ESMOND E. SNELL 



acetate for Lactobacillus casei but was far more active on a weight 

 basis. This observation thus provided an assay method of known 

 specificity for the substance. Independently, O'Kane and 

 Gunsalus (38) had observed that cells of Streptococcus faecalis 

 grown in synthetic medium required an unidentified substance in 

 yeast extract to permit oxidation of pyruvate to acetate. Placed 

 in juxtaposition, these findings suggested identity between the 

 two unidentified substances, and following comparative tests of 

 concentrates, Snell and Broquist (68) concluded that the two 

 substances were identical. 



In an independent line of investigation, Kidder and Dewey 

 had successfully sorted out the complex nutritional requirements 

 of a protozoan, Tetrahymena geleii, and showed it to require one 

 or more unidentified substances. In 1949, Stokstad and co- 

 workers (78) showed that this unidentified substance contained 

 pyridoxal, copper ions, and an unidentified growth factor that 

 was named protogen, all of which were necessary for growth in the 

 basal medium. Supplementation of this medium with the 

 former two substances thus permitted development of an assay 

 of known specificity for protogen, and the preparation of con- 

 centrates. Such concentrates also were highly active in pro- 

 moting growth of L. casei — a fact that led Snell and Broquist (68) 

 to conclude that the acetate-replacing factor for L. casei and 

 protogen were identical. 



By use of these assay methods, isolation of an active crys- 

 talline substance was first achieved by Reed et al. (52), who 



CH2 

 HsQ^ ^CHCHgCHsCHsCHgCOOH 



HS SH 



(a) 



/CH2 

 HaC^ ^CHCHgCHaCHsCHaCOOH 



s s 



(6) 



;XV) Lipoic acid, (a) reduced and (6) oxidized 



104 



