SPECIFICITY OF MALONATE INHIBITION IN THE CYCLE 117 



bition, but actually fumarate alone increases the uptake to 71% above the 

 control. Thus malonate inhibits a significant amount even in the presence 

 of fumarate, pointing to an action other than on succinate oxidation. 

 Another example would be the malonate inhibition of cell division oiArhacia 

 eggs (Barnett, 1953). Cleavage is inhibited almost completely by 60 milf 

 malonate and an equimolar concentration of fumarate abolishes this. It is 

 likely that fumarate overcomes the cycle block but one cannot conclude 

 that the action of malonate is specific on succinate oxidase. Malonate, can 

 also inhibit to some extent steps in the utilization of fumarate, but due to 

 the high concentration of fumarate enough oxalacetate is formed to allow 

 cleavage. Indeed, succinate atGOmJf also abolishes the inhibition, indicating 

 that as a result of the competitive nature of the inhibition enough succinate 

 has broken through the block to restore cleavage. It must be remembered 

 that a complete reversal of a metabolic block is not always necessary for a 

 cell function to proceed normally. 



SPECIFICITY OF MALONATE INHIBITION IN THE CYCLE 



At this point we may summarize some of the conclusions with respect to 

 the specificity of action of malonate on the succinate dehydrogenase. Possi- 

 ble effects of malonate outside the cycle will be discussed in a later section. 

 We have seen that malonate can inhibit enzymes other than succinate dehy- 

 drogenase rather potently (Table 1-12), that the oxidations of certain cycle 

 substrates are suppressed more than predicted on the basis of a selective 

 action on succinate oxidation (Table 1-14). that the accumulation patterns 

 of cycle intermediates are sometimes distorted by malonate in ways implying 

 inhibition at more than one site, and that fumarate is seldom able to reverse 

 the actions of malonate completely. Of particular significance are the clear 

 demonstrations of the inhibition of reactions related to the entry of acetyl- 

 CoA into the cycle, particularly those of Pardee and Potter (1949) pointing 

 to an inhibition of the condensation reaction, those of Stoppani et al. 

 (1958 b) reporting a marked inhibition of acetate utilization apparently 

 unrelated to an inhibition of succinate oxidase, and our own results on rat 

 heart mitochondria where malonate inhibits p^Tuvate oxidation in the 

 presence of malate and with the a-ketoglutarate oxidase completely blocked. 

 Another susceptible site is a-ketoglutarate oxidation, especially in view 

 of the clear proof by Price (1953) that specific inhibition can not even be 

 obtained in the simple system, 



a-Ketoglutarate -^ succinate ->■ fumarate 



There is thus a large amount of evidence that malonate can at certain 

 concentrations in various conditions inhibit other reactions than succinate 

 oxidation. 



