1 6 OLOV LINDBERG et al. 



was able to render the reaction insensitive to dinitrophenol. Hence, 

 Wadkins and Lehninger [86] proposed that the dinitrophenol-insensitive 

 ATP-ADP exchange reaction represents the terminal step of phosphate 

 transfer in respiratory chain phosphorylation. Since a similar conclusion 

 concerning the Mg + ' -activated ATPase was previously reached in our 

 laboratory [75-78] it was of interest to investigate whether the ATPase 

 activity of the preparation of mitochondrial fragments was paralleled by 

 an ATP ADP exchange. Such a connection between the two reactions 

 has recently been postulated by Bronk and Kielley [87] from data obtained 

 with phosphorylating fragments of sonicated mitochondria. If such a 

 connection existed, it was of interest to investigate whether the ATP-ADP 

 exchange reaction was also sensitive to desaminothyroxine. 



TABLE III 



Influence of Some Agents on the ATPase and ATP-ADP Exchange Reactions 

 IN Mitochondrial Fragments Prepared According to Kielley and Kielley 



[80]. 



Conditions : for ATP-ADP exchange see [87] and forATPase see exp. in Fig. 2. 

 Incubation for 4 min. at 30 . 



The ATP-ADP exchange was measured by using terminally labelled 

 ^^P-ADP following the procedure described bv Bronk and Kielley [87]. 

 Table III summarizes some properties of the mitochondrial fragment 

 preparation regarding ATPase and ATP ADP exchange activities. In 

 accordance with the findings of Wadkins and Lehninger [86] the exchange 

 reaction was not inhibited by a concentration of azide which strongly 

 inhibited the ATPase. A similar effect was obtained with sodium fluoride. 

 Conversely, however, AMP at a concentration of 2 x 10^^ m strongly 

 inhibited the exchange reaction but left the ATPase activity practically 

 unaffected. 



In Table IV the effect of desaminothyroxine on the ATP-ADP 

 exchange reaction is shown. The exchange was inhibited almost completely 



