ACETIC ACID AND ACETATE 



277 



i. The Formation of Acids Related to Citric Acid. The identity of the 

 six-carbon acid formed in the citric acid cycle has been a matter of dispute. 

 The tricarboxylic acids, which are known to be formed, namely, citric, iso- 

 citric, and m-aconitic, are interconvertible as shown below: 



COOH 



I 

 HCH 



HOC COOH 



HCH 



COOH 



Citric 

 acid 



Interconversions of Tricarboxylic Acids 



When the enzyme, aconitase, is present, all three acids will appear in the 

 system/'*^'!^^ Although Krebs and Johnson^^^ originally proposed that 

 citric acid was the initial condensation product, this hypothesis had to be 

 abandoned when Evans and Slotin,^^^ as well as Wood et al.,^*^ demon- 

 strated that the labeled carbon in newly-synthesized a-ketoglutarate 

 formed from labeled CO2 and pyruvate in pigeon liver had the isotopic 

 carbon only m the carboxyl group adjacent to the carbonyl group. The 

 tricarboxylic acids could not have originated from a symmetrical citric 

 acid molecule, w^hich would have been formed according to the original 

 hypothesis. This must also be the case with the precursors of the di- 

 carboxylic acids obtained after the reaction of acetate or acetoacetate and 

 oxaloacetate.^*'^'^^^ 



Although it is certain that a symmetrical tricarboxylic acid is not the 

 initial condensation product, it is not definite which unsymmetrical acid 

 is the primary product. Lynen^^° points out that the formation of labeled 

 citrate and succinate in yeast'^^ also rules out symmetrical citric acid. 

 However, the presence of citric acid in yeast, which does not contain the 

 enzyme, aconitase, and which is necessary for transforming isocitrate or 

 cis-aconitate to citrate, would seem to indicate that citric acid is the primary 

 condensation product is this case. According to Ochoa,'^^ oxalosuccinic 



i« C. Martins, Z. physiol. Chem., 257, 29-42 (1938). 

 1^' H. A. Krebs and W. A. Johnson, Enzymologia, 4, 148-156 (1937). 

 '« E. A. Evans, Jr., and L. Slotin, J. Biol. Chem., Ul, 439-450 (1941). 

 '" H. G. Wood, C. H. Werkman, A. Hemingway, and A. O. Nier, J. Biol. Chem., U2, 

 31-45(1942). 



ISO F. Lynen, Ann., 552, 270-306 (1942). 



1" S. Ochoa, J. Biol. Chem., 159, 243-244 (1945). 



