Studies of the biological production and utilization of formate in the 

 tissues of rats were shown to invoh'e "labile methyl" groups. By the use of 

 isotopically labeled compounds it has been found that glycine could give 

 rise to formate which could combine with a second molecule of glycine to 

 form serine. Furthermore, the carbon atom of formate, the jS-carbon atom 



"^ G. W. Clark, Am. J. Med. Sci. 209, 520 (1945). 



"» J. A. Stekol and K. W. Weiss, J. Biol. Chem. 186, 343 (1950). 



"« E. L. Oginsky, Arch. Biochem. 26, 327 (1950). 



1" J. Stekol, K. W. Weiss, and S. Weiss, Federation Proc. 10, 252 (1951). 



