88 1. MALONATE 



The mechanisms for such stimulations must vary with the particular sub- 

 strate used, but it may be useful to suggest some possible ways in which 

 malonate could produce this apparently anomalous effect. (1) If the prepara- 

 tion has an active oxalacetate decarboxylase, this may reduce the concentra- 

 tion of oxalacetate for condensation with acetyl-CoA. Malonate is able to 

 inhibit this enzyme in some cases (Table 1-12) (Pardee and Potter, 1949), 

 in which case oxalacetate may be protected so that cycle entry of acetyl- 

 CoA is facilitated. (2) If ADP concentration is low and ATP concentration 

 high normally in the preparation, malonate by inhibiting certain phases of 

 the cycle might increase ADP concentration and thus stimulate electron 

 transport in other oxidative processes by providing more phosphate ac- 

 ceptor. (3) If there is competition between the different oxidative reactions 

 in the cycle for some common coenzyme or cofactor (e.g. NAD, NADP, or 

 Co A), inhibition of some oxidations by malonate might allow these factors 

 to be used more readily by other systems. Such competition between the 

 pyruvate and a-ketoglutarate systems has been suggested in heart mito- 

 chondria (Montgomery and Webb, 1956 a). (4) If malonate is metabolized by 

 the preparation, it is possible that a product would accelerate the utiliza- 

 tion of some cycle intermediate. In some organisms malonate can form acetyl- 

 CoA and acetate, as well as other products. (5) In intact cells, malonate 

 might increase the permeability of the cell membrane so that certain sub- 

 strates, such as pyruvate, citrate, or a-ketoglutarate, could enter more 

 readily. (6) By chelation with inhibitory metal ions that may occur in the 

 preparation, malonate might accelerate the rates of certain reactions. All of 

 these mechanisms are purely hypothetical, since in no case has the actual 

 mechanism been established. 



Intracellular Concentrations of Cycle Intermediates 



Interpretation of intracellular inhibition by malonate and other inhi- 

 bitors acting on the cycle should ideally involve in many cases a knowledge 

 of the concentrations of certains intermediates. Data collected for different 

 types of cells are shown in Table 1-15. These figures were calculated on 

 the basis of intracellular water contents. However, it is likely that these 

 substances are not distributed homogeneously throughout the cell water. 

 There is evidence that some intermediates may occur within the mito- 

 chondria at different concentrations than in the surrounding medium. Thus 

 the mitochondria/medium ratios for sheep kidney mitochondria under 

 certain conditions were found to be: pyruvate 0.84, fumarate 7.42, a-keto- 

 glutarate 1.0, citrate 0.83, and oxalacetate 0.13 (Bartley and Davies, 

 1954). Furthermore, the values given in the table are all abnormal since 

 truly normal tissues were not used. The rats were fasted for 24 hr while 

 the suspensions of E. coli and yeast were metabolizing acetate rather than 

 a more normal substrate. In normal rat tissues the concentrations may well 



