484 VITAMIN Bi2 



active than vitamin B12. From this work^- it thus appears that a highly 

 microbiologically active form of vitamin B12 exists which under assay con- 

 ditions is more stable than existing known forms. 



3. Methionine 



Davis and Mingioli,^^ studying mutants of E. coli, have demonstrated 

 that vitamin B12 and methionine serve as alternate growth factors for 

 certain mutants. These authors interpret this as meaning that vitamin B12 

 functions as a coenzyme in the synthesis or transfer of labile methyl groups, 

 since all the vitamin Bi2-requiring mutants are blocked between homo- 

 cysteine and methionine. E. coli mutants which can utilize either homo- 

 cysteine or methionine can synthesize their requirements for vitamin B12, 

 since their synthetic mechanism is at an earlier stage in the pathway of 

 biosynthesis of homocysteine. The observation that two compounds are 

 interchangeable for growth of a microorganism does not at all reveal which 

 compound functions in the synthesis of the other. The existence of two 

 E. coli mutants, Nos. 113-30 and 26-20, which grow very rapidly with 

 methionine but considerably more slowly with vitamin Bi2^^ strongly sug- 

 gests that vitamin B12 is the biocatalyst and methionine the product. An 

 impermeability to vitamin B12 was ruled out by the observation that the 

 above mutants readily absorb vitamin B12 from the medium and hence a 

 biochemical defect in vitamin B12 utilization exists. Davis and Mingioli^" 

 postulate that this defect is a limited ability to form a vitamin B12 coen- 

 zyme. Choline, betaine, and creatine do not produce growth with the 

 methionine-vitamin B12 mutants and therefore do not serve in E. coli as 

 methyl donors for the methylation of homocysteine. Desoxy ribosides, in 

 marked contrast to the situation with lactobacilli, do not substitute for 

 vitamin B12 in the nutrition of E. coli mutants. Since desoxyribosides exist 

 in the cells of both groups of organisms, and it seems quite unlikely that 

 they would be synthesized by totally different mechanisms, this is indeed 

 an interesting relationship. Davis and Mmgioli^^ consider it improbable 

 that methionine-vitamin B12 mutants, for example, E. coli 113-3, have 

 only a partial block in the synthesis of vitamm B12 allowing enough for 

 synthesis of desoxyribosides but not for that of methionine. 



Cell suspensions of the mutant E. coli 113-3 \vd\e been studied by Oginsky 

 et al}^ regarding the oxidative capacities of the cells as influenced by vita- 

 min B12. After storage for 5 days or longer of cells grown under moderately 

 anaerobic conditions, the rates of oxidation of acetate, pja'uvate, oxalace- 

 tate, succinate, glutamate, malate, fumarate, acetoacetate and stearate 



"> B. D. Davis and E. S. Mingioli, J. Bacteriol. 60, 17 (1950). 



16 E. L. Oginsky, P. H. Smith, N. E. Tonhuzy, W. W. Umbreit, H. C. Lichstein, and 

 S. F. Carson, /. Bacteriol. 61, 581 (1951). 



