472 5. QUINONES 



lipid solvents abolislies succinate oxidase and succinate:cytochrome c 

 reductase activities, and restoration of activity can be achieved by adding 

 the extract or functional ubiquinones, but not with the simpler benzo- 

 quinones or naphthoquinones (F. L. Crane, 1960; Lester and Fleischer, 

 1961). The exact role of the naphthoquinones is not yet clear but they 

 appear to function in a comparable region of the electron transport chain, 

 although it is not certain whether they are generally on the main sequence 

 or on bypass chains. In bacteria it has been postulated that the two groups 

 of quinones function as the terminal components of the converging arms 

 of the electron transport system: 



NADH -► fp, -> vit Kg 



\ 



cyto-bi -> cyto-aa -> Oj 



/' 

 Succinate -> fpj -> U-Qg 



(Kashket and Brodie, 1963 b), but the situation is different in plant and 

 animal tissues. There are only small amounts of quinones in tissues — e. g. 

 the ubiquinones are usually in the range 1-10 mg/100 g wet weight (Thom- 

 son, 1962) — but the concentrations within the electron transport sys- 

 tems may be high. 



It is necessary to emphasize the remarkable specificity exhibited by the 

 quinones in these systems. We have noted the relative abilities of various 

 quinones to accept electrons from NADH in a reductase preparation from 

 peas (page 434) (Wosilait and Nason, 1954). A bacterial Fe++- and FAD- 

 activated nitrate reductase operates maximally with menadione, or the 

 2,3- and 2,6-dimethyl-l,4-naphthoquinones, but much less rapidly with 

 lapachol or phthiocol, and not at all with vitamin Kj (Wainwright, 1955). 

 Ernster (1961) found that menadione is the best mediator of electrons in 

 the oxidation of glutamate by rat liver mitochondria, but that various 

 benzo- and naphthoquinones can function to varying degrees and in dif- 

 ferent pathways; e. g., p-benzoquinone will function in electron transfer 

 but the system is no longer amobarbital-sensitive. The oxidation of NADPH 

 by a submitochondrial fraction from liver, designated a DT diaphorase, 

 is rapid with menadione, but vitamins K^ and K2, lawsone, 1,2- and 1,4- 

 naphthoquinones, and various benzoquinones including the ubiquinones, 

 are either very slightly active or inactive (Conover et al., 1963 a). These 

 few examples will suffice to illustrate the specificity and to indicate that 

 the interactions between the quinones and the various components of the 

 electron transport chains are controlled by factors such as lipid solubility 

 or enzyme catalysis and not entirely by the relative redox potentials. 



The roles played by the quinones in electron transport are discussed 

 in detail in the Ciba Foundation Symposium "Quinones in Electron Trans- 

 port" (1960), "The Biochemistry of Quinones" edited by Morton (1964), 

 and the reviews by Dam and Sondergaard (1960) and Thomson (1962). 



