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Discussion 



Lehninger : These experiments carried out by Dr. Packer bring into focus one 

 of the apparent discrepancies which can be seen in our respective approaches. 

 Dr. Packer is interested in the physiological description of reversible, low- 

 amplitude swelling and shrinking in tightly coupled mitochondria. Our own 

 interest has been in dissociating the high amplitude swelling and contraction 

 processes from all the other enzymic machinery in mitochondria which are not 

 necessarily tightly coupled. Actually we can make mitochondria contract with 

 ATP after respiration and phosphorylation are irreversibly lost. We have a different 

 approach and the interesting thing is that our drastically swollen mitochondria 

 contract with ATP but not ADP and Dr. Packer's slightly swollen mitochondria 

 contract on the addition of ADP but not ATP. I think that his experiments point 

 to the reason for this apparent discrepancy if I understand him right. It is now the 

 current conception that the mitochondrial membrane in vivo is relatively imper- 

 meable to ATP. The internal and external nucleotides have different turnover 

 rates. Such compartmentation has been invoked in explaining the Pasteur reaction. 

 I think it is possible then under conditions of very drastic swelling such as we use 

 that the permeability of the mitochondrial membrane has in fact changed, though 

 I can hardly agree with Dr. Packer that exposure with KCl can be regarded as 

 unphysiological. It is a more physiological substance than sucrose. In any case it 

 is possible that for shrinking to occur ATP must penetrate inside, or that it must 

 be generated inside from ADP. Do you believe that ADP can penetrate inside and 

 there generate ATP which can drive the contraction ? 



Packer: I think the reconstruction experiment shows that it is very difficult 

 in isolated mitochondria to retain the ATP synthesized by oxidative phosphoryla- 

 tion ; it almost all comes out, bvit when a change in permeability or when a large 



