482 VITAMIN Bi2 



found that L. ladis 8000 was affected in this manner by reducing agents 

 and that the requirement for vitamin B12 was increased by oxidizing con- 

 ditions or oxidizing agents. Kitay et al.^ made a detailed study of ascorbic 

 acid with several microorganisms. Ascorbic acid, thymidine, and vitamin 

 B12 all promoted growth of three out of ten lactobacilli. For several others, 

 for which thymidine and vitamin B12 gave similar effects, ascorbic acid was 

 ineffective. For one organism, L. acidophilus 204, ascorbic acid permitted 

 growth which was heavier and more rapid than that obtained with thymi- 

 dine whereas vitamin B12 was inactive. L. delbrueckii 730 is an example of 

 an organism for which neither vitamin B12 or ascorbic acid was effective 

 in replacing thymidine. 



Welch and Wilson^ found that ascorbic acid replaced vitamin B12 for the 

 growth of L. leichmannii only if the test medium contained an enzymatic 

 casein digest. The potency of ascorbic acid was increased by autoclaving 

 it with the medium. These observations served as the basis for the sugges- 

 tion that ascorbic acid converted inactive oxidized products of vitamin 

 B12 to microbially active forms. Other workers^ found that for ten of four- 

 teen cultures studied ascorbic acid must be autoclaved with the medium 

 and that most organisms would not respond to ascorbic acid unless enzy- 

 matic casein digest was added. Four cultures of lactobacilli grew equally 

 well or better when the ascorbic acid was added aseptically, and three 

 strains would utilize ascorbic acid in the absence of enzymatic casein digest. 

 These exceptions led Kitay et at} to question the interpretation of Welch 

 and Wilson^ and to conclude that ascorbic acid could not be acting solely 

 through the production of vitamin B12 from oxidized products in the en- 

 zymatic digest of casein since (a) it was active for several organisms, for 

 example, L. acidophilus S., in the absence of such digests, and (b) it pro- 

 moted growth of certain organisms typified by L. acidophilus 204 that 

 could not utilize vitamin B12 in place of ascorbic acid. A partial explana- 

 tion of the effects of ascorbic acid and other reducing agents may be that 

 only below a certain oxidation-reduction potential can vitamin B12 be syn- 

 thesized by organisms of the above type. This postulate must also include 

 the assumption that the cells of organisms which utilize ascorbic acid, but 

 not vitamin B12, are impermeable to extracellular vitamin B12 and must 

 rely on vitamin B12 synthesized within the cell in response to the presence 

 of ascorbic acid. As has been pointed out^ there is marked variation among 

 microorganisms of the same genus in the quantitative requirement of vita- 

 min B12 which most probably represents a difference in permeability to this 

 high-molecular-weight vitamin. 



The effect of reducing agents is closely connected with the relative micro- 

 biological activity of vitamins B12 and Biob under various conditions. For 



8 A. D. Welch and M. F. Wilson, Arch. Biochem. 22, 486 (1949). 



