134 Essays in Biochemistry- 



converted by such particle-free extracts to acetoacetate. Furthermore, 

 Drysdale and Lardy were able to demonstrate (1) the formation of 

 acetohydroxamic acid in the presence of hydroxylamine and (2) the 

 quantitative formation of citrate when oxalacetate was supplied. This 

 latter reaction suggested quite strongly the involvement of acetyl CoA, 

 since Stern and Ochoa ' had isolated the enzyme capable of condensing 

 this active intermediate with oxalacetate to form citrate. 



Acetyl CoA + Oxalacetate ^ Citrate + CoA 



In view of the evidence presented by Stern, Coon, and Del Campillo 8 

 concerning the formation of acetoacetyl CoA from acetyl CoA: 



2 acetyl CoA ^± Acetoacetyl CoA + CoA 



there could no longer be any doubt concerning the nature of the active 

 2-carbon fragment derived from the oxidation of fatty acids. 



At least three separate enzyme systems appear to be involved in the 

 activation of fatty acids: (1) an enzyme system described by Korn- 

 berg and Pricer," which is capable of activating acids containing 12 

 to 20 carbon atoms; (2) enzymes derived from beef liver capable of 

 activating C4-C12 acids (Mahler et al.) ; and (3) the well-known 

 acetate activating system. All three appear to require ATP and CoA 

 as well as Mg++. 



The currently accepted mechanism of oxidation of fatty acids is 

 schematically represented in Fig. 1. Major contributors in this area 



Fatty acid oxidation 

 r ► RCH„CH 2 CO-CoA 



1H» 



I 



RCH:CHCO-CoA 



1 l+H 2 



RCHOHCH 2 CO-CoA 



1 

 I 



II- 



2H 



RCOCH 2 CO - CoA 



I +CoA 

 1 — RCO - CoA -* 



-*- +CH3CO-C0A 

 (recyclized) (generated) 



Fig. 1. Generation of acetyl CoA by oxidation of activated fatty acids. 



have been Lynen, Ochoa, and Green. The details of this oxidative 

 mechanism have been fully reviewed elsewhere. 10 



