274 VI. ACETIC, FORMIC, AND PROPIONIC ACIDS 



The experiments of Weinhouse et al.^^^ have been interpreted as indicative 

 of the dehydrogenative coupling of acetic acid and of pyruvic acid to form a- 

 ketoglutaric acid.^^* When isotopic acetate was incubated with kidney 

 shces or kidney mince, it was found, at the end of the experiment, that no 

 isotope was incorporated in the citric acid and czs-aconitate which were 

 added as carriers, in spite of the presence of aconitase.'^® While this 

 might be considered as evidence for the direct synthesis of a-ketoglutarate, 

 Bloch^ suggests that the data exclude only citric acid itself and not neces- 

 sarily the tricarboxylic acid cycle. The possibility exists that, in systems 

 containing tricarboxylic acids and aconitase, the conversion of as-aconitate 

 proceeds more rapidly to isocitrate than to citrate; the conversion of 

 isocitrate to a-ketoglutarate is likewise a rapid reaction. 



In a later report of Weinhouse and co-workers,^" it was shown that, when 

 labeled acetoacetate and ordinary oxaloacetate were incubated with kidney, 

 the citric acid contained enough isotopic carbon to indicate that two-thirds 

 of the citrate arose from acetoacetate. Since the latter compound is 

 directly converted to acetate, it is difficult to explain the diametrically 

 opposite results in the two sets of experiments. Wood^^^ is of the opinion 

 that there are insufficient data to rule out the direct dehydrogenative 

 synthesis of cc-ketoglutarate. However, although there are reactions in 

 which carbon-to-carbon bonds are formed, such as the formation of hexose 

 from triose, or of acetoacetate from acetate, or of the carboxylation of 

 pyruvate to oxaloacetate, all these reactions involve at least one carbonyl 

 group in the reacting molecules. On the other hand, Bloch^ was unable 

 to cite a single instance in which the formation of a carbon-to-carbon group 

 was established by the elimination of hydrogen between methyl or methy- 

 lene groups. This would seem to render it highly improbable that a 

 direct dehydrogenative synthesis of a-ketoglutarate results from acetate 

 and pyruvate. 



h. The Formation of Citric Acid. As indicated in the equations on page 

 171, citric acid is formed by the condensation of acetate with oxaloacetic 

 acid; the six-carbon tricarboxylic acid is then successively degraded to 

 Q!-ketoglutaric acid and to each of the four-carbon dicarboxylic acids. 

 This series of reactions is now recognized as a cycle whereby the acetate 

 molecule can be oxidized in the animal body. Virtanen and Pulkki^^^ 

 were the first to suggest this method of synthesis of citric acid from acetate 

 by way of sugar in the fungus (Aspergillus niger). A similar pathway was 



1" S. Weinhouse, G. Medes, and N. F. Floyd, /. Biol. Chem., 166, 691-703 (1946). 

 1^* A. I. Virtanen and L. Pulkki, Ann. Acad. Set. Fennicae (Suotnalainen Tideaketemia, 

 Helsingfors, Toimiaksia), Ser. A. 33, No. 3, 1-19 (1930). 



