I 62 LARS ERNSTER 



require an intramitochondrially generated supply of high-energy com- 

 pounds. As will be indicated below, this is probably due to compart- 

 mentation phenomena in the intact mitochondrion. 



It was of special interest to establish whether the inhibitory effect of 

 antimycin A was merely due, like those of cyanide and anaerobiosis, to a 

 general block of the energy-generating system, or whether this compound 

 inhibited the succinate-linked reduction of DPN per se. The latter con- 

 clusion has recently been reached by Chance and Hollunger [lo] and led 

 them to postulate that the succinate-linked DPN-reduction involves 

 cytochrome b. The reaction scheme proposed by us (cf. Fig. 9) is not 

 compatible with such a conclusion. 



TABLE XII 



Phosphorylation Coupled to the Antimycin A Insensitive Oxidation 

 OF Succinate by Ferricyanide in Rat Liver Mitochondria 



(Azzone, Ernster and Weinbach, unpublished) 



In each flask: mitochondria from 300 mg. liver, 50 mM KCl, 20 mM glycyl- 

 glycine, pH 7-5, 12-5 mM P^, 8 mM MgCL, 10 mM succinate, 20 mM ferricyanide, 

 0-5 mM KCN, I niM ATP, 15 mM glucose, hexokinase, and, when indicated, 

 I /xg./ml. antimycin A. Final volume, 2 ml. Incubation at 30 for 20 min. 



Copenhaver and Lardy [48] reported in 1952 that oxidation of succinate 

 in rat liver mitochondria with ferricyanide as terminal electron acceptor in 

 the presence of cyanide gave rise to a phosphorylation with a VJ2e ratio of 

 0-6, and that both the oxidation and the coupled phosphorylation were 

 insensitive to antimycin A. These findings were now confirmed (Table 

 XII)* and, using this system, it could be shown that the succinate-linked 



* Subsequent to the studies of Copenhaver and Lardy [48], Pressman [49] 

 reported that, in his hands, antimycin A did inhibit the oxidation of succinate by 

 ferricyanide. In an attempt to explore the reason for this discrepancy, we found 

 (cf. also Discussion, p. 168) that the antimycin A-sensitivity of this system is 

 dependent on the Pj/ATP ratio prevailing in the incubating medium; high concen- 

 trations of Pj favour insensitivity, and high concentrations of ATP favour sensi- 

 tivity, to antimycin A. Furthermore, both Pj and ATP were found greatly to 

 stimulate ferricyanide reduction. These findings are strikingly parallel to those 

 reported by Hatefi [50] concerning the reduction of coenzyme Q in beef heart 

 mitochondria and the antimycin A-sensitivity of this reaction. 



