EFFECTS ON TRICARBOXYLIC ACID CYCLE 87 



Laties (1953) found that increasing the citrate from 1 raM to 10 mM 

 would reduce the inhibition by 10 mM malonate from around 52% to zero in 

 cauliflower homogenates, and Pierpoint (1959) reported that a 10-fold rise 

 in citrate concentration brings about a decrease from 62 to 45% in the 

 inhibition by 21 mM malonate in tobacco leaf mitochondria. Laties pointed 

 out that this could not be due to a competitive effect because of an increased 

 production of succinate, since at the malonate concentration used the 

 oxidation of 10 mM succinate is completely inhibited. However, if what 

 was suggested in the previous section regarding the difficulty in the inhi- 

 bition of the oxidation of endogenously produced succinate is valid, data 

 on the inhibition of exogenous succinate may not be applicable. Laties felt 

 that the explanation might lie in the interference between electron-trans- 

 port systems, so that when citrate concentration is high, the contribution 

 made by succinate oxidation to the total respiration is less. Clear-cut effects 

 of concentration on inhibition have not been observed with a-ketoglutarate 

 (Grafflin et al., 1952; Pierpoint, 1959), pyruvate (Smyth, 1940), acetate 

 (Jowett and Quastel, 1935 c), or malate (Pierpoint, 1959). 



In experiments of this type, it is well to remember that the pattern of 

 oxygen uptake may change with the concentration of the substrate. The rela- 



xO yO 



S -> succinate ->• P 



tive amounts of oxygen taken up before and after the succinate step may be 

 altered by substrate concentration due to factors previously discussed in 

 this chapter. Thus the ratio x/y in the above equation will vary and hence 

 the effect of an inhibitor of succinate oxidation. If the inhibition on succinate 

 oxidation is i and the ratio xjy = r, the over all inhibition on the total oxy- 

 gen uptake will be ijil + r), so that anything that changes r will change the 

 inhibition. If high concentrations of the substrate produce an accumulation 

 of some intermediate (as citrate rises when pyruvate enters the cycle rapidly), 

 r will increase, at least over the initial period, and the inhibition will be 

 less at lower substrate concentrations. 



(e) Stimulation of cycle substrate utilization by malonate. In a number of 

 cases there is clearly a stimulation of the utilization of pyruvate, acetate, 

 or citrate by malonate. Sometimes this is recognized in an increased oxygen 

 uptake but occasionally the disappearance of the substrate is accelerated 

 while the oxygen uptake is inhibited. Often this effect is very marked. In 

 the mycelia of Ashbya gossypii, oxygen uptake due to addition of acetate is 

 stimulated 48% by 4 m.M malonate, whereas 40 m.M malonate inhibits 

 53% (Mickelson and Schuler, 1953). The increase in the respiration from 

 pyruvate in bull sperm by 10 mM malonate is almost as great (Lardy and 

 Phillips, 1945). Table 1-14 cites a number of other instances. 



