Discussion 127 



control is present in the whole cell, but the extent of it is largely un- 

 decided. 



Chance : One of the specific objects of our experiments was an attempt 

 to determine directly whether there was a metabolic state which would 

 demonstrate the extent of respiratory control of the living cell. With 

 yeast, it was clear that we were not able to demonstrate anything better 

 than a 50 per cent change of respiration from activity to rest. With the 

 ascites cell we were able to get ratios of 20-fold or more for cells about 30 

 seconds after their removal from the mouse. I do not think that anyone 

 would question that a muscle will give a ratio of at least 50, perhaps 

 more in insect flight muscle. The thesis that a mitochondrion in a living 

 cell can exhibit a very high ratio of respiratory rates in the presence of 

 phosphate acceptor or phosphate compared to that in the absence of 

 either of these substances, is probably a valid one. One has to consider 

 that perhaps the environment of the mitochondrion within the cell is 

 much more favourable than we can obtain in our polarograph or in the 

 Warburg manometer. 



Mcllwain: Prof. Chance reported, at the beginning of his talk, the 

 concentrations of reactants to which respiratory processes were sensi- 

 tive. He pointed out that the processes were sensitive to ADP levels of 

 the order of 20 [xm, whereas with inorganic phosphate, sensitivity of the 

 respiratory processes was in a different concentration range of some mM. 

 It might appear from this that the two controlling factors came into 

 operation at different stages of breakdown of ATP. I would like to 

 point out the way in which the presence of the enzyme creatine phos- 

 phokinase actually operates to bring these two reactants into action 

 at the same time, because of the nature of the equilibrium at the kinase : 



CrP + ADP ;f^ Cr + ATP 



The reaction normally goes almost entirely to the right. This gives an 

 explanation of how, in neural or muscular tissues, there can be consider- 

 able breakdown of ATP and yet concentrations of ADP remain of the 

 order of 10-20 [jlm: for the tissue contains creatine phosphate concen- 

 trations of perhaps 2-3 mM. This means that one can have a breakdown 

 of creatine phosphate through ATP to inorganic phosphate, involving 

 quantities of the order of 1 m-mole of creatine phosphate per g. of 

 tissue, while the change of ADP concentration is of the order of a few 

 [i,M, which would still leave the systems operating at the normal cell 

 levels of inorganic phosphate, creatine phosphate and ADP. (Incident- 

 ally, the balance in production and utilization of labile phosphate in 

 tissues containing the kinase is reflected largely in their level of creatine 

 phosphate, which is perhaps 100 times^ that of ADP : a point which has 

 analytical conveniences.) 



Gutfreund: Prof. Chance, do you not consider that the half-optimum 

 concentrations of ATP, ADP, phosphate, magnesium and calcium are 

 not independent of one another? If the concentration of available ATP 

 is controlled by the magnesium concentration, then it is possible to get 

 half-optimum Mg^ + concentration at a fixed concentration of all 



