84 1. MALONATE 



50%, depending on the relative rates of the reactions and the period during 

 which the oxygen uptake is measured. If the fumarate is further oxidized, 

 more oxygen will be consumed and the inhibition by malonate may be 

 greater than 50%. Furthermore, it is essentially impossible to inhibit the 

 succinate dehydrogenase completely, especially as succinate will accumulate 

 and progressively overcome the inhibition. 



The use of malonate to study a-ketoglutarate oxidation in mitochondria 

 involves the assumption that malonate does not significantly affect the a- 

 ketoglutarate oxidase directly. Unfortunately, no investigations of the in- 

 hibition of a-ketoglutarate dehydrogenase by malonate have been report- 

 ed, and thus it is difficult to compare the sensitivities of the two dehydro- 

 genases. Several studies have determined the effects of malonate on the 

 disappearance of a-ketoglutarate (a-KG) during periods when the oxygen 

 uptake is depressed, and it is rather strange that some effect, either positive 

 or negative, has always been reported. The pertinent data have been sum- 

 marized in the accompanying tabulation, and it is seen that an inhibition 



of a-ketoglutarate disappearance is observed in some cases and a stimulation 

 in others. The oxidation of a-ketoglutarate depends on Mg++ and it is 

 possible that the differences are related to the degree of Mg++ requirement 

 and the concentrations of Mg++ used in the assay media. Price (1953) 

 found that even 1 mM malonate would inhibit a-ketoglutarate utilization 

 in pea seedling mitochondria and that 30 raM had a very marked effect 

 (60% inhibition). This is not a competitive type of inhibition because 

 increasing the a-ketoglutarate concentration does not lower the inhibition, 

 and even increases it somewhat. The possibility of Mg++ reduction was 

 explored and it was found that the calculated drop in the Mg++ concen- 

 tration could not have been responsible for the inhibition. Furthermore, it 

 was not possible to reverse the inhibition by increasing the Mg^^ concen- 

 tration from 1 mM to 4 mM. An inhibition by a Mg-malonate complex 

 was considered to be the most likely explanation, but yet no inhibition 

 was seen when the concentration of Mg++ in the medium was reduced to 

 zero, although the enzyme was still partially complexed with Mg+"'". What- 



