THE MECHANISM OF COENZYME Q REDUCTION IN HEART MITOCHONDRIA 255 



In contrast to the primary flavoprotein the CoQ reductase contains 

 haem and Hpid. All the haem can be extracted with acid-acetone and its 

 reduced pyridine haemochromogen is identical with that of protohaem. 

 The spectrum of the dithionite reduced enzyme (Fig. i), indicates that 

 the haem prosthetic group is similar to the mitochondrial cytochrome b; 

 however, not all the cytochrome b in mitochondria can be associated with 

 the succinic flavoprotein since mitochondria contain about three times as 

 much protohaem as succinic flavoprotein. 



The cytochrome present in the CoQ reductase is not reduced by 

 succinate (Fig. i) so it cannot function as an electron carrier between the 



6- 



0-4- 



Q 0-2 



O 



-0- 



400 



450 



500 550 



(m// , 



Fig. I. Difference spectra (jf succinic-CoQ reductase (oxidized vs. reduced). 

 The enz\Tne was dissolved in o • i M phosphate buffer pH 7 -4 at a final concentra- 

 tion of I -16 mg. protein per ml. The enzyme was first reduced with succinate 

 (100 /tmoles /ml.) and then with dithionite. 



flavoprotein and CoQ. If, however, the enzyme is first reduced with 

 limiting amounts of dithionite the cytochrome is reoxidized by fumarate 

 (Fig. 2) — an observation which suggests that a functional link still exists 

 between the flavin and haem groups. We have not been able to define the 

 function of the cytochrome associated with the flavoprotein. One possi- 

 bility suggested by the work of Conover and Ernster [14] is that cvtochrome 

 b is an intermediate electron carrier between extramitochondrial oxidative 

 enzymes and the electron transport system. 



Beinert and Sands have examined the succinic-CoQ reductase by 

 electron paramagnetic resonance spectroscopy and they have reported that 

 the enzyme contains a paramagnetic species that can be reduced by 

 succinate and reoxidized by CoQ [7]. Since iron is the only transition metal 



