COMPONENTS OF THE ENERGY-COUPLING MECHANISM 43 



contract quite well in a butfered KCl medium on addition of ATP. The 

 addition of ATP + Mg ^ + + serum albumin was found to cause immediate 

 contraction of mitochondria swollen by a wide variety of agents, including 

 thyroxine, oleate, phlorizin, calcium, PCAIB, phosphate, and many others 

 [44] and, curiously, mitochondria swollen by digitonin and by carbon 

 tetrachloride. Actual extrusion of water was demonstrated by gravimetric 

 methods to accompany the optical changes. It was shown that several 

 hundred moles of water could be extruded per mole of ATP split [34]. 



The molecular mechanism of the ATP-induced contraction can be 

 shown to be completely independent of respiration and net phosphoryla- 

 tion, since it proceeds perfectly well in a medium containing sufficient 



P:0 = 2- 



K^ = i2m/^M/MG 



P:O = 00 

 K^=OOI 



Q 03 



[PO^OO 

 iK* =002 



Time (rnin) 



Fig. 5. Independence of ATP-induced contraction from oxidative phosphoryla- 

 tion and K ~ binding. .Allowing mitochondria to stand in swollen state at 25 for 

 extended periods abolishes phosphorylation and K ^-binding, without affecting 

 abilitv to contract. 



cyanide or other respiratory inhibitors to block respiration or in the 

 presence of sufficient dinitrophenol to completely uncouple oxidative 

 phosphorylation, as long as ATP is in excess [34]. On the other hand, it 

 is clear that the ATP-induced contraction must employ at least a portion 

 of the energy coupling machinery, since this contraction is blocked by 

 inhibitors such as azide, which disconnects the ATP-ADP exchange 

 reaction from the dinitrophenol sensitive site, and is also inhibited 

 characteristically by sucrose and many other sugars and polvhvdroxvlic 

 alcohols, which are also known to inhibit oxidative phosphorylation and 

 the ATP-P^'- exchange reaction as well as ATP-ase [3^]. Furthermore, 

 contraction of mitochondria by ATP is not dependent on any specific 

 ionic environment and can occur in mitochondria whose abilitv for K + 

 transport is completely inactivated [40] (Fig. 5). 



It appears likely that mitochondrial contraction induced bv ATP 



