TRIGLYCERIDES AND FATTY ACIDS 



133 



above hypotheses, Coon and Gurin^'* have proposed an alternate pathway 

 to explain the formation of acetoacetate from leucine. It is suggested 

 that only the a. and ^ carbons of leucine split off to form the C2 intermediate 

 which is capable of condensing to yield acetoacetate. The conclusions 

 were reached on the l:)asis of the results of in vitro studies with liver slices, 

 employing leucine labeled with C'^ in the /3-position, and of the results 

 obtained in in vivo studies with a phlorhizfni'zed rat fed this labeled leu- 

 cine. The following metabolic pathway is proposed; 



CH3 \ 



CH3/ 



CH— (?) 



CH3 CH3 



\ / 



CH 



CH2 



->• 2(— CHo-COOH) 



COOH 



CH3COCH2COOH 

 The Conversion of Leucine to Acetoacetate as Proposed by Coon and Gurin"* 



Coon and Gurin^^^ are of the opinion that the isopropyl residue is not com- 

 pletely oxidized to CO2 in the liver, and that it does not, to any appreciable 

 extent, furnish a C2 fragment similar to that arising from the «- and /3- 

 carbons of leucme. The earlier results of Bloch,^i^ in which it was shown 

 that only 0!5 mole of acetoacetate arose from one mole of leucme, would 

 appear to support this new concept of the intermediary metabolism of 

 leucine. Plant and Lardy^^^ suggested the possibility that the conversion 

 of the isopropyl group of isovaleric acid to acetoacetate may be the result 

 of a stoichiometric fixation of CO2. It was also demonstrated that CO2 

 could be incorporated into the carboxyl group of acetoacetate formed from 

 crotonate, butyrate, nonylate, and pyruvate by rat liver slices. This 

 incorporation did not occur in the acetoacetate formed from rf/-/3-hydroxy- 

 butyrate and valerate. 



b'. Norleucine: There is a lack of agreement on the question of whether 

 or not norleucine, CH3-CH2CH2-CH2-CH(NH2) -COOH, is a ketogenic 

 amino acid. Although Edson^" found that norleucine was converted to 



"5 M. J. Coon and S. Gurin, J. Biol. Chem., 180, 1159-1167 (1949). 



3i« K. Bloch, J. Biol. Chem., 155, 255-263 (1944). 



317 G. W. E. Plant and H. A. Lardy, J. Biol. Chem., 186, 705-716 (1950). 



