EFFECTS OF THYROXINE AND RELATED COMPOUNDS ON LUTR MITOCHONDRIA 17 



by 2 X 10'^ M desaminothyroxine. The extent of inhibition was thus 

 comparable to that of the ATPase activity. It was found on the other hand 

 (Table IV) that an ATP-ADP exchange reaction occurred also in the 

 supernatant obtained in this preparation, and that also this reaction was 

 strongly inhibited by desaminothyroxine. This fraction was free of ATPase 

 activity, in agreement with Kielley and Kiellev [80]. The exchange 

 activity found in the supernatant was actually higher than that of the 

 sediment. It was distinct from the latter as indicated by the fact that the 

 residual activity could not be removed from the pellet by washing. The 

 possibility was considered that the ATP-ADP exchange activity of the 

 supernatant might be a reflection of the adenylate kinase reaction, which 

 is recovered in this fraction in the present procedure. However, the follow- 

 ing findings indicated that the two acti\ities were not correlated: (i) The 



TABLE IV 



Effect of Desaminothyroxine on the ATP-ADP Exchange and ATPase 

 Reactions of Submitochondrial Fractions Prepared According to Kielley 



AND Kielley [80]. 

 Conditions as in Table III, except that time of incubation was 2 min. Sediment 

 and supernatant assayed in equivalent amounts in terms of wet weight liver. 



Preparation Additions 



ATPase activity Exchange activity 

 /xmoles P hydrolyzed /Lmioles P exchanged 



0-56 



o*o6 

 I -46 



adenylate kinase reaction (as measured with ADP as substrate and hexo- 

 kinase and glucose as trapping agent for the ATP) was unafl:"ecced bv 

 2x10 ^.M desaminothyroxine whereas the ATP-ADP exchange was almost 

 completely inhibited (cf. Table IV). (2) The adenylate kinase reaction is 

 inhibited by 2 x 10 - m sodium fluoride [88], whereas the ATP-ADP 

 exchange was virtually unaftected (cf. Table III). (3) The ATP-ADP 

 exchange activity of the supernatant compared with the net adenvlate 

 kinase activity of the same fraction was considerablv higher than the 

 corresponding ratio of the two activities in a purified preparation of muscle 

 myokinase. 



It would seem from these data that a desaminothyroxine-sensitive 

 ADP-ATP exchange reaction is present in subfractions of rat liver mito- 

 chondria; however, the relation of this reaction to the ATPase is not 

 clear, since it is present both in the fraction in which the ATPase is 

 concentrated and in the fraction devoid of ATPase activitv. 



