8o 



J. B. CHAPPELL 



The same dependence of order of addition on the immediate effect 

 of oxaloacetate on succinate oxidation has been observed with intact 

 liver and kidney mitochondria. Mitochondria which had been preincu- 

 bated with arsenate, DPN and amytal, oxidized succinate at rapid rates. 

 However, if i mM oxaloacetate were added as little as 2 sec. before the 

 succinate, oxygen uptake was severely and sometimes completely inhibited, 

 whereas if the inhibitor were added after the succinate no significant effect 

 was observed (Fig. 10). 



2 3 4 

 Time (mm) 



Fig. 10. Effect of adding oxaloacetate (i mM), both before and after succinate, 

 on the rate of oxygen uptake of liver mitochondria. 2 mM arsenate and i • 8 mM 

 amytal were present at zero time. Other conditions as in Fig. 7. 



These observations enable an explanation of the effect of Azzone and 

 Ernster [12] to be given. Preincubation with DNP and arsenate leads to 

 the accumulation of oxaloacetate from endogenous substrates, the keto- 

 acid then forms a stable complex with the succinate dehydrogenase, which 

 dissociates with difficulty. Very little can be said of how ATP reverses this 

 inhibition ; it may be that in some way ATP dissociates the dehydrogenase- 

 oxaloacetate complex, but this is unlikely since ATP had no effect on the 

 oxaloacetate inhibition of succinate oxidation by kidney preparations 



