STABLE STRUCTURAL STATES OF RAT HEART MITOCHONDRL^ 99 



additions of small amounts of ADP to expanded mitochondria with mag- 

 nesium present. A succession of stable structural states is established with 

 increasing concentrations of ADP until an equilibrium contracted con- 

 dition is reached. 



Parallel measurements of respiration rates carried out at the same time 

 as the spectrophotometric observations with the same preparation of 

 mitochondria under identical experimental conditions showed that the 

 mitochondria in their expanded condition exhibited respiratory control [6] 

 with ADP temporarily accelerating their respiration in a characteristic way. 

 This fact suggested a second question. 2. Is the influence of ADP concen- 

 tration on structural change quantitatively similar to its effect on respiration 

 rate ? 



We have found that the two relationships are quantitatively very 

 similar. The results given in Fig. 2 are repeated in a different form in 

 Fig. 3 in order to show the relation between the total concentration of 

 added ADP and the total change of extinction caused by its addition. 

 Half-maximal shrinkage was effected at a concentration of 18 fiM ADP. 

 Respiration measurements show that half-maximal acceleration of respira- 

 tion is brought about at approximately the same concentration of ADP 

 (e.g. 26 juM for pigeon breast muscle mitochondria [7]). 



The hypothesis that the respiration rate of mitochondria in vivo is 

 controlled by the concentration of ADP in the cytoplasm has been well 

 ventilated recently [8]. Both the work of Packer and his associates and the 

 above experiments emphasize that reversible alterations in mitochondrial 

 structure probablv represent a feature of the same control mechanism. 

 They provide evidence supporting the suggestion of Ernster [9] that "a 

 reversible labilization of the mitochondrial structure may constitute a 

 physiological principle of metabolic control." 



References 



1. Packer, L. Ann. N.Y. Acad. Sci. 72, 518 (1959)- 



2. Packer, L. 7- biol. Chem. 235, 242 (i960). 



3. Beechey, R. B., and Holton, F. A., Biochein.J. 73, 29P (i959)- 



4. Chance, B., and Williams, G. R., J. biol. Chem. 217, 409 (i955)- 



5. Holton, F. A., Hiilsmann, W. C, Myers, D. K., and Slater, E. C, Biuchem.J. 



67. 579 (1957)- 



6. Chance, B., in " Ciba Foundation Symposium on the Regulation of Cell 

 Metabolism", ed. G. E. W. Wolstenholme, C. M. O'Connor, London, 

 Churchill, 91 (1959). 



7. Chance, B. (i960), personal communication. 



8. Wolstenholme, G. E. W., and O'Connor, C. M., " Ciba Foundation Symposium 

 on the Regulation of Cell Metabolism". London, Churchill (i959)- 



9. Ernster, L., Exp. Cell. Res. lO, 721 (1956). 

 10. Holton, F. A., Biochem.J. 66, 37P (1957)- 



