ACCUMULATION OF SUCCINATE 91 



El Dein, 1962), tobacco leaves (Vickery, 1959; Vickery and Palmer, 1957), 

 potato slices (Romberger and Norton, 1961), avocado mitochondria (Avron 

 and Biale, 1957), pea leaf particulates (Smillie, 1956), barley roots (Laties, 

 1949 b), Colpidium (Seaman, 1949), Trypanosoma (Bowman et al., 1963), 

 carp liver mitochondria (Gumbmann and Tappel, 1962 b), rat heart ho- 

 mogenates (Lehninger, 1946 b), rat liver slices (Elliott and Greig, 1937), 

 human heart slices (Burdette, 1952), ascites carcinoma cells (Dajani et al., 

 1961), and in many tissues of rats and rabbits (Busch and Potter, 1952 a; 

 Forssman, 1941). In the experiments leading to the results in Table 1-16, the 

 preparations were incubated for one to several hours with malonate and the 

 succinate analyzed at the and of the incubation, so that the rates of succinate 

 formation at any time are difficult to evaluate, and may well have been 

 greater initially. The over all succinate concentrations may be estimated 

 from the volumes in which the expriments were run and in most cases the 

 final succinate concentrations range between 0.5 and 2.5 mM. 



Several points are brought out by the results in Table 1-16. It is seen 

 that succinate can be formed from essentially all the cycle substrates and 

 intermediates in the presence of malonate. Rapid rates are found when 

 oxalacetate or some substance forming oxalacetate is added with pyruvate, 

 as would be expected, because in the absence of a source of oxalacetate, the 

 malonate would reduce the incorporation of pyruvate into the cycle and 

 hence the rate of formation of succinate. It may be noted in some cases 

 that, in the absence of added substrates or malonate, some succinate ac- 

 cumulates (yeast, Avena coleoptile, spinach leaves, and dog heart), which 

 implies that under the experimental conditions succinate is formed more 

 rapidly than it can be oxidized. This is somewhat surprising because it is 

 usually assumed that the activity of succinate oxidase is quite high in 

 most tissues. The possibility of the accumulation of sufficient oxalacetate 

 to inhibit succinate dehydrogenase when little acetyl-CoA is available 

 cannot be ignored. This phenomenon is also evident in the analyses for 

 succinate given in Table 1-15. The interesting effects of malonate concen- 

 tration are seen in two investigations. In spinach leaves, the maximal suc- 

 cinate accumulation occurs at malonate concentrations around or below 

 50 milf; at higher concentrations, the malonate is apparently acting on 

 other enzymes in the cycle and reducing the rate of formation of succinate. 

 Likewise, in brain minces, the high concentration of 200 mM malonate is 

 seen to depress succinate accumulation. 



Quantitative conversion of cycle substrates to succinate in the presence 

 of malonate is generally not observed. In fact, in most cases in which the 

 disappearance of substrate was determined simultaneously with the for- 

 mation of succinate, only a small fraction appeared as succinate. For exam- 

 ple, Speck et al. (1946) found in malarial parasitized erythrocytes that 

 only 22% of the pyruvate utilized in the presence of 20 mM malonate was 



