MISCELLANEOUS WATER-SOLUBLE GROWTH FACTORS 



Factor W 



The factor W of D. V. Frost and C. A. Elvehjem ^ has ah^eady been 

 discussed (page 349). It would appear to be an impure preparation of 

 the filtrate factor. Another filtrate factor, possibly identical with 

 factor W, is factor By^, claimed by H. Kringstad and G. Lunde ^ to be 

 essential for the growth of rats. This factor was present in both liver 

 and yeast and differed from pantothenic acid in being stable to alkali 

 and not extracted at pH. i by ether. Liver and yeast also contained 

 a closely related factor, termed factor B^, which prevented grey hair 

 in rats. The precise status of these factors has never been determined. 



Factors R and S 



Schumacher et al. ^ designated as factors R and S two unidentified 

 factors necessary for the growth of chicks. Extracts of these factors 

 were prepared by Hill et al.^ from dried brewers' yeast by aqueous 

 extraction at 80 to 85° C, by extraction with acid or by digestion 

 with takadiastase. The extracts were concentrated under reduced 

 pressure, the _/)H was adjusted to i-6 and factor S precipitated by the 

 addition of ten volumes of alcohol. After adjusting the pH of the 

 filtrate to 7-0, factor R separated on standing ; the filtrate contained 

 folic acid. Thus neither factor was identical with pteroylglutamic 

 acid but factor R is possibly a conjugated form of folic acid. Factor 

 S is now known to be identical with strepogenin.^^ 



Strepogenin 



In 1944, H. Sprince and D. W. Woolley ^^ prepared concentrates 

 from solubilised liver extract by six different methods and tested them 

 as growth factors for haemolytic streptococcus, Streptococcus lactis 

 and L. helveticus. Since the relative activities of the concentrates 

 towards these organisms were substantially the same, it was concluded 

 that one and the same factor was probably responsible for the growth 

 stimulation in each instance. It was given the name of " strepogenin " 

 since " its presence is necessary for streptococci of group A to generate ". 

 Trjrptic digests of many pure proteins, e.g. insulin, trypsinogen, 

 trypsin, chymotrjrpsin and chymotrypsinogen, ribonuclease, tobacco 

 mosaic virus, haemoglobin and casein, were found to be good sources 

 of strepogenin. 12 -^q^ o^ly was strepogenin a growth factor for micro- 

 organisms, but it also stimulated the growth of mice.^^ jj^g growth 

 rate of mice was reduced when the animals were fed on a diet containing 

 hydrolysed casein together with cystine and tryptophan and restored 

 either by replacing these substances by intact casein or by supple- 

 menting them with a tryptic digest of casein.^* This indicated that 

 strepogenin was associated with a protein molecule, and this was 



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