EFFECTS OF THYROXINE AND RELATED COMPOUNDS ON LIVER MITOCHONDRIA 7 



be described, which differ from those outhned above in being both 

 consistent and instantaneous. Furthermore, some of these effects can also 

 be demonstrated in mitochondrial fragments and even at the level of the 

 purified enzvme. 



The reactions studied can be divided into two categories. The first 

 category involves the mitochondrial ATPase reactions (both the dinitro- 

 phenol- and the Mg + ^-activated ATPases), the P-ATP exchange 

 reaction, and an ATP-ADP exchange reaction catalyzed by certain 

 mitochondrial subfractions. In the second category belong certain flavin- 

 catalyzed electron-transfer reactions, such as the DPNH diaphorase, 

 DPNH-cytochrome c reductase, and the DPNH oxidase reactions, as well 

 as a second diaphorase reaction, which is non-specific with respect to 

 pyridine nucleotides. From the data presented the conclusion is derived 

 that thyroxine and related compounds inhibit, in a consistent and in- 

 stantaneous manner, reactions which involve a part or the whole of the 

 fiavin-linked respiratory chain phosphorylation. Some implications of 

 these results as to the mode of action of thyroxine analogues on mito- 

 chondria in vitro will be discussed. 



(a) ATPase reactions 



Agents which uncouple oxidative phosphorylation in li\er mito- 

 chondria usually also evoke an increased ATPase activity [64-68]. Two 

 types of ATPase reactions may be distinguished. One is elicited by 

 dinitrophenol and related uncoupling agents. This ATPase reaction 

 occurs in structurally intact mitochondria and requires no addition of 

 Mg ^ ^ to exhibit maximal activity. Another type of liver-mitochondrial 

 ATPase appears when the structure of the mitochondria is damaged by 

 physical or chemical means, so as to disrupt the obligatorv coupling 

 between respiration and phosphorylation. This ATPase reaction is 

 strictly dependent on added Mg ^ . According to a widely held opinion 

 [65, 67, 69-78] one or both of the ATPase reactions reflect, in a modified 

 form, a part of the reaction sequence involved in oxidative phosphorylation. 



Early considerations that thyroxine and related compounds may act as 

 uncouplers of oxidative phosphorylation were paralleled by the assump- 

 tion that these agents would also evoke a high mitochondrial ATPase 

 activity. Data presented by Lardy and Maley [10] and by Malev [79] 

 showed that this was the case, even though the ATPase activity appearing 

 in rat liver mitochondria in the presence of thyroxine was relatively low 

 as compared to that induced by dinitrophenol. While recently confirm- 

 ing these data in our laboratory, the rather unexpected finding was made 

 that certain thyroxine analogues markedly inhibited the ATPase activities 

 of rat liver mitochondria, both that induced by dinitrophenol and that 

 elicited by destruction of the mitochondrial structure. 



