COMPONENTS OF THE ENERGY-COUPLING MECHANISM 37 



of the latter with the exchange activity observed in intact mitochondria. 

 Addition of adenylate kinase, which also catalyzes an ATP-ADP exchange, 

 to fresh digitonin particles does not confer DNP-sensitivity on this re- 

 action, for example. 



800 



|=DNP-SENSITIVE PORTION 



<j a 

 X t- 



UJ < 



FRESH _ 



PARTICLES 



AGED 



u 



DP SOL 



COMB- 

 INED 



DP SOL 



COMB- 

 INED 



Fig. 2. Failure of aged digitonin particles (48 hr. at 2 ) to confer DNP- 

 sensitivity on soluble ATP-ADP exchange enz\Tne. 



We have concluded that the digitonin particles have lost, during the 

 course of preparation, a significant fraction of the molecules of the ATP- 

 ADP exchange enzyme present in mitochondria. However, the binding 

 sites to which these molecules are normallv attached are still functional 



400 



■|=DNP SENSITIVE PORTION 

 CONTROL 



+ 0.002 M 

 AZIDE 



DP SOL COMB- 

 INED 



DP 



SOL COMB- 

 INED 



Fig. 3. Elffect of 0002 m azide on "recombination " of ATP-ADP exchange 

 enzyme with digitonin particles. 



and capable of " rebinding" the soluble form of the exchange enzyme in a 

 specific manner so as to bring the nucleotide exchange reaction it catalyzes 

 into equilibrium with a DNP-sensitive reaction. In consonance with this 

 conclusion we have found that there is an upper limit to the capacity of 



