COMPETITIVE ANALOGUE-METABOLITE INHIBITION 465 



to an extent such that another secondary enzyme system requiring the 

 factor became the limiting reaction of the biological system. Since the 

 rate of synthesis of pantothenic acid under the testing conditions is 

 determined by the ratio of cysteic acid to aspartic acid, the addition of 

 such a product of a secondary enzyme system would be expected to result 

 in an increase in the inhibition index — a type 3 effect. 



Such an effect is obtained with citric acid, as-aconitic acid, or a-keto- 

 glutaric acid. The inhibition index determined over a range of aspartic 

 acid concentrations is increased approximately tenfold when one of these 

 substances is added to the medium of Escherichia coli. Although inactive 

 alone, supplements of oxalacetic and pyruvic acids together increased the 

 inhibition index slightly. Acetate had a similar slight effect. These effects 

 were not comparable to those of the tricarboxylic acids or of a-ketoglu- 

 taric acid. Pantoic acid was inactive. Since the sparing action of citric 

 acid, ra's-aconitic acid, or oc-ketoglutaric acid cannot be duplicated by 

 the precursors of the tricarboxylic acids, it appears that in Escherichia 

 coli pantothenic acid functions in the biosynthesis of the tricarboxylic 

 acids, citric and as-aconitic acids, from oxalacetic acid and pyruvate or 

 acetate. It has since been shown that coenzyme A, which was found to 

 contain pantothenic acid while the work just mentioned was being com- 

 pleted, 13 functions in the oxidation of acetate in yeast 14 and in the forma- 

 tion of citric, acid by pigeon liver from oxalacetic acid and acetate. 15 

 Phenylpantothenone has been reported to prevent the conversion of 

 pantothenic acid to coenzyme A in yeast. 16 



The "sparing action" of a-ketoglutaric acid on pantothenic acid and 

 the precursor effect of glutamic acid in the biosynthesis of aspartic acid 

 result in an unusual effect of glutamic acid on the toxicity of cysteic acid 

 for Escherichia coli. Apparently a very rapid transamination with 

 oxalacetic acid results in the formation of essentially equivalent amounts 

 of aspartic acid from supplementary glutamic acid. The presence of a-keto- 

 glutaric acid results in a "sparing action" on the product of the system 

 such that glutamic acid is 3 to 10 times as effective as aspartic acid in 

 preventing the toxicity of cysteic acid. 9 This puzzling situation of an 

 apparent precursor of a metabolite being more effective than the metabo- 

 lite itself led to the elucidation of the complete cycle in which glutamic 

 acid simultaneously acts as a limiting precursor and as the product of a 

 secondary enzyme system utilizing the product of the blocked enzymatic 

 reaction as indicated in Figure 4. 



At an inhibition index of approximately 3,000, N-pantoyl-n-butylamine 

 inhibits the utilization of pantothenic acid by Lactobacillus arabinosus 

 17-5. A supplement of either oleic acid or "Tween 80" increases the inhibi- 

 tion index approximately tenfold to a value of 30,000. Since either acetate 



