56 THE BIOCHEMISTRY OF B VITAMINS 



The basal medium as originally proposed has been modified by the 

 addition of more glucose and sodium acetate, the elimination of half of 

 the biotin, and the introduction of y>-aminobenzoic acid. 82 In the study 

 referred to earlier in connection with the riboflavin assay, Roberts and 

 Snell 50 found that the new medium for Lactobacillus casei also supports 

 heavy growth of Lactobacillus arabinosus in 16 hours, and suggest that 

 this improved medium may be made the basis of a nicotinic assay. In 

 spite of improvements which may be made in the basal medium, no 

 serious errors are introduced by using the medium as originally proposed. 



Recently two new microbiological assay methods for nicotinic acid 

 have been developed, one using a yeast, Torula cremoris, 83 the other a 

 nonpathogenic bacteria, Proteus HX19. 8i Both methods are more sensitive 

 and more rapid than the Snell-Wright method, and it remains to be seen 

 how widely they are used. One sure advantage, from the standpoint of 

 research, of having several available methods is the fact that the different 

 organisms respond characteristically to different natural derivatives. 



Biological Tests. Although assays for nicotinic acid have been made 

 to a limited extent using dogs 85, 8G ' 78 and chicks 86 as experimental 

 animals, the methods have not been standardized. As a result of recently 

 accumulated knowledge regarding the common transformation of trypto- 

 phan to nicotinic acid (p. 353) in mammals, it is obvious that no biolog- 

 ical test which leaves out of consideration this transformation can be 

 specific for nicotinic acid. 



Pantothenic Acid 



Although attempts have been made to determine pantothenic acid 

 chemically by lead tetraacetate oxidation of the lactone derived from it, 

 no serviceable method was developed, 87 and the only available assay 

 methods are microbiological and biological. 



Microbiological Methods. Many microorganisms require pantothenic 

 acid for growth or are stimulated by it, and all of these are potential test 

 organisms. 



The yeast test served adequately in the discovery and isolation of 

 this vitamin 88, 89 but later was not used, partly because of the effective- 

 ness of /^-alanine, a cleavage product, as a yeast growth stimulant, and 

 also because the pantothenic acid concentration governed the rate of 

 growth rather than the total growth, as in the case of the lactic acid 

 bacteria. 90, 91 Because of this latter fact, the multiplicity of yeast nutri- 

 lites, 92 and the complicated effects of amino acids, 93 it was feared that 

 the yeast method would not be as specific as would be required in dealing 

 with all sorts of materials. 



Recently the yeast test has been revived: a different strain of yeast 



