474 THE BIOCHEMISTRY OF B VITAMINS 



unidentified substances related to p-aminobenzoic acid, folic acid or re- 

 lated factors. 5 



Thymidine. One particular factor was especially interesting since it 

 prevented the toxicity of methylfolic acid for Leuconostoc mesenteroides 

 8293 and the toxicity of either sulfanilamide or 6,7-diphenyl-2,4-diami- 

 nopteridine for Lactobacillus arabinosus 17-5. 5, 55 Since an inhibition anal- 

 ysis indicated that the factor was a product of the biological functioning 

 of folic acid and p-aminobenzoic acid, the high activity of refined liver 

 extracts suggested the possibility that the factor might be a conjugated 

 form of folic acid. However, on isolation of the factor from hog liver, a 

 colorless crystalline compound was obtained and identified as thymidine 

 which was present in some refined liver extracts to the extent of 1 per 

 cent of the solids. 55 



The inhibition index for Leuconostoc mesenteroides inhibited by meth- 

 ylfolic acid is increased from 3,000 to 30,000 by the addition of thymi- 

 dine to the medium. Although thymine and thymidine are interchangeable 

 for many organisms, thymine is inactive in the above tests. Consequently, 

 the biosynthesis of thymidine does not appear to take place through the 

 intermediate formation of thymine. 55 



The Vitamin B^. Group. Of the many tests developed for factors 

 occurring in refined liver extracts, five were found to involve a func- 

 tionally related group of factors involved in the biochemical functioning 

 of p-aminobenzoic acid in Escherichia coli. 5 One testing method utilized 

 sulfanilamide in a concentration sufficient to prevent the biosynthesis of 

 methionine in Escherichia coli grown in a salts-glucose medium supple- 

 mented with known vitamins, xanthine, thymine, serine and glutamic 

 acid. Under these conditions, the organism responded to very small 

 amounts of refined liver extract; and by use of this assay technique, a 

 crystalline red compound was isolated from refined liver extracts after 

 a 20,000-fold concentration. 5 Because of its distinctive color and biological 

 properties, the factor was termed "erythrotin." This factor is apparently 

 identical with or closely related to vitamin B 12 . 5, 56 Still another factor, 

 which moves more slowly in organic solvents used as eluants during 

 various chromatographic separations, is also active in the Escherichia 

 coli assay; this has been tentatively named "erythrotide." 5 The root 

 prefix erythro- has been suggested as a basis for naming the individual 

 members of this group of factors which have the same biological function 

 but slightly different chemical structures. 



Although methionine replaces erythrotin for Escherichia coli under the 

 testing conditions, the latter compound is from 100,000 to 300,000 times 

 as active. Consequently, no interference by methionine was encountered 



