KARL FOLKERS 



of pimelic acid in promoting the biosynthesis of biotin. Cysteine or 

 cystine, as sources of organic sulfur, enhanced the effect of pimelic 

 acid. The studies by du Vigneaud, Dittmer, Hague, and Long (57) 

 on the growth-stimulating effect of biotin for the diphtheria bacillus 

 in the presence and absence of pimelic acid led to the interpretation 

 that pimelic acid was being utilized as a precursor by the diphtheria 

 bacillus for the biosynthesis of biotin. 



Experiments described by Dittmer, Melville, and du Vigneaud 

 (3) on the activity of desthiobiotin (XXI) for stimulating the growth 

 of S. cerevisiae showed that desthiobiotin disappeared from the incu- 

 bating yeast cultures and was replaced by an equivalent amount of 

 a substance possessing growth activity for L. casei. To these investi- 

 gators, the most logical interpretation was that desthiobiotin (XXI) 

 was transformed to biotin (XX) by the growing yeast cell. Further 

 support for this interpretation was supplied by the experiments of 

 Stokes and Gunness (54), which showed that, when extracts of yeast 

 grown with desthiobiotin were treated with Raney nickel, the process 

 (60) for converting biotin to desthiobiotin, the activity of the yeast- 

 formed substance for L. casei was destroyed and activity for the yeast 

 was retained. Avidin also neutralized the yeast-formed substance, as 

 it does biotin. 



Another example of biosynthesis was found by Stokes, Keresz- 

 tesy, and Foster (55), who reported that the S.L.R. factor was con- 

 verted by S. lactis R into a substance which was active for the growth 

 stimulation of L. casei. 



The possibility that alanine might be a biological precursor 

 of vitamin Be was recognized two years ago by Snell and Guirard (49) 

 when it was found that vitamin Be was not required for the growth of 

 S. faecalis R if sufficient alanine was added to the medium. Subse- 

 quent studies by Snell (48) showed that an enzymic digest of casein 

 contained an unknown biological precursor which, together with dl- 

 alanine, permitted the growth of L. casei in the absence of vitamin Be. 

 Since fl?( — )alanine was active and /(4-)alanine was almost inactive, 

 these data seem to be the first which indicate that the "unnatural" 

 amino acids may be essential for normal metabolic processes. 



It is possible that future research on biological precursors and 

 biosyntheses of the vitamins will make these substances available by 

 new methods of startling simplicity. 



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