488 



5. QUINONES 



inhibitors are substituted in both 2- and 3-positions, e. g., the 3-thioalkyl 

 derivatives of menadione, which are usually more inhibitory than mena- 

 dione. It should be noted that 1,2-naphthoquinone is also a very potent 

 inhibitor and yet does not readily react with SH groups. 



EFFECTS ON RESPIRATION 



The responses of respiration to the quinones are variable and complex, 

 and depend on the species or tissue used, the type of quinone, and the con- 

 ditions of the experiment. It is probably safe to state that in all the in- 

 stances of respiratory inhibition reported the site or sites of action are 

 unknown. Surprisingly little is known of the effects of the quinones on 

 the oxidation of pyruvate or the operation of the cycle, and this impedes 

 our understanding of respiratory inhibitions since the cycle is the most 

 likely general site for the action. There are actually so many ways in which 

 the quinones could alter oxygen uptake that a detailed discussion of the 

 effects on respiration would be meaningless without more information on 

 these mechanisms. We shall therefore confine our discussion to the most 

 general topics and attempt to illustrate briefly the types of effect com- 

 monly observed. 



One characteristic response of the respiration to the quinones is imme- 

 diately evident; this is the tendency for the respiration to be increased, 

 especially at low quinone concentrations, the action being biphasic if a 

 sufficiently wide range of concentrations is used. Hydroquinones often 

 stimulate oxygen uptake simply by acting as substrates, but we have seen 

 that many quinones can elevate electron transport through oxidation- 

 reduction reactions in various steps of the sequence, this not necessarily 

 involving direct effects on the participating enzymes. A few examples of 

 diphasic response are given in the accompanying tabulation. Another 



Preparation 



Quinone 



Reference 



Flaig and de Jong (1960b) 

 Gaffron (1945) 



Bueding et al. (1947) 



Lardy and Philhps (1943 a) 

 Kisch and Leibowitz (1930) 



Supniewski et al. (1936) 

 Chipperfield and Marrian (1962) 



