86 III. OXIDATION AND METABOLISM 



occur. They take place simultaneously. A. Some /3-oxidized Cn-2 

 fatty acid molecules are cleaved between their a- and /3-carbons (C4 and C5 

 of original Cn molecule) yielding a Cn_4 residue and C2 fragments which 

 enter the same metabolic pool referred to in step (2). B. An orientation 

 occurs between the C2 fragments in the liver pool and the Cn-2 fragments 

 in such a way that, when a,^ cleavage of the Cn-2 residue occurs, simul- 

 taneous condensation with the oriented C2 units occurs. The acetoacetate 

 would be predominantly labeled in the carboxyl group. 



In a later study, Goldman et oL^" studied the decarboxylation of the 

 acetoacetate formed during the oxidation of carboxyl-labeled palmitic and 

 octanoic acids by rat liver slices. In the presence of sufficient non-isotopic 

 acetoacetate carrier, the thermal and catalytic procedures yielded essen- 

 tially equivalent values for the specific activity of the carboxyl carbon. 



The asymmetric labeling of acetoacetate was explained by Beinert and 

 Stansley^^'^- by the enzymatic acyl interchange between CoA and /3-keto 

 acids. The acetoacetate cleavage enzyme, E, catalyzes the condensation 

 of acetoacetate from two molecules of AcCoA. If AcCoA is labeled, the 

 acetoacetate formed is symmetrically labeled. However, in the presence 

 of added unlabeled acetoacetate, Ac*CoA gives rise to acetoacetate labeled 

 preferentially in the carboxyl half (Ac-Ac*0~). The total amount of 

 label incorporated mider these conditions is fifty times that obtained in the 

 absence of added acetoacetate. Both of these effects can be duplicated 

 with higher /?-keto acids. 



The production of asymmetrically labeled acetoacetate is related to the 

 rate of the following reactions: 



a) E + AcCoA , EAc*CoA 



b) EAc*CoA + Ac*CoA , EAc*Ac*CoA 



c) EAc*Ac*CoA + AcAcO- , EAcAcCoA + Ac*Ac*0- 



d) EAcAcCoA , EAcCoA + AcCoA 



Under certain conditions, EAcCoA can react with Ac-CoA to form EAc • - 

 Ac*CoA; this results from a positional difference between the C2 units, 

 rather than from a variation in chemical reactivity. This accounts for the 

 fact that asymmetrically labeled acetoacetate may arise during fatty acid 

 oxidation. 



Brown and Chaikoff^-^ postulated that two types of C2 derivatives arise 



«> D. S. Goldman, G. W. Brown, Jr., H. R. Matheson, and I. L. Chaikoff, /. Biol. 

 Chem., 195, 415-422 (1952). 



«i H. Beinert and P. G. Stansley, Federation Proc, 12, 176 (1953). 



" H. Beinert and P. G. Stansley, /. Biol. Chem., 204, 67-76 (1953). 



«3 G. W. Brown, Jr., and I. L. Chaikoff, Biochim. et Biophys. Acta, 11, 37-51 (1953). 



