102 Britton Chance 



sevenfold (Chance, 1956). Proof that the deceleration of 

 respiration is due to the restitution of respiratory control and 

 not to an unknown inhibition of electron transfer is demons- 

 trated by the fact that ADP addition gives an even more 

 rapid respiratory rate (a 14-fold increase). 



To determine the magnesium concentration needed for 



aerated 



medium |> 400^M ADP 



6mM \ \\ 



^[0^] =0^ 



Fig. 8. An illustration of the increased 

 respiratory control occasioned by the addi- 

 tion of magnesium to rat liver mitochondria 

 suspended in sucrose-EDTA-phosphate med- 

 ium. Mw= mitochondrial suspension. (Expt. 

 no. 474b) (From Chance, 1956. Reproduced 

 by permission of the Editors, Proc. Ill int. 

 Congr. Biochem.) 



restoration of half-maximal respiratory control in the suspen- 

 sion described above, we have plotted, in Fig. 9, the respira- 

 tion rate for a number of magnesium concentrations. As 

 shown in Fig. 8, the addition of 6 mM magnesium causes a 

 maximal deceleration of respiration of more than sevenfold. 

 Half-maximal effect is obtained with roughly 0-5 mM mag- 

 nesium. Thus, inhibition of respiratory activity can be 

 obtained by additions of low concentrations of magnesium to 

 the mitochondria. Cyclic responses of respiratory activity 

 have not been demonstrated for increases and decreases of 

 magnesium concentration, as they have for ADP (Fig. 4). 

 However, it is reasonable to assume that such responses would 



