10 OLOV LINDBERG et al. 



give a maximal inhibition of about 30 and 60" q, respectively. A similar 

 pattern of inhibition could be obtained also in the case of the dinitrophenol- 

 induced ATPase if this reaction was measured in the presence of added 

 Mg + + (Fig. 3). Thus, whereas added Mg + + did not alter the inhibition 

 of the dinitrophenol-induced ATPase by desaminothyroxine, it rendered 

 the inhibition with thyroxine less efficient and with a maximal inhibition 

 of about only 30",,. It would seem that this effect of Mg + + was not due 



None 



Desamino 

 thyroxine 



mi n 



Fig. 4. Time-course of inhibition of the dinitrophenol-induced ATPase 

 reaction by desaminothyroxine. When indicated desaminothyroxine was added 

 in a final concentration of 0-2 mM. Other experimental conditions as in Fig. i. 



primarily to a binding of thyroxine (in which case the protection by Mg + + 

 should have been overcome with higher concentrations of thyroxine), 

 but rather to an ability of Mg + + to restrict the number of active sites in 

 the preparation accessible to thyroxine. For this reason the investigations 

 to follow were performed with desaminothyroxine. 



The inhibitory effect of desaminothyroxine on the dinitrophenol- 

 induced ATPase reaction was instantaneous, as shown in Fig. 4. Pre- 

 incubation with desaminothyroxine prior to the addition of ATP did not 

 influence the extent of inhibition, neither in the case of this reaction, nor 

 in the case of the Mg + +-activated ATPase reaction catalyzed by mito- 

 chondrial fragments (Fig. 5). 



