24 OLOV LINDBERG et al. 



a consistent manner and it would appear that it occurs only in a very 

 narrow range of thyroxine /mitochondrial protein ratio (cf. Expt. 2a). 



Thus the situation especially as far as thyroxine is concerned seems to 

 be very complicated indeed. This is further emphasized by the recent 

 findings of Bronk [104] and of Dallam et al. [105, 106] that in their systems 

 thyroxine was even able to cause an increase of the phosphate uptake 

 coupled to the oxidation of jS-hydroxybutyrate. 



3. Concluding remarks 



Evidence has been presented above that thyroxine analogues inhibit 

 in a consistent and instantaneous manner the P^-ATP exchange and 

 dinitrophenol-induced ATPase reactions taking place in intact liver mito- 

 chondria, as well as the Mg ^ -activated ATPase and ATP-ADP exchange 

 reactions observed in mitochondrial fragments. A common denominator 

 of all these reactions is that they are considered to include one or several 

 steps of the reaction sequence involved in phosphorylation coupled to 

 electron transport. The succinate-linked reduction of mitochondrial DPN, 

 a process which is also considered to involve a partial reaction of electron- 

 transport-coupled phosphorylation, has recently been reported by Chance 

 and Hollunger [107] to be highly sensitive to thyroxine. 



It has been concluded from previous work in this laboratory [76-78, 

 85] that the mitochondrial P--ATP exchange and ATPase reactions 

 reflect predominantly only one of the three phosphorylations occurring 

 along the respiratory chain, that located in the DPN-flavin region. Also 

 the succinate-linked reduction of mitochondrial DPN is thought to involve 

 primarily a partial reversal of this phosphorylation [51, 53, 107, 108]. It 

 would therefore seem that the observed instantaneous effects of thyroxine 

 and related compounds concern primarily the DPN-flavin-coupled phos- 

 phorylation. The finding that these compounds inhibited the diaphorase 

 component of the amytal- and antimycin A-sensitive DPNH oxidase 

 system, is consistent with this conclusion, and may indicate that the 

 effect of thyroxine and related compounds on the flavin-linked phos- 

 phorylation consists of a direct action on this enzyme. The DPNH- 

 cytochrome c reductase, which is insensitive to amytal and antimycin A, 

 and which probably represents a non-phosphorylating pathway of electron 

 transport [38, 39, 47, 91-95], was only marginally inhibited by the com- 

 pounds studied. The significance of the observed inhibition of the DT 

 diaphorase cannot be understood as yet, since the role of this enzyme in 

 mitochondria is unclear (cf. [109-112]). 



The effect of thyroxine and related compounds was clearly less re- 

 producible on the integrated processes of respiration and phosphorylation 

 than it was when studied with the above component reactions as test 



