EFFECTS ON THE ENDOGENOUS RESPIRATION 167 



onate, as is evident from the range of K^ values observed (page 33). It is, 

 perhaps, too often assumed that in every organism the succinate dehydro- 

 genase wiU be readily blocked by malonate and that the inhibition of the 

 endogenous respiration depends only on the importance of the enzyme in 

 the total oxygen uptake. It is mandatory to demonstrate the sensitivity 

 of succinate dehydrogenase to malonate in the preparation being studied. 



(b) Degree to which intracellular succinate dehydrogenase is inhibited. In 

 addition to the intrinsic susceptibility, there are other factors which can 

 alter the inhibition occurring within the cell. The concentration of succinate, 

 both initially and following the accumulation resulting from the inhibition, 

 may be high enough to oppose the malonate effect appreciably. The relative 

 stability of plant respiration to malonate has been attributed to the high 

 concentrations of succinate and other organic anions in plant cells. It is 

 probably very seldom that an inhibition even approaching completeness 

 can be achieved in cells at concentrations likely to be specific. 



(c) Specificity of malonate inhibition. Inhibition of the endogenous res- 

 piration can, of course, arise from actions other than on succinate dehydro- 

 genase. If the object of the study is to evaluate the contribution of the 

 cycle to the oxygen uptake, inhibitions on noncycle pathways must be 

 eliminated. At the high malonate concentrations often used (see Table 1-26), 

 there is certainly no assurance that the inhibition is specific. 



(d) Intracellular concentration of malonate. Malonate does not penetrate 

 readily into most cells, especially at physiological external pH values, so 

 that the internal concentrations of malonate may be far below those in 

 the surrounding medium (see page 190). The degree of respiratory inhibition 

 observed is probably often more of a measure of malonate penetrability 

 than of the nature or susceptibility of the metabolic systems. There are 

 several instances in Table 1-26 in which the inhibition rises with destruction 

 of the normal tissue structure or the removal of permeability barriers. In 

 general, the inhibition is greater in homogenates than in minces, and greater 

 in minces than in slices or intact cells. The results of Bonner (1948) on 

 Avena coleoptiles are interesting in this regard. Soaking in water for 24 hr 

 increases the susceptibility to malonate and removal of the endosperm 

 further increases the inhibition. The many observations that a lowering 

 of the pH augments the inhibition also provide evidence of the importance 

 of permeability. 



(e) Metabolism of malonate. Many tissues and organisms can metabolize 

 malonate to acetyl-CoA, the oxidation of which contributes to the oxygen 

 uptake (see page 228). Some of the respiratory stimulations noted with 

 malonate must be due to this, and it is likely that the experimentally de- 

 termined inhibition in other cases is reduced from that which would be ob- 

 served if malonate were not metabolized. The best way to test for and 



