The Functional Link of Succinic Dehydrogenase with the 

 Terminal Respiratory Chain 



Giovanni Felice Azzone* 



The Wemier-Gren Institute for Experimental Biology, 

 University of Stockholm, Sweden 



It is our purpose to examine some energetic aspects of the electron 

 transport system catalyzing the aerobic oxidation of succinate in intact 

 liver mitochondria. It has been generally accepted that the oxidation of 

 succinate is completely independent of electron transport and phosphoryla- 

 tion in the DPN-flavin region of the respiratory chain. Support for this 

 concept came from the findings that mitochondria either depleted of DPN, 

 or in the presence of amytal, as well as non-phosphorylating submito- 

 chondrial preparations are fully capable of catalyzing the aerobic oxidation 

 of succinate. Therefore it seemed likely that cytochrome b was the site of 

 entrance for the electrons coming from succinic dehydrogenase; the two 

 phosphorylations in the cytochrome region of the respiratory chain could 

 then account for the net phosphate uptake occurring during succinate 

 oxidation. 



Renewed interest in this question has emerged subsequent to the recent 

 work of Chance and Hollunger [i, 2], and of Klingenberg et al. [3, 4], who 

 found that the extent of reduction of mitochondrial pyridine nucleotide is 

 greatly increased by the addition of a flavosubstrate, succinate, or glycerol 

 I -phosphate. 



A different approach to the question of the interaction between succinic 

 dehydrogenase and the DPX-flavin region of the respiratory chain recently 

 has been possible because of the finding [5, 6] that intact liver mito- 

 chondria, when depleted of high energy phosphate, are no longer capable 

 of oxidizing succinate at any appreciable rate unless ATP is added, or 

 synthetized in the system. Furthermore, the beneficial effect of ATP is not 

 abolished in the presence of uncoupling agents. 



The depletion of mitochondrial high energy phosphate and 

 the inhibition of succinate oxidation 



An experiment illustrating the depression of the capacity for succinate 

 oxidation in rat liver mitochondria is shown in Fig. i. Addition of arsenate 



* Fellow of the Consiglio Nazionale delle Ricerche. Permanent address : 

 Istituto di Patologia Generate, Universita di Padova, Italy. 



VOL. II. — o 



