Metabolic Cycles and Decarboxylation 



E. A. EVANS, JR. 

 University of Chicago^ 



IF I WERE to adhere rigidly to a discussion of the topic assigned 

 me I should be compelled to mention almost every aspect of 

 our present knowledge of intermediary metabolism. I am claim- 

 ing, therefore, the traditional privilege of discussing those matters 

 that seem of particular interest and importance. 



From what we know of the chemical constitution of most cells, 

 it is to be expected that those interactions of cell constituents by 

 which the organism obtains the energy necessary for continued 

 existence should exhibit certain characteristics of continuity and 

 recurrence, as does the cell itself. The utilization of foodstuffs by 

 the cell frequently involves a cyclic chain of chemical transforma- 

 tions in which certain cell constituents, usually present in small and 

 apparently constant amounts, facilitate the transformation of larger 

 quantities of other metabolites in reactions releasing energy or 

 leading to the formation of the actual protoplasmic fabric of the 

 cell itself. The synthesis of urea, the transforaiation of glycogen into 

 lactic acid, and, in a broader sense, the transport of the respiratory 

 gases by the blood are familiar and typical examples of the cyclic 

 mechanisms by which the organism maintains the balance necessary 

 for its existence in the midst of the dynamic processes by which it 

 functions. 



In the past few years such a cyclic series of reactions has been 

 proposed for the mechanism of oxidation of carbohydrate in various 

 tissues— more specifically for the oxidation of pyruvic acid in volun- 

 tary musculature. This scheme, the so-called citric acid cycle, was 

 proposed by Krebs (1, 2, 3, 4). In its original form the theory was 

 concerned with the oxidation of pyruvic acid by minced pigeon 

 breast muscle, and although the generalizations of the theory have 

 been extended in part to other tissues and species (5, 6), the most 

 convincing and complete data are those derived from suspensions of 

 muscle tissue. 



* The original work reported in this paper was aided in part by grants 

 from tlie John and Mary R. Markle Foundation and from tlie Dr. Wallace C. 

 and Clara A. Abbott Memorial Fund of the University of Chicago. 



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