VITAMIN AND GROWTH FACTOR RESEARCH 



casei; following acid hydrolysis the value given by the former 

 organism rises to the constant value obtained with the latter. 

 This observation of Wright and Skeggs (85) demonstrated the 

 natural occurrence of a biotin conjugate and illustrates again 

 the utility of quantitative comparative bioassays in the detection 

 of naturally occurring vitamin derivatives. Isolation (86) and 

 characterization (40) of the compound, which has the structure 

 illustrated in formula XIII, was followed by means of this 

 same assay procedure. 



O 



II 



HC CH NH2 



I I I 



HoC^ GHCH2CH2CH2CH2CONHCH2CH2CH2GH2CH 



S I 



COOH 



(XIII) Biocytin 



This compound replaces biotin as a growth factor for 

 animals, and for some but not all bacteria (84). Its metabolic 

 significance is not yet known, but its unusual structure and the 

 high proportion of the total biotin in some materials comprised by 

 it indicate that it is more than a metabolic oddity. 



VITAMIN B12 



This spectacular substance is exceptional in all ways : in the 

 complexity of its structure, in the variety of active forms that can 

 be obtained from nature, in its content of a metal ion (cobalt), in 

 its unusally high (even for vitamins!) biological activity, and in 

 the fact that one group of investigators succeeded in isolating it 

 through the exclusive use of man as a test animal (aided greatly 

 in the latter phases, undoubtedly, by the distinctive red color of 

 the vitamin!). Nonetheless, its isolation in this country was 

 greatly aided by discovery that certain lactic acid bacteria re- 

 quired concentrates of it for growth under some conditions — an 

 observation first made by Shorb (59). Because of its activity in 

 controlling pernicious anemia in man, in partially alleviating 



101 



