II GLUCOSE, FATTY ACIDS CATABOLISM 7 



{Green and Beinert, 1955; Crane and Beinert, 1954; Mahler and Green, 1954). 



The unsaturated fatty acid ester is next converted io a liydroxyacyl fatty acid 

 ester due to the action of enoyl-hydrase enzymes (Stern et al., 1956c). One of these, 

 crotonase, which has been crystaUized from ox Uver, has been demonstrated in 

 most animal tissues, bacteria and in spinach. The crystalline enzyme-hydrates- 

 CoA esters of chain length to at least C^-Cg. It also acts upon ^-methyl-crotonyl- 

 CoA and hydrates cis as well as trans isomers. 



The beta hydroxy fatty acid ester is next dehydrogenated to the ^-keto fatty 

 acid ester by a pyridine nucleotide-dehydrogenase. It is of interest that the [3- 

 hydroxyacyl dehydrogenase of liver or heart is specific for the L-hydroxy fatty 

 acid esters of CoA whereas the enzyme which attacks free [3-hydroxy-butyric acid 

 acts upon the D-diasterioisomer. 



The [ii-keto acid is next cleaved in the presence of coenzyme A by the enzyme, 

 ^-ketoacylthiolase, to acetyl-CoA and to a fatty acid ester of having two less 

 carbon atoms than the original fatty acid. Pig heart thiolase is highly specific 

 for acetoacetyl-GoA whereas ox liver thiolase acts upon [3-keto fatty acid-CoA 

 derivatives of chain len.gth of from 4-12 carbon atom,. The latter enzyme pre- 

 parations have been studied in crude form only so that it is likely that there exist 

 a number of thiolases with different chain length specificity. 



It is significant that the fatty acid esters derived from the action of thiolase are 

 already in the activated coenzyme A form. The process of oxidation, hydration, 

 oxidation, and thiolytic cleavage may thus be repeated until the entire fatty acid 

 molecule has been converted by beta oxidation to acetyl-GoA units. 



Although the activation of fatty acids by liver enzymes requires the presence of 

 ATP and GoA, heart, kidney, adrenal gland and skeletal muscle enzymes can 

 activate acetoacetic acid by a somewhat different mechanism (Stern^'i a/., 1956a). 

 This entails the action of soluble enzymes known as thiophorases and involves the 

 transfer of GoA from succinyl-CoA as shown in the following equation: 



Succinyl-GoA + GHj-GO-CH^-COOH - — . succinate + CH3-CO-CH2-CO-SC0A 



In the presence of coenzyme A, the acetoacetyl-GoA may next be cleaved by the 

 thiolase enzyme to two molecules of acetyl-GoA. As a result, acetoacetate, which 

 is produced in liver in excess under certain conditions, may be activated and 

 oxidized in peripheral tissues at the sacrifice of the thioester linkage of succinyl- 

 GoA. The enzyme found in heart muscle is capable of transferring GoA from 

 succinyl-GoA to [3-ketovalerate and [3-ketocaproate, as well as to acetoacetate 

 (Stern et al., 1956b). A thiophorase enzyme also occurs in Clostridium kluyveri. 

 However, the bacterial enzyme has a somewhat different specificity in that it 

 catalyzes the reversible transfer of GoA from acetyl-GoA to saturated fatty acids of 

 chain length Gj-Gg, to vinylacetic acid and lactic acid, but not to acetoacetic acid. 



5. Tricarboxylic acid cycle {Citric acid cycle: Krebs cycle) 



The acetyl-GoA, generated by the oxidation of fatty acids or pyruvate, is oxidized 

 to GO2 and water through the citric acid cycle. The cycle begins with the conden- 

 sation of acetyl-GoA with oxalacetate to form citrate. The citrate is transformed 



Literature p. 124 



