276 VI. ACETIC, FORMIC, AND PROPIONIC ACIDS 



was likewise found to be ineffective in bringing about citrate syn- 

 thesis."'^*^'^*^ Buchanan et al.^*^ also observed that acetate and aceto- 

 acetate were equally effective as precursors of a-ketoglutarate when kidney 

 homogenates were used. 



Although it has generally been considered that acetate condenses with 

 oxaloacetate to form isocitric acid, it has been suggested that acetoacetate 

 may be the compound which condenses with oxaloacetate. ^■*'' The greater 

 reactivity of acetoacetate as compared with acetate could then be explained 

 by the fact that the rate of the reaction, acetate — >- acetoacetate, might 

 to some extent be a hmiting factor. Wieland and RosenthaF*" postulated 

 that acetoacetate combines with oxaloacetate to yield pro-citric acids such 

 as citroylacetic or acetylcitric acid. This might likewise explain the fact 

 that Weuihouse et al., using kidney homogenates, were able to demonstrate 

 citric acid formation when acetoacetate was the substrate, '^^ but only 

 a-ketoglutarate when acetate was present in the incubation medium. ^^^ 



There are several arguments agamst the hypothesis that acetoacetate 

 is required for condensation with oxaloacetate. Thus, citrate is formed 

 in yeast by the tricarboxyhc synthesis from acetate, but this fungus cannot 

 change acetate to acetoacetate. According to Medes, Floyd, and Wein- 

 house,^^^ acetate, which is completely oxidized by heart muscle, does not 

 pass through acetoacetate as an intermediate; the same is at least partly 

 true of the acetate utilized by kidney. Moreover, Lehninger" reported 

 that, in washed liver cells, acetoacetate, in contradistinction to octanoic 

 acid, was incapable of yielding citrate. This may be due to the inabiUty 

 of this tissue to convert acetoacetate to the active two-carbon fragment. 

 On the opposite side of the picture, Buchanan et aU*'^ showed that, when 

 labeled acetoacetate and unlabeled acetate were incubated with rabbit 

 kidney, the acetate remaining had a lower isotope content than did the 

 succmate resulting from the oxidation of a-ketoglutarate. Bloch' suggests 

 that, from thermodynamic considerations, an energy-rich acetyl would be 

 expected to arise more readily from acetoacetate than from acetate. 

 Lehnmger," in testing for the identity of the compound which combines 

 with oxaloacetic acid, showed that the following compounds are inactive: 

 glycolate (HOCHa-COOH), glyoxalate (OCH-COOH), oxalate, acetalde- 

 hyde, ethanol, acetamide (CHrCONHa), glycme (NHzCHs-COOH), 

 acetyl phosphate (CH3COPO3H2), and acetyl pyrophosphate (CH3CO- 



(P03)2H). 



1** J. M. Buchanan, W. Sakami, S. Gurin, and D. W. Wilson, Federation Proc, 5, 

 126(1946). 



i« G. Medes, N. F. Floyd, and S. Weinhouse, J. Biol. Chem., 112, 1-9 (1946). 



