THE FOLIC ACID COMPLEX 



preparation (see page 467). This had the properties of a purine- 

 pyrimidine dinucleotide or a mixture of the two mononucleotides, and 

 could be replaced as a growth factor by a mixture of guanine and 

 thymine, although larger amounts of these were required in order to 

 produce the same response (page 513). Compared with the folic acid 

 of Mitchell, Snell and Williams, Snell and Peterson's factor and 

 Stokstad's factor were relatively crude, and this in part accounts for 

 the conclusion reached at the time by Mitchell et al. that folic acid 

 was not identical with either of these factors. 



The norit eluate factor from liver was still further purified by 

 Hutchings et al* who showed that it was essential for the nutrition of 

 the chick, although there was an element of doubt as to the identity 

 of the bacterial and chick factors, as the one preparation was not tested 

 for both types of activity. The method of isolation and the behaviour 

 towards inactivating agents were, however, identical with those 

 reported by earlier workers, and it was subsequently observed ^ that 

 purified preparations of this chick factor not only promoted growth, 

 but also resulted in increased haemoglobin formation and normal 

 feathering of chicks. 



The picture then became a trifle confused, following the publication 

 of a paper by E. L. R. Stokstad,^ describing the preparation of a 

 norit eluate factor from yeast. Although the product stimulated the 

 growth of L. helveticus to the same extent as did the liver factor, it 

 had only half the activity of the liver factor towards 5. faecalis R. 

 Stokstad, therefore, concluded that the two factors were different 

 and suggested that the liver factor might be identical with the vitamin 

 Be of Pfiffner et al.'^ (see page 469). The intimate relationship between 

 the L. casei factor (in its various forms) and vitamin Be received 

 further support from a report ^ that a pure preparation of the norit 

 eluate factor made by fermentation (see page 468) had been found 

 to stimulate the growth of L. helveticus and 5. faecalis R and also to 

 increase the growth rate of chicks. This fermentation product has 

 sometimes been referred to as " the third L. casei factor ". 



SLR Factor 



A fourth factor, described by Keresztesy et al.^, differed from the 

 three L. casei factors in being inert towards L. helveticus (L. casei), 

 although effective in stimulating the growth of S. lactis R (S. faecalis 

 R). Accordingly it was named the S. lactis R factor, abbreviated to 

 SLR factor. Subsequently, Stokes et al}^ found that folic acid could 

 replace the SLR factor for all bacteria that could utilise the latter 

 and that folic acid was produced when 5. faecalis R was grown on a 

 folic acid-free medium containing the SLR factor. The SLR factor 

 failed to produce folic acid when incubated with rat liver suspensions, 



458 



