METABOLIC CYCLES AND DECARBOXYLATION 201 



to 5 involves interruption of the cycle at the succinate-fumarate stage 

 by poisoning with malonic acid. Implicit here is the assumption that 

 malonate acts specifically on this particular reaction, i.e., the oxida- 

 tion of succinate to fumarate. The inhibition of succinic dehydro- 

 genase by malonic acid in isolated enzyme preparations has been 

 recognized and demonstrated by various investigators (7, 8). The 

 inhibition is competitive in nature; that is, it depends not upon the 

 absolute concentration of malonate but on the relative quantities of 

 succinate and malonate. Since the basis of this competitive inhibition 

 is the resemblance in chemical structure between succinic and 

 malonic acids, it follows that malonate will probably inhibit the 

 enzymic transformation of any substrate bearing some chemical 

 resemblance to its own structure. However, the available data 

 suggest that the effect of the poison is most pronounced with the 

 succinic dehydrogenase. Expressed quantitatively, with 1 : 10 suspen- 

 sions of pigeon breast muscle in calcium-free phosphate sahne, 0.001 

 M malonate inhibits pyruvate utilization about 20 per cent. Higher 

 concentrations of malonate, around 0.025 M, inhibit to more than 

 90 per cent. 



As equation 1 indicates, fumaric acid will abolish the inhibitory 

 eflFect of malonate in muscle tissue with the simultaneous utilization 

 of one mole of pyruvate and 2 moles of oxygen to give 1 mole of 

 succinate and 3 moles of carbon dioxide. The extent of this fumarate 

 eflFect, however, depends upon the relative concentrations of malo- 

 nate and fumarate. Krebs, in a long series of experiments (2), has 

 shown very clearly that the demonstration of the stoichiometric 

 relationships of equation 1 depends upon the presence of an ade- 

 quate concentration of malonic acid. When the malonate concentra- 

 tion is too low, as, for example, with 0.001 M malonate in the 

 presence of 0.0025 M fumarate, the gradual conversion of fumaric 

 acid to succinic acid will so increase the succinic acid concentration 

 as compared with that of the malonate that succinic acid will be 

 oxidized to fumaric acid at a rate sufficient to provide for the 

 continuation of the cycle at the full rate. On the other hand, with 

 0.025 M malonate the stoichiometric relationships of equation 1 are 

 realized. It is obvious, then, that the relationships expressed in 

 equations 1 to 5 can be demonstrated only in the presence of ade- 

 quate concentrations of malonic acid. When this is done, the experi- 

 mental data are in fairly close approximation to the expected values. 



In similar experiments with pig heart muscle, Smyth (5) has ob- 

 served an occasional failure with citric acid to obtain the quantities 



