90 1. MALONATE 



be higher, and in yeast metabolizing sugar the values may be lower. One 

 value for oxalacetate in normal rat liver is available (0.036 mikf) and in 

 this case fasting for 24 hr did not alter this appreciably (Kalnitsky and 

 Tapley, 1958). 



These values do not represent a thermodynamic equilibrium based on dif- 

 ferences in free energy, but rather a dynamic or kinetic equilibrium, depend- 

 ing mainly on the relative rates of the cycle reactions and competing process- 

 es. The higher concentration of fumarate compared to malate in rat tissues 

 illustrates this because in a thermodynamic equilibrium there would be 

 about one-fourth the concentration of malate. It is interesting that the values 

 differ so widely from tissue to tissue and certainly this must be one factor in 

 determining the different responses to malonate or other competitive inhi- 

 bitors. The levels of succinate are generally low, except in yeast, but as 

 pointed out above the concentrations within the mitochondria or at the re- 

 gion of the active center of succinate dehydrogenase may well be higher. 

 The effects of malonate on the concentrations of these intermediates in cells 

 will be taken up in the following section. 



Analyses of plant tissues have not been presented here because there is 

 some doubt as to the significance of the figures. Most plants contain large 

 amounts of organic acids, including the cycle intermediates. Beevers (1952) 

 postulated that the cj^cle in plants is less readily blocked than in animal 

 tissues because of these high concentrations of succinate and other cycle 

 intermediates. This could well be an important factor, but actually the 

 concentrations of these acids in the plant cytoplasm are not known in 

 most cases, total analyses including the vacuolar fluid, which is often of 

 greater volume than the cytoplasm and contains most of the organic acids. 

 There is another way by which these plant acids could protect against 

 malonate. The presence of large amounts of fumarate or malate, or of any 

 substance capable of forming oxalacetate, would allow pyruvate to be incor- 

 porated into the cycle even in the state of complete block of succinate oxi- 

 dation. Examples of the overcoming of malonate inhibition by fumarate 

 and malate will be presented shortly. 



ACCUMULATION OF SUCCINATE DURING MALONATE 



INHIBITION 



An effective inhibition of succinate oxidation should lead to a rise in 

 the concentration of succinate under conditions in which succinate can still 

 be formed. Such accumulation of succinate has been frequently observed 

 and some of the more quantitative results are summarized in Table 1-16. 

 In addition to the examples in the table, accumulation of succinate has 

 been reported in the following species and tissues: Shigella (Yee et al., 

 1958), Nocardia (Cartwright and Cain, 1959), Aspergillus (Shimi and Nour 



