b INTERMEDIARY METABOLISM AND GROWTH I 



to isocitrate and the isocitrate is oxidized to oxalosuccinic acid. The latter is 

 decarboxylated to a-ketoghitaric acid. The oxidative decarboxylation of a-keto- 

 glutaric acid is analogous to that of pyruvate. Lipothiamide pyrophosphate, 

 DPN^, and coenzyme A are required and succinyl coenzyme A is formed as a 

 product of the overall reaction. The succinyl-CoA is further metabolized to fuma- 

 rate, and malate. The malate is next dehydrogenated, thereby regenerating 

 oxalacetate. The oxalacetate may react with a second molecule of acetyl-CoA so 

 as to start the cycle once again. With each turn of the cycle, two molecules of 

 CO-, are formed. 



It is apparent that the rate at which acetyl-CoA can be oxidized through the tricar- 

 boxylic acid cycle will depend in part on the concentration of available oxalacetate. It is 

 therefore of interest to inquire as to the origin of the latter substance. Two general sources 

 may be mentioned. First, aspartic acid may readily be converted to oxalacetic acid, by 

 transamination. Likewise, tricarboxylic acid cycle intermediates or closely related com- 

 pounds such as a-ketoglutarate or glutamate may be utilized for the synthesis of oxalacetic 

 acid. Secondly, oxalacetic acid may be generated from the glycolytic intermediates, 

 pyruvate and phosphoenolpyruvate. The synthesis of dicarboxylic acids from pyruvate 

 and phosphoenolpyruvate is shown in equations i) and 2) : 



Mn^" or Mg"^ DPN^ 

 i) Pyruvate + CO2 + TPNH2 ^ ' malate ^ ^ oxalacetate 



CDP CTP 



2) Phosphoenol pyruvate + CO2 + Ynp " ' jTP ^ oxalacetate 



Reaction i) is catalyzed by "malic enzyme" (Ochoa et al., 1950; Ochoa and Kaufman, 

 1 951), and involves a reductive carboxylation of pyruvate to malate; the malate may then 

 be oxidized to oxalacetate. 



Reaction 2) is catalyzed by the mitochondrial enzyme, oxalacetic carboxylase. (Ban- 

 durski and Lipmann, 1956; Utter and Kurahashi, 1954a, 1954b; Utter et al., 1954). Inosine 

 and guanosine pyrophosphate function as acceptors of the phosphate group of phospho- 

 enolpyruvate during this reaction. 



6. The formation of CO 2 



The Embden-Meyerhof glycolytic sequence and the tricarboxylic acid cycle con- 

 stitute pathways for the complete combustion of glucose to COj and water. Like- 

 wise, the fatty acid spiral and the tricarboxylic acid cycle provide pathways for 

 the complete rjxidation of fatty acids. However, the release of COj from organic 

 compounds ordinarily takes place at only three points on these metabolic sequences. 

 These are: 



i) Pyruvate -* CO2 + acetyl-CoA 



2) Oxalosuccinic — > CO2 + a-ketoglutarate 



3) a-Ketoglutarate -^ CO2 + succinyl-CoA 



Equation i ) refers to the generation of acetyl-CoA from the pyruvate which 

 is formed during glycolysis while equations 2) and 3) refer to steps of the citric 

 acid cycle which account for the formation of CO, from the acetyl-CoA. The 

 latter is generated either via glycolysis or from the oxidation of fatty acids. Under 



