Cylochronie b in the Respiratory Chain 591 



The chain for succinate oxidation in both phosphorylation and non- 

 phosphorylating preparations is rather similar to the phosphorylating DPNH 

 chain, but it cannot be adequately described in the shorthand. 



Although the proposed mechanisms are certainly not the only ones which 

 fit the observations, and the details arc in many cases highly speculative, 

 we should like to emphasize that it is our view that the true mechanisms are 

 not less complicated than the ones proposed here. 



1. At the same time. Slater (1953) suggested that the back reaction (oxidation of reduced 

 cytochrome b by fumarate) could not be ignored in any kinetic treatment of the reduction 

 of cytochrome b by succinate, under these conditions. Chance (1958) has recently calculated 

 from his new data that the back reaction was negligible in his earlier experiments. While we 

 agree that this is a valid conclusion to be drawn from Chance's recent measurements, it is 

 necessary to point out that Chance's calculations are based on a misreading of Slater's (1953) 

 comment and include an arithmetical mistake of a factor of 100. He has calculated 

 the ratio of the rates of reduction and oxidation of cytochrome b for the case where 

 [succinate]/[fumarate] = 015, instead of [fumarate]/[succinate] = 0-15, which had been 

 calculated by Slater from Chance's (1952) experiments. Under the conditions chosen by 

 Chance (1958), rate of reduction/rate of oxidation = 10 x 0-15 = 1-5, not 150 given by 



Chance. For the conditions calculated by Slater (1953), the ratio of the rates = — — = 67, 

 which is indeed strongly in favour of reduction. ""'^ 



2. The fact that the results plotted in this way fall on a straight line shows that one 

 molecule of succinate reduces two molecules of ferricytochrome b, under these conditions. 



3. Indeed, it seems not unlikely that the oxidation of DPNH in a normal heart-muscle 

 preparation proceeds partly along this pathway involving reduction of cytochrome b, and 

 partly by the sequence of reactions (4)-(16) shown above. We have observed that 

 cytochi-ome b is reduced more rapidly and completely in those preparations which have the 

 highest DPNH oxidase activity. 



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