The adenosine triphosphate-adenosine diphosphate exchange reaction of intact rat liver mitochondria.

Studies of the adenosine triphosphate-adcnosine diphosphate exchange reaction of phosphorylating submitochondrial particles first described by Cooper and Lehninger (l), and subsequently studied in detail by Bronk and Kielley (2), and Wadkins and Lehninger (3) have provided evidence that this exchange reaction is a manifestation of the terminal step of oxidative phosphorylation in which ADP reacts with an unknown “highenergy” intermediate generated during coupled electron transport to form ATP. This conclusion is based on the specificity for ATP and ADP, the absence from digitonin particles of other reactions known to catalyze such an exchange reaction, and the marked inhibition of the ATP-ADP eschange reaction by uncoupling agents such as dinitrophenol, Dicumarol, and gramicidin. Aside, an uncoupling agent of osidative phosphorylation and an inhibitor of the dinitrophenoland Dicumarol-stimulated ATPase, prevents the inhibition of the ATP-ADP exchange by dinitrophenol and Dicumarol, although azide alone has no effect on the rate of the ATP-ADP exchange reaction (3). Aging the particles results in the loss, at similar rates, of dinitrophenol sensitivity of the ATP-XDP exchange, of oxidative phosphorylation, and of the dinitrophenol-stimulated ATPase activity, whereas there is no significant loss of total ATP--4DP exchange activity. In addition to indicating that the ATP--4DP exchange reaction and the labile, aside-sensitive site of artion of dinitrophenol are two separate steps of the coupling mechanism, these results suggest that efficient intrraction of the two steps, perhaps through a common intermediate, is a prerequisite for the inhibitory effect of dinitrophenol on the hTP-.4DP eschange rate. The present communication describes some of the properties of the ATP-ADP exchange reaction as it occurs in intact rat liver mitochondria and compares the inhibition of the exchange reaction and the stimulation of hTPase activity by several uncoupling agents. In general, the results obtained support the view that there is a functional relationship between the ATP-ADP exchange reaction, the ATPase reaction, and the overall coupling mechanism of osidative phosphorylation. These relationships are as clearly evident in intact mitochondria as in digitonin fragments.