190 E. R. REDFEARN 



21. Chance, B., " Ciba Foundation Symposiuni on Quinones in Electron Trans- 

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 London, 1961. 



22. Hatefi, Y., Biochim. biophys. Acta 31, 502 (1959). 



23. Harrison, K., Nature, Loud. 181, 1131 (1958). 



24. Clark, V. M., Kirby, G. W., and Todd, A., Nature, Land., 181, 1650 (1958). 



25. Chmielewska, I., Biocliim. biophys. Acta 39, 170 (i960). 



26. Chance, B., and Williams, G. R., J', biol. Chetn., 217, 395 (1955). 



27. Green, D. E., Jarnefelt, J., and Tisdale, H. D., Biochim. biophys. Acta 31, 34 

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Discussion 



ZiEGLER : I would like to point out that it would be difficult to measure the 

 initial rates of endogenous Q reduction by the method Dr. Redfearn used. The 

 particles contain a large excess of Q relative to the cytochromes or flavoprotein 

 and only part of it may be rapidly reduced in a blocked system. In order to measure 

 initial rates at 22' the reaction would have to be stopped in less than a second and 

 I believe your reaction times were of the order of several seconds. Later in this 

 Symposium we will present data which show that the rate of reduction of exogenous 

 Q is fully compatible with the assumption that it functions as an electron carrier 

 between the flavoprotein and cytochrome c. The turnover of the flavoprotein with 

 Q as the acceptor is more rapid than it is in the intact particle with oxygen as the 

 acceptor. 



Redfe.'VRN : This is a derivative particle that you are using ? 



Ziegler: The naturally occurring quinone, Qjo, is reduced as rapidly as the 

 synthetic Q homologues. 



Redfearn : We have measured the rate of reduction of exogenous ubiquinone 

 in our preparation, and found the rate to be very much slower than that of the 

 reduction of the endogenous material. 



Ziegler: You do require lipids. Coenzyme Qj,, is extremely insoluble in water, 

 and by adding a mixture of phospholipids you can increase its effective concentra- 

 tion to the point where you can use it as the final electron acceptor. 



Redfearn : I would like to add that in experiments we did with Dr. Chance we 

 measured the rate of reduction of endogenous ubiquinone in the double beam 

 spectrophotometer at the same time as we measured the rate by the extraction 

 procedure, and we got very close agreement. 



Chance : We have been interested in the maximal rate at which the endogenous 

 Q could be reduced. I think that Dr. Redfearn and I had already observed rates 

 at 22' of about 5 micromoles per hour per milligram protein for the reduction of 

 endogenous Q on adding succinate to the CN-inhibited system. By using a rapid 

 flow apparatus and more active preparations we have observed values of Q-reduc- 

 tase activity up to this level, which is a rather high activity, but this is still some- 

 what less than the rates of oxygen reduction. 



Singer: I was wondering, since you did not commit yourself, which of the 

 three possible hypotheses of the mode of action you favour, and whether the sum 

 total of the data presented today plus those you published in the Biochemical 

 Journal do not point to a possible function of Q in interchain electron transport ? 



