Cytochrome b in the Respiratory Chain 585 



with the possible relationship between the antiraycin-sensitive site in the 

 respiratory chain, and the 2:3-dimercaptopropanol (BAL)-sensitive site 

 (Slater, 1949). In the first place, it was found that treatment of the heart- 

 muscle preparation with BAL, in the presence of air, caused the destruction 

 of cytochrome b' , and therefore abolished the extra absorption obtained by 

 the addition of antimycin, in the presence of succinate. However, it is not 

 yet known whether this destruction of cytochrome b' is related to the inhi- 

 bition of the respiratory chain brought about by incubation with BAL, or is 

 a side reaction. 



The second observation was that, although cytochrome b was readily 

 reduced by succinate after BAL treatment, and by succinate or DPNH after 

 antimycin treatment, the addition of antimycin to a BAL-treated preparation 

 prevented the reduction of cytochrome b by succinate or DPNH. This 

 observation supports other indications that, although the sites of action of 

 antimycin and of BAL are closely connected, they are not identical. 



POSSIBLE MECHANISM OF ACTION OF CYTOCHROME b 



IN THE RESPIRATORY CHAIN 



To conclude this paper, we should like to suggest a mechanism, which is 



treated in greater detail elsewhere (Colpa-Boonstra, 1959), for the function 



of cytochrome b in the respiratory chain, which is based on the following data: 



(1) In phosphorylating preparations of the respiratory chain, cytochrome b 

 is rapidly reduced by added succinate or by endogenous DPNH. 



(2) In non-phosphorylating preparations, cytochrome b is rapidly reduced 

 by succinate, or by high concentrations of DPNH, but only slowly by low 

 concentrations of DPNH. 



(3) Cyanide inhibits the reduction of cytochrome b by succinate without 

 affecting the reduction of the other cytochromes. 



(4) In phosphorylating preparations, it is probable that one molecule of 

 adenosine triphosphate (ATP) is synthesized for each pair of hydrogen atoms 

 (or electrons) transferred from DPNH to cytochrome b, and also one for each 

 pair of electrons transferred from cytochrome b to cytochrome c. 



The fact that phosphorylation is coupled with the respiratory chain shows 

 that the latter is much more complicated than merely a succession of hydro- 

 gen- or electron-transferring reactions. Moreover, we think that it is im- 

 portant to keep in mind that although the oxidation of the iron atom of 

 cytochrome b by the iron atom of cytochrome c is a single-electron reaction, 

 one molecule of ATP is synthesized for each two electrons transferred. 



For some time now, we have been formulating oxidative phosphorylation 

 by the type of mechanism shown in reactions (l)-(3b) 



i AHa -f B -}- I ^ A -- I + BHg (1) 



A^I-f-X^X^I-fA (2) 



