EFFECTS ON TRICARBOXYLIC ACID CYCLE 73 



(a) Inhibition of pyruvate oxidation. The oxidation or disappearance of 

 pyruvate in cellular preparations is usually not depressed very much by mal- 

 onate at concentrations less than 10 miH , whereas in mitochondrial prepara- 

 tions the expected degree of inhibition is usually observed. This may be 

 partly explained by poor penetration into the cells and partly by the alter- 

 nate pathways that may reduce the importance of the cycle. One type of 

 correction that can be applied for a more accurate determination of cycle 

 inhibition by malonate is that used by Speck et al., (1946). In malarial 

 parasitized erythrocytes, pyruvate is oxidized without the appearance 

 of acetate, but in the presence of malonate, some acetate in formed. Cor- 

 rection was made for that pyruvate that went to acetate, since this fraction 

 of the pyruvate utilization is not dependent on the cycle. The inhibition of 

 over all pyruvate utilization was 12% but corrected for the acetate it was 

 31%. In the free parasites, the over all inhibition was 33% and the cor- 

 rected inhibition 76%. Of course, pyruvate here or in other cells may be 

 metabolized in other ways, so that the correction for acetate alone may 

 not give the true cj^cle inhibition, but at least is provides a better value. 



Malonate should inhibit the oxidation of pyruvate more strongly when 

 there is a low concentration initially of oxalacetate or a substance forming 

 oxalacetate (see page 70). This was shown in homogenates of rat tissues 

 by Pardee and Potter (1949). In each case the inhibition by 4 mM malonate 



is less when oxalacetate, is present. Since the oxygen uptake was deter- 

 mined from 10 to 30 min after the start of the experiments and inasmuch as 

 the concentrations of pyruvate and oxalacetate were 3.5 mM, it is unexpect- 

 ed that so much inhibition would be exerted when the mixture is present. 

 This might be due to the decarboxylation of sufficient oxalacetate so that 

 it was less effective than anticipated, or even at this low concentration 

 malonate may have been inhibiting some reaction other than the oxidation 

 of succinate. In rat heart mitochondria we found that 5 mM malonate 

 inhibits the oxidation of 5 mM pyruvate only about 10% in the presence of 



