ACCUMULATION OF SUCCINATE 97 



What effects succinate concentration might have on these reactions are 

 unknown, but some inhibition during succinate accumulation is possible, 

 although a simple backing-up of the a-ketoglutarate -^ succinate reaction 

 would be thermodynamically unlikely. 



The relative potential rates of a-ketogluti^rate oxidase and succinate 

 oxidase under normal intramitochondrial conditions are not known, but the 

 oxidation of succinate is certainly one of the most rapid reactions seen in 

 mitochondrial suspensions. It may be that there is no accumulation of suc- 

 cinate in the cycle under physiological conditions, and that the small 

 amounts of succinate found in tissues do not truly indicate the situation in 

 the regions of succinate oxidase. Succinate oxidase is, perhaps, the one 

 enzyme that has never been considered as normally limiting the cycle rate. 

 If the maximal rate of succinate oxidation is much higher than the rate at 

 which succinate can be formed, it would require a fairly high inhibition of 

 the oxidase before succinate accumulates markedly. For example, if the rate 

 of succinate formation is one-tenth the rate at which it can be oxidized, 

 90% inhibition of the succinate oxidase would make the rates equivalent, 

 and the succinate concentration would not rise very much (probably not 

 more than 0.02 0.05 n\M). Under any likely conditions, calculations from 

 Eqs. 1-7-8 and 1-7-9 make it clear that succinate oxidation must be inhibited 

 fairly strongly to produce a significant succinate accumulation. The com- 

 m.on assumption that succinate must accumulate rather quantitatively 

 when sufficient malonate has been added to inhibit succinate oxidase 

 75-90% is thus unjustified. If alternate pathways for succinyl-CoA or suc- 

 cinate exist, the accumulation of succinate would be even less evident. 



Several instances of failure of succinate to accumulate during malonate 

 inhibition have been reported. Hanly et al. (1952) found that in only two 

 of six experiments with carrot root slices did succinate accumulate, and 

 in these the rise was insignificant. One might suspect a lack of penetration, 

 but 15-50 mM malonate was used at pH 4; under these conditions, malonate 

 depressed respiration strongly. Weil-Malherbe (1937) found no succinate 

 accumulation in guinea pig brain slices with malonate 4-40 mM, respiration 

 being markedly reduced at the higher concentrations. A depression of suc- 

 cinate level in Streptomyces due to 10 vaM malonate was noted by Coch- 

 rane (1952), with either malate or citrate as substrate. Since the incubation 

 was 16 hr and the pH 5.1-5.4, it is possible that a nonspecific acid damage 

 from malonic acid penetration was responsible for the cycle depression, or 

 it might be that in this organism malonate is not specific at 10 mM, or, 

 as Cochrane suggested, succinate may not be formed via the cycle. The fail- 

 ure of succinate to accumulate, even when the respiration is suppressed by 

 malonate, is difficult to explain except on the basis of actions other than 

 on succinate dehydrogenase. 



