22 II. BIOSYNTHESIS 



experiments of Morehouse, ^^ and in those of Morehouse and Deuel. ^^ 

 When caproate labeled with deuterium in the /3,7-positions was fed to fast- 

 ing rats, the resulting |S-hydroxybutyrate isolated from the urine had twice 

 the isotope content of that of the same product prepared from the urine 

 after a-jS-dideuterocaproate had been fed. If the acids were split into three 

 C2 fragments, one would expect a similar proportion of deuterium in the /?- 

 hydroxybutyrate in both cases. On the other hand, if the caproate breaks 

 down by jS-oxidation to form "primary" acetoacetate, and the ketone bodies 

 are formed only in part by recondensation of acetate units, the higher iso- 

 tope content of the /3,7-, as compared with that of the a,/S-compound, would 

 be expected. 



On the basis of results obtained with partially purified bacterial phospho- 

 transacetylase, Stadtman, Doudoroff, and Lipmann^^ postulated that two 

 activated C2 units are used in the synthesis of acetoacetate, with the aid of 

 an enzyme system present in pigeon liver. These workers are of the 

 opinion that 2 moles of acetylphosphate are first converted to 2-acetyl-CoA 

 molecules by transacetylase. It was suggested that, following this, the 

 pigeon liver enzyme catalyzes the head-to-tail condensation of these 

 molecules. 



According to Lynen,**'' the acetoacetate synthesis involves a modified 

 Claisen condensation. After combination of two molecules of acetyl-CoA, 

 one molecule of the thiol alcohol-CoA splits off, leaving acetoacetyl-CoA, 

 which, on hydrolysis, yields acetoacetate. This is shown in the equation 

 on the following page. 



In the scheme suggested by Lynen,*" acetyl-CoA functions to bring about 

 the synthesis of acetoacetate, while the reverse reaction is catalyzed by 

 the thiol alcohol HS-CoA. This combines with acetoacetyl-CoA on the 

 carbonyl group, and splits it by the reaction described as "thiolysis." 

 The split occurs between the methylene carbon and the carbonyl carbon, 

 whereby the hydrogen attaches to the methylene group and the sulfur, 

 with the rest of the coenzyme molecule, combines with the carbonyl carbon. 



One must conclude that, although the animal has the power to condense 

 acetyl groups to yield acetoacetate, this reaction accounts for only a portion 

 of the acetoacetate normally produced in the animal, inasmuch as the latter 

 compound can also be formed directly from 4-, 6-, or even 8-carbon acids. 

 The acetoacetate must in all probability be in equilibrium with acetate 



« M. G. Morehouse, /. Biol. Chem., 129, 769-779 (1939). 



93 M. G. Morehouse and H. J. Deuel, Jr., Proc. Soc. Exptl. Biol. Med., 45, 96-98 

 (1940). 



9" E. R. Stadtman, M. Doudoroff, and F. Lipmann, /. Biol. Chem., 191, 377-382 (1951). 



