198 A SYMPOSIUM ON RESPIRATORY ENZYMES 



A great variety of chemical transformations involving pyruvic 

 acid has been demonstrated in different species of Hving cells, iso- 

 lated enzyme systems, etc. (representative reactions are listed in 

 Table 1 which is not, however, complete). In some cases several of 

 these diverse ways of treating pyruvic acid can be demonstrated to 

 occur in the same cell. In such circumstances there is considerable 

 advantage in a working hypothesis in which reactions involving the 

 same compound are regarded as components of an integrated sys- 

 tem. In a tissue such as muscle, where specialization of function 

 might be expected to reflect a similar specialization of metabohsm, 

 an hypothesis of this type should be especially fruitful. The citric 

 acid cycle represents, then, an attempt to summarize the available 

 information with respect to the metabolism of pyruvic acid in pigeon 

 breast muscle. Such a summary is of value only so long as it does 

 not conflict with experimental observations and to the extent that it 

 adequately represents the experimental foundation on which it rests. 

 At the moment there are no observations that can be regarded as 

 invalidating the theory, although objections to one or another feature 

 of the scheme have been made. At the risk of adding little that is 

 new to what you already know, I should like to discuss several 

 aspects of the citric acid theory, particularly in regard to its experi- 

 mental basis. The cycle itself is shown diagrammatically in Figure 1. 



The essential experimental support for the theory can be sum- 

 marized in the following equations: 



In the presence of malonate: 



( 1 ) Fumarate + pyruvate + 2 O2 — > succinate + 3 CO2 + H2O 



(2) Malate + pyruvate + 2 O2 -^ succinate -\- 3 CO2 + 2 H2O 



(3) Oxalacetate + pyruvate -f- IJ2 O2 -^ succinate + 3 CO2 + H2O 



(4) Citrate + O2 -^ succinate + 2 CO2 +H2O 



(5) a-Ketoglutarate + )k O^-^ succinate + CO2 + H2O 



In the absence of malonate: 



(6) Succinate + M O2 -^ fumarate -|- H2O 



( 7 ) Pyruvic acid + 2Yi O2 -> 3 CO2 -|- H2O ( Equations 1 -|- 6 ) 



In nitrogen: 



( 8 ) 2 Oxalacetate -j- pyruvate — > citrate + CO2 + malate 



(9) Oxalacetate + citrate — > a-ketoglutarate -|- CO2 + malate. 



The demonstration, in suspensions of muscle tissue, of the stoi- 

 chiometric relationships expressed in these equations constitutes the 

 experimental proof on which the citric acid cycle is based. This 

 demonstration has two aspects— one concerned with the qualitative 

 presence of these reactions, i.e., with the existence of the necessary 



