ANTAGONISM WITH FUMARATE 113 



data for fumarate reversal were reported by Quastel and Wheatley (1935), 

 who showed that the inhibition of acetoacetate utilization by rat liver in 

 the presence of malonate is mainly abolished when fumarate is added, 

 and they used this as evidence that malonate does not act directly on the en- 

 zyme involved in the breakdown of acetoacetate. The use of fumarate im- 

 mediately became popular and many studies since 1935 have included its 

 addition for the purpose of demonstrating that the observed effect of mal- 

 onate is indeed due to its block of succinate oxidation. 



Most of the tests for fumarate reversal, it must be admitted, have not 

 been done properly and the results have been evaluated uncritically, so that 

 little of value has been demonstrated. There are several points that must 

 be considered in the planning and interpretation of such experiments. 



(a) An increase in the oxygen uptake upon addition of fumarate to a 

 malonate-inhibited preparation is not, by itself, very meaningful. Let us take 

 a typical experiment similar to many reported in the literature. The nor- 

 mal Qq of a tissue preparation is 10 and this is decreased to 4.5 by mal- 

 onate. When malonate and fumarate are added together, the Qq is 8.7. 

 It has been generally stated in such cases that fumarate is capable of 

 antagonizing the malonate inhibition. Actually, an increase in the respira- 

 tion could have been brought about by the addition of any substrate that 

 is oxidized by the preparation, including substrates completely unrelated 

 to the cycle. One can say that fumarate has overcome the malonate inhibi- 

 tion but the results are of no particular significance with respect to malonate 

 specificity or the site of the inhibition. What one has shown is that fumarate 

 can be oxidized in the presence of malonate and this need not imply that 

 the operation of the cycle has been even partially restored. If fumarate 

 had been added to the uninhibited preparation and a Qq of 14.2 found, the 

 following might have been concluded: malonate inhibits the endogenous 

 respiration 55% and the respiration in the presence of fumarate either 13% 

 (with respect to the endogenous control) or 39% (with respect to the rate 

 with fumarate alone present), in both cases the inhibition being less than that 

 for the endogenous respiration. Actually, it has been shown that the oxi- 

 dation of fumarate is unaffected by malonate, since the same absolute 

 rise in the Qq^ is obtained from fumarate in the uninhibited and inhibited 

 preparations. If the oxygen uptake resulting from the addition of fumarate 

 results mainly from the oxidation of malate, the results would have little 

 bearing on the mechanism or selectivity of malonate inhibition. 



(6) The addition of fumarate would not in any case restore the complete 

 cycle, since oxidation of succinate would still be depressed and less energy 

 would be available from this step. It is possible in some instances that the 

 energy from succinate oxidation, rather than the over-all energy production 

 from all oxidations, is important, and this would not be restored by fu- 

 marate. 



