THE ROLE OF CYTOCHROME b IN THE 

 RESPIRATORY CHAIN 



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



Laboratory of Physiological Chemistry, 

 University of Amsterdam, Amsterdam* 



Cytochrome b was one of the three cytochromes whose bands were first 

 seen with the microspectroscope by McMunn (1886) and Keihn (1925). In 

 the reduced state, it shows strong absorption bands at 564 m/« (a), 530 mfi {(i) 

 and 432 m/i {y). It is one of the five cytochromes known to be present in 

 the mitochondria of animal tissues and is also present in many micro- 

 organisms. This distribution makes it highly probable that it is a component 

 of the main cytochrome system which is involved in the main respiratory 

 pathway of most aerobic cells. Nevertheless, we know less about this 

 cytochrome than of many other cytochromes, including those in the b group 

 (e.g. cytochrome Z^a or b^). One of the reasons for this is that it has never 

 been isolated in a purely soluble, and enzymically active form. 



The first studies of Keilin (1925) showed that cytochrome b has some unique 

 properties. When a suspension of yeast cells v/as aerated in the presence of 

 urethane, the b band remained visible (i.e. cytochrome b was reduced), while 

 the a and c bands disappeared (i.e. cytochromes a and c were oxidized). This 

 indicated rather strongly that cytochrome b is the member of the chain which 

 acts nearest the substrate. This was further supported by Ball's (1938) 

 measurements of the oxidation-reduction potential of the three cytochromes, 

 which showed that cytochrome b had a much lower potential (—40 mV at 

 pH 7-4) than the other two (+260 mV and +290 mV, respectively). It has, 

 perhaps, not always been sufficiently emphasized that the fact that cyto- 

 chrome b acts nearer to the substrate than the other cytochromes implies that 

 some kinetic diff"erences between this cytochrome and the others are to be 

 expected, and need not be abnormal. 



Keihn and Hartree (1939) emphasized the close association of succinate 

 dehydrogenase with cytochrome b and at one time (see, for example, Ball, 

 Anfinsen and Cooper, 1947; Slater, 1949) the possibility that the two were 

 identical was seriously considered, especially when it appeared from the 

 studies of Slater (1950) that cytochrome b was not involved in the oxidation 

 of reduced diphosphopyridine nucleotide (DPNH). However, Tsou (1951) 



* Postal address: Jonas Daniel Meyerplein 3, Amsterdam-C. 



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