40 INTERMEDIARY METABOLISM AND GROWTH I 



D. The Activation of Fatty Acids and Cholic Acid 



Fatty acids must be activated to the corresponding CoA thioester derivatives 

 prior to oxidation. ATP is required in the activation process. Several examples of 

 fatty acid activation reactions are shown in Group IV (reactions 70-72), Group V 

 (reactions 84, 85). 



Likewise, cholic acid must be activated to cholyl-CoA prior to utilization for 

 the synthesis of taurocholic acid or glycocholic acid. An enzyme which catalyzes 

 the activation of cholic acid has been observed in liver microsomes; ATP is 

 required as shown in reaction 86 (Siperstein and Murray, 1956; Elliott, 1955, 

 1956). A second enzyme of the supernatant fraction of liver catalyzes the 

 condensation of cholyl-CoA with taurine : 



Cholyl-CoA + taurine -^ taurocholic + CoA 



The microsome enzyme is also capable of activating the formation of other bile 

 acid-CoA derivatives. Deoxycholyl-CoA and lithocholyl-CoA have been prepared 

 in micromole amounts in this way. 



An enzyme capable of activating acetic acid, but not the longer chain fatty 

 acids, has been obtained from yeast (Hilz and Lynen, 1953), animal tissues, 

 plants, and rhodospirillum (Berg, ig56a). The mechanism of the activation process 

 has been clarified by Berg (Berg, 1956b, c). The yeast enzyme catalyzes an acetate 

 or propionate dependent exchange of 32p_32p ^{^h ATP. On the other hand. 

 Coenzyme A inhibits the exchange of ATP with 32p_32p_ Acetohydroxamic acid 

 is formed from ATP, acetate, and hydroxylamine in the absence of added CoA. 

 The exchange of AMP-''*C with ATP is dependent on the presence of acetate and 

 CoA and the exchange of acetate-^'*C with acetyl-CoA requires both AMP and 

 PP. These facts svipport the following mechanism for acetate activation: 



i) AMP ~ PP + acetate - — . adenyl ~ acetate + 32p_32p 

 2) Adenyl ~ acetate + CoA-SH - — . acetyl-CoA + AMP 



Sum: 3) ATP + acetate + CoA-SH ' — . acetyl-CoA + AMP + P-P 



Adenylacetate, which is postulated as a product of reaction i, has been syn- 

 thesized chemically by Berg. In the presence of ^^P-^-^P, adenylacetate, and the 

 yeast enzyme, ATP labelled with -^^P is formed. Likewise, acetyl-CoA is formed from 

 adenylacetate and coenzyme A; while in the presence of oxalacetate and the 

 condensing enzyme, citrate is formed. Coenzyme A, by competing for adenylace- 

 tate, inhibits ATP formation from adenylacetate and 32p_32p_ 



It seems likely that the same mechanism underlies the activation of other fatty 

 acids. Jencks has demonstrated a CoA independent acyl activation of octanoate 

 by pig liver enzymes in the course of which pyrophosphate is formed from ATP 

 (Jencks, 1953). It has also been shown that in the presence of the butyrate acti- 

 vating enzyme, butyryl-CoA can be formed from synthetic adenylbutyrate and 

 CoA while ATP can be formed from the adenylbutyrate and pyrophosphate. 



