THE FUNCTIONAL LINK OF SUCCINIC DEHYDROGENASE 201 



sulphinate to prevent the inhibition of succinate oxidation when added to 

 the mitochondria during the incubation with arsenate and dicoumarol, can 

 be explained on the basis of the abihty of cysteine sulphinate to maintain, 

 by rendering the oxaloacetic carboxylase reaction inactive, high energy 

 compounds in the substrate level phosphorylation compartment of the 

 mitochondria. 



On the basis of the present findings it appears that oxaloacetate besides 

 its known function as competitive inhibitor of succinic dehydrogenase, 

 possesses also the capacity of competing, through the oxaloacetic car- 

 boxylase reaction, in the utilization of the high energy phosphate com- 

 pounds required for succinate oxidation. This additional property of 

 oxaloacetate appears to be of particular significance in conditions where 

 the generation of high energy phosphate compounds from respiratory 

 chain phosphorylations is blocked by the addition of uncoupling agents. 

 In these conditions the oxidation of succinate, which otherwise can 

 dispose of the high energy intermediates formed in the last two respiratory 

 chain phosphorylations, will be dependent, as sole source of energy, on 

 the ATP which originates from the a-ketoglutarate-linked substrate-level 

 phosphorylation. 



The activation of succinate oxidation 



The experimental evidence reported above supports the concept that in 

 intact mitochondria the aerobic oxidation of succinate must proceed 

 through a thermodynamically unfavourable reaction which requires the 

 investment of energy. This energy-demanding reaction appears to be 

 characteristic only of phosphorylating mitochondrial preparations, since no 

 such requirement has been demonstrated for non-phosphorylating 

 succinoxidase preparations. 



Although no conclusive evidence exists at present regarding the nature 

 of the high energy intermediate formed, it seems conceivable that this 

 intermediate must possess the following properties : (a) it must be different 

 from the two high energy intermediates which provide the two net phos- 

 phorylations occurring during the aerobic oxidation of succinate ; (/;) the 

 energy of this additional intermediate can be used directly or indirectly for 

 the reduction of the mitochondrial pyridine nucleotide; this conclusion is 

 supported by the findings made in collaboration with KHngenberg [ii] 

 that the stimulation of succinate oxidation by ATP in arsenate-dicoumarol 

 pretreated mitochondria is paralleled by a reduction of mitochondrial 

 pyridine nucleotide ; (c) the formation of this high energy intermediate is 

 not impaired by the presence of uncoupling agents. This finding suggests 

 that the energy required for activation of succinate oxidation and for 

 reduction of pyridine nucleotide cannot be supplied by a non-phos- 

 phorylated intermediate of the X ~ I type, because the latter has the 



