CARBON DIOXIDE 



formed earlier can be converted to pyruvic acid by reductive carboxyl- 

 ation, i. e., by reversal of reaction Ilia. We would thus have a cyclic 

 mechanism whereby carbon dioxide and hydrogen entering at various 

 points would emerge as pyruvic acid. The di- and tricarboxylic acids 

 would only act catalytically as carriers of carbon dioxide and hydrogen. 

 This is a reversal of the so-called tricarboxylic acid cycle, which is 

 considered to be an important pathway for the oxidative breakdown of 

 carbohydrate and fat in cells. Scheme II presents this metabolic cycle, 

 incorporating recent findings concerning some of the intermediate 

 reactions (1,4,13,28). For a more detailed discussion, see Lardy and 

 Elvehjem (14). 



Scheme n 



Reversible Tricarboxylic Acid Cycle 

 Carbohydrate 



fatty acids 



pyruvate ^ 



lactate 



malate 



-H2O 



czi'-aconitate. 



+H2O 



citrate 



+H2O 



-H2O 



-H:0 



fumarate 



-HK5 



+H2O 



isocitrate 



-2H 



+ 2H 



+ 2H 



■2H 



succinate 



oxalosuccinate 



•2H 

 -CO2 



+ 2H 

 + CO2 



+ CO2 



-CO2 



a-ketoglutarate 

 Another mechanism recently suggested for photo- and chemo- 

 syntheses involves a sequence of carboxylations and reductions leading 



183 



