Control of Oxygen Utilization 101 



and further oxidation in response to further additions of 

 phosphate. It would, therefore, appear that phosphate affinity 

 is considerably lower than ADP affinity. The results of these 

 studies are summarized and compared in Fig. 7 (Chance and 

 Connelly, 1957), where pyridine nucleotide shows a half- 

 maximal response at 56 (xm ADP and 1 mM phosphate. These 

 constants determine the response of the mitochondria of the 

 intact cell to metabolic changes involving concentration 

 changes of ADP and phosphate. ADP is the more effective: 

 the mitochondria respond to much smaller changes of its con- 

 centration, and a low concentration of ADP will be more 

 rapidly expended and brought down to a low final concentra- 

 tion. The response to phosphate will not occur until a high 

 concentration has accumulated, and will be prolonged due to 

 both the large amount and the slow utilization of concentra- 

 tions below 1 mM. An increment of phosphate that would 

 stimulate half-maximal respiration would cause an interval of 

 respiration 20 times greater than one stimulated by an increase 

 of ADP concentration. In other words, the response to phos- 

 phate will be sluggish, and the response to ADP will be rapid. 



Effect of magnesium 



It has been recognized for some time that maximal phos- 

 phorylation efficiency of isolated mitochondria may require 

 added magnesium (Cross et al., 1949). More recently, however, 

 it has been shown that mitochondria may initially contain 

 sufficient magnesium to permit a high phosphorylation 

 efficiency, though one of short duration (Baltscheffsky, 1956). 

 It has also been found that either endogenous or added 

 magnesium also increases the respiratory control ratio (Chance, 

 1956). An example of the effect of magnesium upon the 

 respiratory control of rat liver mitochondria is given in 

 Fig. 8. In this case, mitochondria suspended in a sucrose- 

 EDTA-phosphate medium show respiratory stimulation upon 

 addition of succinate. After about a minute, 6 mM magnesium 

 causes an immediate reduction of respiratory rate of about 



