EFFECTS OF THYROXINE AND RELATED COMPOUNDS ON LIVER MITOCHONDRIA 29 



112. Conover, T. E., this volume. 



113. Boyer, P. D., Falcone, .A. B., and Harrison, W. H., Xature, Lniid. 174, 401 



(1954)- 



114. Bover, P. D., Luchsinger, \V. E., and Falcone, A. B.,jf. biol. Cheni. 223, 405 



(1956). 



115. Ernster, L., Ljunggren, M., and Lindberg, O., Acta chem. scaiid. 8, 658 



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116. Tapley, D. F., and Basso, N., Bioc/iim. biophys. Acta 36, 486 (i960). 



117. Grabe, B., Biochifn. biophys. Acta 30, 560 (1958). 



iiS. Lindberg, O., Cirabe, B., Low, H., Siekevitz, P., and Ernster, L., Acta 

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Discussion 



Hess : I would like to mention earlier experiments regarding the protecting 

 activity of thyroxine on mitochondrial function. Martius and I found that with 

 small concentrations of thyroxine (lo"'^ m) the phosphorylating activity of rat liver 

 mitochondria was conserved over a period of 16 hr. in the cold in comparison to the 

 control which lost an appreciable amount of activity. This effect could be due to an 

 inhibition of a latent ATP-ase activity, which is activated by ageing or chemical 

 actions like DXP or Mg"^ (Biocheni. Z. 326, 191 (1955)). On the other hand, I 

 am wondering about the physiological significance of the action of thyroxine or 

 triiodothyronine on the mitochondrial ATP-ase, because we found that triiodo- 

 thyronine acts on the cytochrome region of oxidative phosphorylation in digitonin 

 particles of rat liver mitochondria. The particles were prepared free from ATP-ase 

 and myokinase and no ATP-ase activity was released by the addition of the 

 hormone. The synthesis of ^'-P-ATP in the presence of ascorbate, cytochrome c, 

 ADP and inorganic phosphate, being activated (max. 35 "o) by triiodothyronine in 

 the concentration range of 5 x io~^ M, responds with half maximal inhibition to the 

 presence of 5 x io~® M triiodothyronine (unpublished experiments). Perhaps, in the 

 light of the data of Prof. Lindberg and ourselves it seems that there can be a 

 common site of hormonal action upon various components of the respiratory chain, 

 whose elucidation depends upon the experimental conditions. I am wondering 

 whether you have data which shed light on the vertebral level of the hormonal 

 action. Is there a phosphate requirement in your diaphorase experiments ? 



Lindberg: No. There is no phosphate requirement. 



Ch.ance : I was very glad to hear Prof. Lindberg mention that the effect of 

 succinate on DPN reduction might be involved in thyroxin action. Hollunger and 

 I did investigate how much thyroxine would be required to inhibit half maximally 

 the effect of succinate on DPN reduction and found that in the absence of magne- 

 sium and without an incubation period of over a minute, that less than 10 •* M 

 thyroxine gave half maximal inhibition with a protein concentration of about one 

 mg. per ml., so it is extremely sensitive to thyroxine. I don't know whether this is 

 the primary site of thyroxine action. 



Lindberg : It is our feeling that it is difficult to reproduce experiments with 

 thyroid compounds on respiration and phosphorylation, but the closer you come to 

 a certain region, the diaphorase region, in the respiratory chain the easier it is to 

 get a good, consistent result. 



