476 5. QUINONES 



accompanying tabulation) (Klapper and Hackett, 1963). This is cer- 

 tainly one of the most potent inhibitions by p-benzoquinone yet reported. 



The inhibition was claimed to be correlated with redox potential but 

 too few inhibitory quinones were used to establish this. There is a great 

 need for more thorough analysis of such systems and the ways in which 

 the quinones can interfere with electron transport. The observation that 

 Mn++ reverses the peroxidase inhibitions, for example, cannot be readily 

 interpreted but could be an important clue in the elucidation of the in- 

 hibitory mechanism. 



Interactions of Quinones with Metalloflavoproteins 



Quinones can serve as electron acceptors from certain flavoproteins 

 (Wieland and Mitchell, 1930; Booth, 1935), and recent work has demonstrat- 

 ed a variety of quinone reductases. A very thorough investigation of the 

 interactions between quinones and metalloflavoproteins was undertaken 

 by Mahler et al. 1955 a) and, although they did not study inhibitions in 

 such systems, the results are of obvious importance for inhibition of elec- 

 tron transport. It was found that p-benzoquinone, p-xyloquinone, 1,4- 

 naphthoquinone, and various substituted 1,4-naphthoquinones.can serve 

 as efficient acceptors for several such flavoenzymes. For example, excess 

 NADH in the presence of reduced NADH:cytochrome reductase is oxidized 

 very slowly by oxygen, but the addition of menadione induces a rapid 

 oxidation of the NADH by the following sequence of reactions: 



NADH + fp + H+ ±? NAD+ + fpH^ 

 fpHa + MD ±5 fp -f- MDH2 

 MDH2 + O2 ^ MD + H^O,, 



The quinones take the place of cytochrome c as acceptors from the flavo- 

 enzymes but, since they are 2-electron acceptors, no metal ion is involved. 

 The rates are in general quite comparable to those when cytochrome c is 

 the acceptor. The reduced quinones can react with cytochrome c if it is 

 present. In the total system there would be competition between the qui- 

 nones and cytochrome c as acceptors from the flavoenzymes, as well as 



