8 OLOV LINDBERG et al. 



Inhibition of dinitrophenol- and Mg + ^-activated A TPases by thyroxine 



and rehited compounds 



In Fig. I, the effects of thyroxine, triiodothyronine and desamino- 

 thyroxine on the dinitrophenol induced ATPase of rat Hver mitochondria 

 are illustrated. Of the three compounds, desaminothyroxine exhibited 

 the strongest inhibition, giving half-inhibition at a concentration of 

 about 0-02 mM. The effect of the same compounds on the Mg + +-acti- 

 vated ATPase is shown in Fig. 2. For the study of this reaction a 

 preparation of mitochondrial fragments, obtained after disruption of 

 mitochondria with a rapidly rotating Super-Thurrax blender was used. 



mM 



Fig. I . Effect of L-thyroxine, DL-triiodothyronine and desaminothyroxine on 

 the dinitrophenol-induced ATPase activity of rat liver mitochondria. For 

 experimental details see [76]. 



The procedure was adapted from Kielley and Kielley [80] who devised it 

 for enriching mitochondrial ATPase free from adenylate kinase. As can 

 be seen in Fig. 2, the ATPase activity of the Kielley and Kielley prepara- 

 tion was also inhibited by the three compounds tested, and again, desami- 

 nothyroxine exhibited the strongest inhibition. The half inhibitory con- 

 centration of desaminothyroxine was roughly the same as in the case of 

 the dinitrophenol-induced ATPase. However, in contrast to this latter 

 reaction, the inhibitions given by the triodothyronine and thyroxine were 

 not progressive with concentration, but levelled off at about • i mM to 



