106 Britton Chance 



sucrose-EDTA-phosphate medium containing the usual 

 amount of magnesium, the mitochondria are quite resistant 

 to calcium. Maximal respiration would therefore appear to 

 require 2 niM calcium, half-maximal effects being obtained at 

 about • 5 mM calcium. 



Respiratory control and oxidative phosphorylation 



Experimental data show that loss of respiratory control is 

 not necessarily accompanied by a significant decrease of 

 oxidative phosphorylation efficiency. Even the earliest 

 studies of phosphorylation efficiency showed that reasonably 

 high P/0 values could be obtained in preparations demonstrat- 

 ing no measureable change of respiratory rate in the presence 

 or absence of phosphate acceptor. Although it is also generally 

 observed that mitochondrial preparations exhibiting a high 

 degree of respiratory control (Lardy and Wellman, 1952) 

 show a high phosphorylation efficiency, recent experiments 

 indicate that the two phenomena are readily dissociable in 

 intact mitochondria. Hoch and Lipmann (1954) have shown 

 that the effect of phosphate acceptor is almost abolished 

 although the P/0 ratio of the same preparation does not 

 decrease. In a preparation whose respiratory control ratio, in 

 the absence of dinitrophenol (DNP), is approximately 7:1, 

 10"^ DNP causes only a 23 per cent decrease of the P/0 ratio 

 and a 72 per cent drop in the acceptor effect. Chance and 

 Williams (1955a) have noted similar phenomena using butyl 

 3 : 5-di-iodo-4-hydroxybenzoate (DIB). For a preparation which 

 in the absence of the uncoupling agent, shows a respiratory 

 control ratio of 7 : 1 and a P/0 value of 3 • 0, the addition of 

 the uncoupling agent causes the respiratory control ratio to 

 fall to 1 • 41 while the P/0 value remains essentially unchanged 

 (3*1). Further experiments on the magnesium-deficient 

 system studied by H. Baltscheffsky (1957) show that lack of 

 magnesium can cause a similar loss of respiratory control 

 under conditions which do not significantly affect the P/0 

 value. It would appear that the "R factor" discussed by 



