VOL. 12 (1953) ENZYMES OF FATTY ACID METABOLISM 3OI 



acetate in this organ. In either case CoA-SH is made available for activation of further 

 fatty acid molecules. 



The equilibrium of reaction (a) is predominantly in favor of the thioclastic splitting 

 of the /?-ketoacyl CoA derivatives. For this reason the name ^-ketothiolase, or simply 

 thiolase, has been proposed for this class of enzymes^. The equilibrium of reaction (b) 

 favors reduction of the j8-keto derivative, hence the name j8-ketoreductase has been 

 proposed for this group of enzymes^. The name crotonase has been suggested^' for the 

 enzyme or enzymes catalyzing reaction (c) and, finally, the name ethylene reductase has 

 been used^^ to designate the enzyme or enzymes catalyzing reaction (d) . 



There are two main mechanisms for activation of fatty acids, i.e., for the synthesis 

 of their S-acyl CoA derivatives: (a) by a reaction with CoA-SH and ATP which, as we 

 shall see later, results in the reversible formation of the corresponding acyl CoA, AMP 

 and PP, and (b) by transfer of CoA from certain acyl CoA compounds such as acetyl 

 CoA or succinylCoA. Animal tissues, such as liver, heart, and kidney, utilize mainly the 

 first mechanism while C. kluyveri utilizes the second. Extracts of this organism catalyze 

 the reversible transfer of CoA from acetyl CoA to such fatty acids as propionate or 

 butyrate^''. Because of the presence of phosphotransacetylase^^, in the presence of CoA 

 such extracts can utilize acetyl phosphate for activation. 



A transferring enzyme of rather limited specificity is present in heart and probably 

 in skeletal muscle and kidney but appears to be absent from liver. This enzyme catalyzes 

 the reversible transfer of CoA from succinyl CoA to acetoacetate^^-^'®^'^^. Through the 

 formation of acetoacetyl CoA the enzyme activates acetoacetate, produced in the liver 

 and carried by the blood stream to the peripheral tissues, for oxidation in these tissues 

 via the tricarboxylic acid cycle (see reference no. 8). The latter in turn generates the 

 necessary succinyl CoA through the oxidation of a-ketoglutarate which, as shown by 

 recent work26. 54,27^ reacts with CoA-SH and DPN+ to f orm succinylCoA.COa.andDPNH. 



S-acyl fatty acid derivatives 



Synthetic S-acetoacetyl and S-crotonyl derivatives of N-acetyl thioethanolamine^* 

 have been found to act as substrates of jS-ketoreductase and ethylene reductase re- 

 spectively^^- ^^. These structural analogues of the natural S-acyl CoA derivatives have 

 therefore provided suitable substrates for the isolation of the two enzymes. Further, the 

 two model compounds have characteristic absorption spectra. This made it possible not 

 only to predict the optical properties of the corresponding natural substrates, i.e., 

 S-acetoacetyl and S-crotonyl CoA, but also to develop convenient optical methods for 

 the assay of several enzymes. Thus, the analogues have greatly facilitated the study of 

 individual steps of fatty acid metabolism. 



S-acetoacetyl-N-acetyl thioethanolamine was obtained as a colourless crystalline 

 compound (m.p., 60°) through reaction of N-acetyl thioethanolamine with diketene. 

 As a solid, the compound is in the keto form but it undergoes rapid enolization in solution 

 as can be shown with the ferric chloride reaction and by the change in its absorption 

 spectrum. At equilibrium the percentage of enol is higher in solutions of the thioester 



00 OH 9 



CHa—fc— CHj— C— S— CHg—CHg— NH— CO— CHa^CHg— C = CH— C— S— CHg—CHj— NH— CO^CHg, 



than in those of acetoacetic ethyl ester, a fact which was first observed by Baker and 

 Reid with acetoacetic thioethyl ester^. 



References p. 313J314. 



