576 E. C. Slater and J. P. Colpa-Boonstra 



showed that it was possible to inhibit succinate dehydrogenase (by incubation 

 with cyanide), without decreasing the amount of cytochrome b reduced, 

 clearly showing that the two were not identical, while at about the same time 

 Morton (1950) obtained an active succinate dehydrogenase in a solution free 

 from haematin compounds. Further purification of the dehydrogenase, 

 particularly by Singer and Kearney (1954) and Wang, Tsou and Wang (1956), 

 has shown quite conclusively that it does not contain a haematin. 



Early observations with the microspectroscope indicated that when suc- 

 cinate was added to a respiratory-chain preparation (e.g. the Keilin and 

 Hartree heart-muscle preparation) saturated with air, the b band rapidly 

 appeared as soon as oxygen was exhausted from the suspension (about 2 sec). 

 With the eye, cytochrome b in active preparations was reduced as rapidly as 

 cytochromes a and c, and there was no reason to suspect that it did not lie 

 on the main pathway between succinate and oxygen. The same result was 

 obtained when succinate was added to the preparation in the absence of 

 oxygen, or in the presence of oxygen and cyanide. On the other hand, 

 DPNH, under these conditions, reduced cytochrome b so much more slowly 

 that it was concluded that cytochrome b was on a side path for the oxidation 

 of DPNH (Slater, 1950). 



In 1952, Chance began his important measurements of the rate of reduction 

 of the components of the respiratory chain, first in the Keilin and Hartree 

 heart-muscle preparation, later in mitochondria. His conclusions concerning 

 the role of cytochrome b in the respiratory chain were : 



(1) In heart-muscle preparations, cytochrome b is reduced by succinate at 

 a rate which is inconsistent with a place for this cytochrome in the main 

 pathway (Chance, 1952). 



(2) In intact yeast cells (Chance, 1954), or in isolated rat-liver mito- 

 chondria (Chance, 1955), cytochrome b is reduced rapidly not only by 

 succinate, but also by diphosphopyridine nucleotide (DPN)-requiring sub- 

 strates. Chance and Williams (1955) concluded that when oxidative phos- 

 phorylation takes place, cytochrome b is in the main pathway for both 

 substrates, and that the non-phosphorylating systems are artefacts, in 

 which cytochrome b has lost its capacity to participate in the respiratory 

 chain. 



(3) The addition of antimycin (or 2-n-heptyl-4-hydroxyquinoline N-oxide) 

 to a heart-muscle preparation causes an increase in the rate of reduction of 

 cytochrome b by DPNH (Jackson and Lightbown, 1958) and by succinate 

 (Chance, 1958), and in the amount of cytochrome b (or similar pigment) 

 reducible by succinate (Chance, 1958). Chance concluded that antimycin 

 alters the system in such a way that the electrons that flow to cytochrome c^ 

 in its absence are routed to cytochrome b in its presence. 



The remainder of this paper will be devoted to a description of our recent 

 experiments relevant to the problem of the role of cytochrome b, and to the 



