464 THE BIOCHEMISTRY OF B VITAMINS 



Application of Inhibition Analysis to Elucidation of Biochemical 

 Transformations 



Biosynthesis of Pantothenic Acid. The specificity of the yeast assay 

 for pantothenic acid which resulted in the discovery of the vitamin is 

 dependent upon the presence of asparagine in the medium. 6 Asparagine 

 prevents the response of the organism to /^-alanine, which in the absence 

 of asparagine replaces pantothenic acid in stimulating the growth of 

 yeast. Similar results have been obtained with a number of a- and /?-amino 

 acids, 7 and with propionic acid 8 (p. 645) . 



The involvement of aspartic acid in the biosynthesis of /^-alanine and 

 of pantothenic acid in Escherichia coli has been demonstrated with two 

 analogues of aspartic acid. Cysteic acid ° and hydroxyaspartic acid 1 

 competitively prevent the functioning of aspartic acid in Escherichia coli 

 cultured in a salts-glucose medium. The inhibition indices are 30-100 and 

 3-16, respectively. If a supplement of either /^-alanine or pantothenic 

 acid is added to the growth medium, cysteic acid does not affect the 

 growth of the organism even at relatively high concentrations; but 

 hydroxyaspartic acid at high concentrations still prevents it. The inhibi- 

 tion index in the latter case is increased to 20-30. Thus, cysteic acid 

 apparently prevents only the conversion of aspartic acid to ^-alanine, 

 whereas hydroxyaspartic acid prevents that reaction and at least one 

 additional transformation involving aspartic acid. 



Inhibitions of the biosynthesis of pantothenic acid by several analogues 

 of pantoic acid have been reported. Salicylic acid has been considered in 

 this category 10 (p. 646). Either cc-hydroxy-/?,/?-dimethylbutyric acid or 

 /?,y-dihydroxy-/?-methylbutyric acid prevents growth of Saccharomyces 

 cerevisiae G. M. stimulated by /3-alanine, but does not affect growth 

 stimulated by pantothenic acid u (p. 646) ; hence, these compounds 

 apparently prevent the biosynthesis of pantothenic acid by preventing 

 the combination of pantoic acid with /^-alanine. 



Biochemical Functions of Pantothenic Acid. A study of the effect of 

 known metabolites on the inhibition of pantothenic acid synthesis by 

 cysteic acid in Escherichia coli has been utilized in an effort to determine 

 the metabolic function of pantothenic acid. 12 As the organism becomes 

 deficient in synthesis of the vitamin and the corresponding coenzyme, one 

 particular secondary enzyme system would be expected to become the 

 first limiting reaction for growth, since the various apoenzymes requiring 

 the coenzyme would be expected to differ in their affinity for the coenzyme 

 and in the quantity of the coenzyme required for normal enzymatic 

 activity. Addition of the product of this secondary enzyme system would 

 allow growth until the rate of synthesis of pantothenic acid was decreased 



