THE MECHANISM OF COENZYME Q REDUCTION IN HEART MITOCHONDRIA 259 



the assumption that the non-haem iron is an intermediary in electron flow- 

 between flavoprotein and CoQ but not between flavoprotein and phenazine 

 methosulphate. 



Spectra of the purified CoQ reductase suggest that the preparation 

 contains, in addition to the flavin and haem prosthetic groups, some other 

 component that can undergo oxidation and reduction. In addition to the 

 fl.avin band at 450 m/x, succinate also reduces a component at approxi- 

 mately 415 m/jL (cf. the difference spectra shown in Fig. i). The change in 

 the spectrum upon reduction of the enzyme by succinate cannot be 

 entirely due to the flavin even in the 450 460 m/a region of the spectrum. 



450 500 



Wovelength (m//) 



600 



Fig. 3. These spectra were obtained under the same conditions as those of 

 Fig. 2 except that CoQ., (0-05 ;umole) was added instead of fumarate. 



Even if all of the flavin is reduced, which is very unlikely, the decrease in 

 optical density at 450 460 m/n is considerably greater than could be 

 attributed to the flavin alone. 



The band at 415 m^a is not contributed by flavin, since it is not re- 

 oxidized by fumarate (Fig. 2). All the haem is reoxidized and essentially all 

 the flavin should be reoxidized by fumarate. It is unlikely that the bands 

 remaining after the addition of fumarate can be attributed to the flavin 

 prosthetic group. In addition to the main band at 415 m/^i a broad band 

 persists at 450 m/^, which again suggests that not all the reduction observed 

 in this region is due to the flavin. 



The components of the enzyme that remain reduced after the re- 

 oxidation of the flavin and haem by fumarate are, however, reoxidized by 



