Electroinelric and Other Studies of Cytochrome 



371 



and Vernon, 1955) and the modification of mammalian heart muscle cyto- 

 chrome c by treatments such as proteolytic digestion (Tsou, 1951a) many 

 varied properties are apparent in pigments having the same or closely similar 

 absorption spectra. 



This variation is observed in the important property of oxidation-reduction 

 potential {Eq) even where the haem prosthetic groups are known to be of 

 the mammalian heart cytochrome c (M. heart c) type (Table 1). It is parti- 

 cularly interesting that a difference in E'q at least as large as that existing 

 between the naturally occurring 'c-type' pigments may be obtained by 

 modification of M. heart c without any significant spectral change (Table 2). 



OXIDATION-REDUCTION POTENTIAL OF YEAST 

 CYTOCHROME C 



The early controversy regarding the £■„ value of yeast cytochrome c centred 

 around the values of +0-26 V (Coolidge, 1932) and +0-12 V (Green, 1934) 

 (see also Table 3). It was generally considered to have been resolved in 



Table 3. Eq values of various yeast cytochrome c samples 



favour of the former by what is now obviously the doubtful procedure of 

 comparing the values already obtained from yeast samples with those from 

 several other sources, especially mammalian heart muscle. Furthermore, 

 Green's criticisms of Coolidge's methods seem quite valid. The problem 

 does not appear to have been attacked again using the isolated pigment until 

 Minakami (1955) reported some potential levels for several ratios of oxidized/ 

 reduced cytochrome c of yeast which were similar to those for M. heart c. It 

 still seemed desirable to carry out a complete electrometric titration of yeast 

 cytochrome c extracted under mild conditions and avoiding the use of 

 Na2S204 as titrant (see, for example, Paul, 1951). This was done using a 

 sample (kindly provided by Prof. R. K. Morton) which had been extracted 

 from dried baker's yeast with NaCl and chromatographed on a resin exchange 



