Cytochrome b in the Respiratory Chain 579 



m//, which Chance ascribes to a cytochrome /j-like pigment which is not 

 reduced by succinate, even in the presence of antimycin. 



OXIDATION-REDUCTION POTENTIAL OF CYTOCHROME b 



Ball (1938) calculated the oxidation-reduction potential of cytochrome b 

 from measurements with the visual spectroscope of the proportion of the 

 total cytochrome b (measured with Na2S204) which was reduced in the 

 presence of mixtures of succinate and fumarate. He found a value of —40 mV 

 at pH 7-4. 



Slater (1953) drew attention to the fact that in Chance's (1952) first experi- 

 ments, the [fumarate]/ [succinate] ratio at the end of the experiment was 0-15, 

 which would be expected to give only 37 % reduction of the cytochrome b, 

 if Ball's (1938) value for the oxidation-reduction potential of cytochrome b 

 were correct. Since it was clear from Chance's data that the reduction had 

 proceeded much further than this. Slater (1953) suggested that Ball's value 

 might have to be raised (see Note 1). 



Hill (1954) reported unpublished experiments suggesting that the oxidation- 

 reduction potential of cytochrome b was in the region of zero. 



The potential has been recently re-measured by Colpa-Boonstra and Holton 

 (1959) by determination of the equilibrium between cytochrome b and 

 succinate/fumarate in heart-muscle preparations from horse and pig. 

 Absorbancy changes were measured spectrophotometrically in Holton's 

 (1957) instrument at 434 m/* and 558 m//, the isosbestic points of the other 

 cytochromes taken together (see above). Interference by myoglobin was 

 eliminated by treating the particles with nitrite. 



The results were calculated by rearranging the equation for the equilibrium 

 constant 



[fumarate] [b^f 

 "~ [succinate] ' [^+-H-]2 



to give 



1 _ 1 1 /[fumarate] 



VI 



[b++] [b] j^i^^y^ f^ ^ [sMCcm^iQ] 



where [/?++] and [^+++] are the concentrations of ferro- and ferricytochrome b 

 respectively, and 



[b] = [b++] + [b+++]. 



Values of K were calculated from the straight line (see Note 2) obtained by 

 plotting 1/A^ against A^^'^^rate] ^ where AA refers to the observed 



absorbancy change associated with the reduction of cytochrome b from its 

 oxidized state to its anaerobic equilibrium with succinate/fumarate. This 



