EFFECTS OF THYROXINE AND RELATED COMPOUNDS ON LIVER MITOCHONDRIA 1 5 



no photosensitizing effect on the Mg + ^-activated ATPase, in contrast to 

 the flavin antagonists, atebrin and chloropromazine. 



From the resuhs presented above it would thus appear that the effect 

 of desaminothvroxine, and probably of thyroxine analogues in general, 

 differs in its mechanism from those of other known inhibitors of the two 

 types of liver mitochondrial ATPase reactions. 



(b) exchange REACTIONS 



P-A TP exchange 



The effect of desaminothyroxine on the P—ATP exchange reaction of 

 liver mitochondria was investigated using the conditions previously 

 established in this laboratory [85]. Under these conditions the ATP and 

 P; concentration are virtually constant during the measurement, and the 

 exchange rate is about 0-35 /xmole/min. per mitochondria from 200 mg. 

 wet-weight liver. As shown in Table II, the P— ATP exchange reaction 



TABLE II 



Inhibition of the Pj-ATP Exchange Reaction of Rat Liver Mitochondria 



BY Desaminothyroxine 



Assay conditions as in [85], except that 50, rather than 20, /xmoles P, were 

 added per sample. 



Desamino- Per cent ^-P in ATP after 

 thyroxine — 



M 3 min. 13 mins. 



o 3-91 12-05 



ID** 3-92 11-30 



10-^ 1-89 . 2-74 



IO-* 1-25 0-72 



was strongly inhibited by desaminothyroxine, 10^^ m giving an almost 

 complete inhibition. Again, the inhibition was present from the onset of 

 the incubation, the preincubation with desaminothyroxine prior to the 

 addition of Pj and ATP had no influence on its extent. 



ATP~ADP exchange 



Wadkins and Lehninger [86] described recently the occurrence of an 

 exchange reaction between ATP and ADP in phosphorylating digitonin- 

 preparations. This reaction was characterized by a sensitivity to dinitro- 

 phenol which was lost, without loss of the exchange activity, when the 

 preparations were damaged so as to lose their phosphorylating capacity. 

 Azide, also, although not inhibitory to the ATP-ADP exchange reaction. 



