The Mechanism of Coenzyme Q Reduction in Heart 



Mitochondria 



Daniel M. Ziegler* 



Institute for Enzyme Research, 

 University of Wisconsin, Wis., U.S.A. 



Our laboratory and the Liverpool group have previously presented 

 evidence [i, 2, 3] that Coenzyme Q (CoQ) is positioned between the flavo- 

 protein and cytochrome c^ in the mitochondrial succinoxidase system. 

 Furthermore, it is generallv accepted that cytochrome b, at least in non- 

 phosphorylating particles, is not an obligatory electron carrier either in the 

 reduction of CoQ or in its reoxidation by cytochrome c^ [i]. However, 

 CoQ does not react directly with the flavoprotein since the primary 

 succinic flavoprotein isolated by the method of Singer et a/. [4] does not 

 catalyze the reduction of CoQ. We have isolated a soluble form of the 

 succinic flavoprotein that can catalyze this reaction [5], and in this report 

 we will present evidence that the non-haem iron associated with the 

 succinic dehvdrogenase functions as an electron carrier between the flavo- 

 protein and the quinone. 



tablp: I 



SucciN!c-CoQ Reductase Activities* 



Preparation 



/imoles CoQ reduced 

 (min., mg. protein) 



Beef heart mitochondria 

 Succinic-CoQ reductase [5] 

 Primary succinic fla\'()protein [4] 



I • I 



560 



o-o 



* The succinic-CoQ reductase activities were measured b) 

 described in ref. [2]. 



the method 



Table I lists the succinic-CoQ reductase activities of heart mito- 

 chondria, the soluble succinic-Co(} reductase and the primary succinic 

 flavoprotein. The succinic-CoQ reductase is about fifty times more active 

 than the starting heart mitochondria while the primary succinic flavo- 

 protein does not catalyze this reaction. It is apparent that the site necessary 

 to link CoQ to the flavoprotein is still present in the Q reductase but is 

 either lost or non-functional in the primary flavoprotein. 



* The author is indebted to Dr. D. K. Green for his advice during the course 

 of this work. 



