VITAMIN AND GROWTH FACTOR RESEARCH 



multiplies indefinitely the number of possible vitamin-like struc- 

 tures that can be obtained. These vary in their activities for 

 different test microorganisms and for animals. The first com- 

 pound isolated, first called vitamin B12 and later cyanocobalamin, 

 contained dimethylbenzimidazole as the nucleotide base, and 

 CN~ as the anion. It is fully active in animals and all micro- 

 organisms so far known, and is the vitamin B12 of commerce. 

 Pseudovitamin B12, which is similarly constituted but contains 

 adenine in place of dimethylbenzimidazole, has full activity for 

 some (not all) microorganisms, but little or none for animals. 

 The significance of this variability in structure remains to be 

 clarified. 



Similarly, the relationship of the intrinsic factor to vitamin 

 B12 remains to be clarified. This substance appears to be a 

 glycoprotein present in normal individuals but not in those with 

 pernicious anemia, and is necessary for the absorption of the 

 small amounts of vitamin B12 ordinarily present in foodstuffs. 

 Whether it functions by combining with the vitamin and pre- 

 venting its absorption by intestinal bacteria, thus conserving it 

 for the host, or whether it functions otherwise is not known. 

 The possibility that it promotes direct absorption through the 

 intestinal wall has been mentioned earlier (see p. 100). 



LIPOIC ACID (tHIOCTIC ACId) 



As early as 1937, Snell, Tatum, and Peterson (76) observed 

 the marked stimulating effects of acetate on growth of lactic 

 acid bacteria, and included this substance in the nutrient 

 medium for the purpose of clarifying other nutritional require- 

 ments of these bacteria. It was observed (75) that the substance 

 not only served as a buffer but played some more specific role in 

 growth, and that it was not required in media supplied with 

 yeast extract (76) . Following elucidation of several other growth 

 requirements of these organisms, Guirard, Snell, and Williams 

 (17) returned in 1941 to an investigation of this substance and 

 demonstrated the existence in yeast and other natural materials 

 of a substance that duplicated the growth-promoting effects of 



103 



