H. A. BARKER 



Tracer experiments indicated that the ethanol is oxidized to the 

 acetate level before being converted to butyrate. Thus acetate 

 or an activated derivative was identified as a major precursor of 

 butyrate. Later, Lynen's acetyl-coenzyme A (19), formed from 

 acetaldehyde and coenzyme A by a DPN-dependent oxidation, 

 was shown to be the activated form of acetate. 



Even before the isolation of acetyl-CoA by Lynen, the 

 evidence for the existence of such a compound was greatly 

 strengthened by Stadtman's discovery that the exchange of in- 

 organic phosphate with acetyl phosphate in C. kluyveri extracts 

 is catalyzed by an enzyme, phosphotransacetylase, which is 

 coenzyme A dependent (33). This exchange could be most 

 easily interpreted by means of the following reaction 



acetyl phosphate + CoA > acetyl-CoA + phosphate 



which was subsequently shown to occur. The phosphotrans- 

 acetylase reaction has been very useful in enzymatic studies of 

 acetate metabolism, since it provides a convenient method of 

 forming acetyl-CoA. 



A central problem in the metabolism of fatty acids was the 

 identity of the intermediates between acetate and butyrate. 

 Early investigations of fatty acid metabolism in animals sug- 

 gested that acetoacetate and j8-hydroxybutyrate might be 

 involved, but conclusive evidence for or against the participation 

 of these compounds was not forthcoming until the development 

 of cell-free enzyme systems that metabolize fatty acids. With 

 extracts of C. kluyveri, acetoacetate, /3-hydroxybutyrate, and other 

 possible C4 compounds in the same oxidation states were shown 

 definitely not to be intermediates in the interconversion of ace- 

 tate and butyrate (16). This and other evidence led to the 

 development of the idea that the intermediates are not simple C4 

 molecules but are C4-coenzyme compounds. This hypothesis 

 has been fully confirmed by the discovery of the role of the 

 acetyl, butyryl, crotonyl, j8-hydroxybutyryl, and acetoacetyl 

 derivatives of coenzyme A in both animal and bacterial systems 

 (19,21,33). 



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