594 DOWSE, NEUMAN, LANE, AND NEUMAN 



centratioii dependency of the effect is to be expected for a competi- 

 tive inhibitor of this type. Addition of fmnarate, after malonate, at 

 levels up to twice that of the malonate concentration used, did not 

 relieve the l^lock, nor did addition of oxaloacetate. The latter was of 

 interest because malonic acid has been shown to be an inhibitor of 

 fumarase as well as of succinic dehydrogenase ( Massev, 1953). It 

 would appear, therefore, that malonic acid was eliminating, bv a 

 competitive inhibition of succinic deh\ drogenase, O- uptake due to 

 succinate oxidation, l)ut was not reducing the availabilitv of dicar- 

 boxylic acids for the functioning of the rest of the cycle. 



A single addition of fluoroacetate at the 5-mM level had no effect 

 on endogenous Qn.^ (Fig. IC), whereas a second addition at the 

 same concentration, or single additions at the 10- or 20-mM level, 

 reduced the Qi,.^ by some 25 per cent. There was no greater effect 

 at the highest concentration. This, as well as the delav period before 

 the effect became manifest, is consistent with the noncompetitive 

 nature of the inhibition, and with the necessity for prior incorpora- 

 tion of the fluoroacetate into the TCA cycle to form fluorocitric acid. 

 The continued oxygen uptake, with fluoroacetate at the 20-mM level, 

 could be due to an incomplete block at the aconitase stage, perhaps 

 because of an adequate supply of endogenous pyruvate. If, instead 

 of being added during the incubation, 10-mM fluoroacetate was pres- 

 ent from the beginning, the reduction in Qi)., could be prevented b\' 

 the coincident addition of p\'ruvate at 10 mM, but not of fumarate 

 at 10 niM or glucose at 17 niM (Table II). This protective property 



TABLE IT. Thk Effects of Fluoroacetate and Substrates 



ON Oxygen Consumption 



(Phosphate medium; 100 per cent Oo, 2-hour incubation; averages of 4 to 8 flasks) 



