PRIMING REACTIONS 199 



a transfer takes place from succinyl-Cox\ to acetoacetate. Thus the aceto- 

 acetate formed in the Hver is activated, brought to the tissues by the blood 

 and oxidized by way of the tricarboxylic acid cycle. 



III. THE TRICARBOXYLIC ACID CYCLE 



Acetyl-CoA is the starting point of a series of transformations known 

 under the name of the tricarboxylic acid cycle. Into this cycle, at various 

 points, are introduced other products of the degradation of cellular 

 nutrients, particularly of the different amino acids. This terminal cycle, 

 common to the main structures forming the organism, is the chief source 

 of the energy-rich bonds required for biosynthesis. 



Our knowledge of this cycle began with some experiments carried out by 

 Szent-Gyorgyi on a mince of pigeon breast muscle. This mince respires 

 vigorously without producing lactic acid. The respiration, at first intense, 

 diminishes with time. Parallel with the fall in respirator}^ activity, the 

 concentration of succinate in the muscle-mince decreases, but the addition 

 of small amounts of succinate (or of fumarate) brings about an increase in 

 the respiration. Since the respirator}^ quotient is equal to 1, we can conclude 

 that carbohydrate is being broken down. The conclusion therefore is that 

 the oxidation of carbohydrates is catalysed by succinate and fumarate; 

 succinic dehydrogenase must also play an important part because malonate 

 blocks the stimulant effect of succinate. 



Szent-Gyorgyi having also observed the depressant effect of malonate 

 (an inhibitor of succinic dehydrogenase) on the respiration of the muscle 

 mince, he saw that the malonate must hinder the restoration bv the 

 succinate of the respiratory system which had become completely oxidized. 



He therefore proposed the following scheme to explain respiration : 



AHa . , II ». ^ reduced cytochrome >. / 



H O 



oxidised cytochrome ' ^ * 



Succinic acid 



Dehydrogenas of AH 2 Succinic Cytochrome 



dehydrogenase oxidase 



REFERENCES 

 Lynen, F. (1954). Acetylcoenzyme A and the "fatty acid cycle". Harvey Lectures, 

 48, 210-244. 



