INHIBITION OF SUCCINATE OXIDATION 51 



the four examples given in Table 1-11. The importance of the endogenous 

 correction is seen to vary with the effect of malonate on the endogenous 

 respiration. When malonate inhibits the endogenous O2 uptake poorly, 

 as in rat liver, or actually stimulates the O2 uptake (due to its metabolism), 

 as in Euglena, the true inhibition of succinate oxidation is much higher than 

 would be calculated simply from the data on succinate and succinate + 

 malonate. 



Comparison of the inhibitions in Tables 1-6 and 1-10 for the same species 

 or tissues, although this is qualitative only, shows that in several instances 

 the inhibitory potency of malonate seems to be significantly less in cellular 

 preparations. This is true for E. coli, Rhodospirilliim, Crithidia, Zygor- 

 rhynchus, pigeon muscle, and rat liver. Moses (1955) points out that in 

 Zygorrhynchus the inhibition of succinate oxidation in cell suspensions is 

 very weak, even at the low pH of 3.4, but when the cells are treated with 

 liquid nitrogen to destroy their structure, malonate inhibits normally. 

 The oxidation of succinate by cell susj^ensions of Bacterium succinicum is 

 not inhibited at all by 5 mM malonate whereas such oxidation in cell-free 

 extracts is inhibited completely (Takahashi and Nomura, 1952). There are 

 also several reports in which malonate was found to be ineffective but the 

 extracted succinate oxidase system was not directly tested; however, in 

 these cases one would certainly expect the enzyme to be sensitive to mal- 

 onate. For example, malonate (2 mM) does not inhibit the oxidation of 

 succinate by barley roots (Honda, 1957), or at 5-20 mM in dried cells of 

 CJdorella (Millbank, 1957), while in beech roots malonate (28.6 mM) actually 

 stimulates the rate of succinate oxidation (Harley and Ap Rees, 1959). 

 However, in some cases comparable inhibitions have been observed in vitro 

 and in vivo. Danforth (1953) showed in Euglena that malonate is quite ef- 

 fective in intact cells if the pH is low enough (around 4.5) and, although it 

 is difficult to compare the results with those obtained from homogenates 

 because of different concentrations of succinate and malonate, it would 

 appear that malonate is equally inhibitory in the two preparations. Similar 

 effects of malonate were also observed in our work (Montgomery and Webb, 

 1956 b) on rat heart slices and mitochondrial suspensions. 



The failure of malonate to inhibit the oxidation of added succinate in 

 cellular preparations well, or at all, has usually been attributed to permea- 

 bility factors. However, it is difficult to understand how permeability could 

 explain these results, inasmuch as the penetration of both succinate and mal- 

 onate would be controlled by the same factors, presumably. That is, if there 

 is some barrier to malonate reaching the succinate dehydrogenase, how can 

 succinate pass this barrier? It is true that the "pK,, for succinate is higher 

 than for malonate but almost identical values of ^K,, have been reported; 

 thus the distribution of the ionic forms around neutrality would be approxi- 

 mately the same (Table 1-3). If only the uncharged forms of these acids — 



