MITOCHONDRIAL LIPIDS AND THEIR FUNCTIONS IN THE RESPIRATORY CHAIN 191 



In weighing the evidence it is well to remember that what matters is not that under 

 certain sets of conditions the rate of cycling of ubiquinone approaches that of the 

 respiratory chain, but rather that it is relatively easy to produce conditions under 

 which the turnover of ubiquinone is lower than the rate of succinate oxidation. 

 The latter type of experiment would not suggest that ubiquinone is an obligatory 

 component of the electron transport chain, but it would by no means exclude its 

 function as an interchain lipid. This function would, of course, also lead to a reduc- 

 tion of cytochromes c and c-^ but not necessarily in the same chain. 



Redfearx : I, of course, rather favour this idea and I discussed it at some length 

 in a recent paper, but I didn't want to commit myself here. I think this possibility 

 fits the results well but we can't exclude the other possibilities. 



EsTABROOK : I have a question of terminology on your very interesting observa- 

 tion. You lose only 20",, of the succinate oxidase activity on removing 96 "o of what 

 you said was extractable Qi,,. Is this total ubiquinone or that extracted bv vour 

 solvent system ? Was there still over 10",, remaining ? 



Redfearn : Yes. 



Williams: I wanted to ask whether it would be a logical consequence of Dr. 

 Singer's hypothesis, that you should be able to isolate chains which do not contain 

 ubiquinone ? 



Redfearx: Yes, that is what we are trying to do. We find it very difficult to 

 remove the remaining few per cent of ubiquinone. As you continue extracting you 

 remove more and more phospholipids and other structural lipids and then you 

 start losing activity. It is very difficult to remove ioo"„ of the ubiquinone without 

 removing other lipids. 



