EFFECTS ON ELECTRON TRANSPORT SYSTEMS 471 



Participation of Quinones in Electron Transport Systems 



It has been known for many years that certain hydroquinones can donate 

 electrons to specific sites in the electron transport chain, and that qui- 

 nones can often act as electron acceptors, either being directly reoxidized 

 or passing electrons on to enzymes within or outside the normal phos- 

 phorylating sequences. Most of the early work was with /j-benzoquinone 

 and its hydroquinone. The enzyme thought to be concerned with the oxida- 

 tion of T^-benzohydroquinone was called indophenol oxidase but Stotz 

 et al. (1938) showed the oxidation to be mediated by cytochrome c or 

 other components of the cytochrome portion of the complex. Many en- 

 zyme systems isolated from a variety of sources have been shown to interact 

 with quinones; for example, (1) succinate:quinone reductase, (2) NAD(P)H: 

 quinone oxidoreductase (quinone reductase), (3) NAD(P)H:2-methyl-l,4- 

 naphthoquinone oxidoreductase (menadione reductase), (4) QHg: cyto- 

 chrome c reductase, (5) QHg: cytochrome c^ reductase, (6) ferrous iron: 

 quinone reductase, (7) DT diaphroase, (8) vitamin K^ reductase, and (9) 

 ubiquinone reductase. Most of these appear to be fragments of normally 

 functioning complete electron transport systems. The quinones occurring in 

 intact electron transport systems are usually benzoquinones of the coen- 

 zyme Q (ubiquinone) series or naphthoquinones of the vitamin K^ series, 

 although other types of quinone, such as the plastoquinones, the vitamin Kg 

 series, and the tocopherylquinones may also participate in some instances. 



o cHj n 



O 



Coenzyme Q (ubiquinone) series 



CH 

 I 

 CHj— CHj— CH-ca 



iJ-H 



The lipid-soluble ubiquinones probably exist in the lipoidal phase of 

 the particulate electron transport systems and transfer electrons between 

 the flavoproteins and cytochrome b^. Extraction of mitochondria with 



