592 Discussion 



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DISCUSSION 



The Crossover Theorem and Sites of Oxidative Phosphorylation 



Chance : Application of the crossover theorem to the respiratory chain indicates that there 

 are three inhibitory interactions 



i i i 



DPNH-^ fp-^ 6-^ Ci-^ c-^ a-^ Qg — O2 



This result immediately places considerable constraint on acceptable hypotheses for 

 oxidative phosphorylation, since chemical equations for the relief of inhibition of 

 electron transfer caused by adding AD? must involve interaction with these couples. 



DPNH + fp -^ DPN + rfp (1) 



b++ + Ci+++ -^ b+++ + ci++ (2) 



C++ + fl+++ -^ C+++ + a++ (3) 



From Slater's paper, the couples involved are DPN and oxidized cytochrome b in 

 his reaction (5), 



DPN ~ IH + 26+++ - I2 ^ DPN+ + 2b++ - Ij (5) 



and b and c, in his reaction (9), 



*+++ - I2 + C+++ ^ 6+++ - I2+ + C++ (9) 



Although the crossover data do not eliminate the possibility of such interactions, it 

 seems only reasonable to consider first the interactions for which experimental evidence 

 is available. 



The Oxidation-reduction Potential of Cytochrome b 



George: Have the various determinations of E^' for cytochrome b all been made under 

 the same conditions of temperature, ionic strength and pH ? These factors influence 

 the magnitude of oxidation-reduction potentials, and small differences might be partly 

 if not wholly responsible for the discrepancies in the values. 



Slater: The main reason for the discrepancy between Ball's (1938) value of —40 mV at 

 pH 7-4 and the mean value of +77 mV obtained by Holton and Colpa-Boonstra at pH 

 70, appears to be that Ball based his oxidation-reduction calculation on the assumption 

 that all the cytochrome b reduced by Na2S204 is functional in the succinate oxidase 

 system. Chance has shown that this is not the case and we have confirmed this. The 

 observations of Holton and Colpa-Boonstra showed that if no fumarate was added 

 cytochrome b was about 98 % reduced, and not 75 % as estimated by Ball. Taking this 



