464 C. F. Strittmatter 



or /^-phenylcne diamine. Following reduction by DPNH or dithionite, the 

 cytochrome of microsomal suspensions could be reoxidized by presence of 

 air, or, more rapidly, by added cytochrome c or ferricyanide ; the microsomal 

 cytochrome might therefore provide a link in a DPNH-cytochrome c reduc- 

 tase system. As the reoxidation by air was not inhibited by cyanide (10"- m) 

 or azide, it appeared that the microsomal cytochrome was not reoxidized 

 via cytochrome oxidase, but might be autoxidizable. The sum of the proper- 

 ties of the microsomal cytochrome suggested that it should be classified in the 

 Z>-group of cytochromes. 



Studies with Isolated Microsomal Cytochromes 



The nature and relations of the microsomal cytochrome were contirmed 

 and extended in studies with isolated microsomal cytochrome by Strittmatter 

 and Velick (1956a, b) and by Vehck and Strittmatter (1956), who first obtained 

 the cytochrome in highly purified state from rabbit liver microsomes, using a 

 procedure involving separation from microsomes by treatment with pancre- 

 atic lipase and purification by ammonium sulphate fractionation. The 

 purified cytochrome, with a molecular weight of about 17,000, possessed as 

 prosthetic group one iron protoporphyrin unit per molecule and contained 

 no appreciable non-haemin iron or flavin (Strittmatter and Velick, 1956a). 

 In its absorption spectra the isolated cytochrome agreed well with the par- 

 ticulate form, showing the Soret band at 413 m^, the oxidized state and peaks 

 at 423, 526 and 556 m/^ after reduction with cysteine or other reducing agents. 

 With the purified substance it was possible also to obtain accurate values for 

 the absorbancy indices of the absorption peaks and to observe new broad 

 absorption bands at 355-370 m^ in the oxidized state and at 320-340 m/^ in 

 the reduced state. 



Oxidation-reduction titrations and equilibrium measurements indicated 

 that the isolated cytochrome behaved as a single, univalent electron acceptor 

 and donor, and from the oxidation-reduction equilibria, the standard 

 potential was calculated to be -f 0-02 V at pH 7 (Velick and Strittmatter, 

 1956). This potential value is more than 0-1 V higher than the values esti- 

 mated for the cytochrome in particulate suspension from either rat or rabbit 

 liver (Yoshikawa, 1951 ; Strittmatter and Ball, 1952), but the significance of 

 the difference is uncertain at present. The significance of observed apparent 

 standard potentials, particularly those for particle-bound components, are 

 necessarily subject to uncertainty. If it is a real difference, the shift in the 

 apparent standard potential of such a reversibly reduced substance on libera- 

 tion from its particulate complex might reflect a differential interaction of the 

 oxidized and reduced cytochrome with some components of the microsomal 

 complex. This would be analogous to the sliift of apparent standard 

 potential of diphosphopyridine nucleotide (DPN) in the alcohol dehydro- 

 genase reaction which occurs in the presence of stoichiometric amounts of 



