470 



5. QUINONES 



tential of the enzyme SH groups would, of course, also be modified by the 

 pH. However, it seems likely that the state of ionization of the SH groups or 

 the changing over-all charge on the enzyme may be of greater importance. 

 Finally we shall inquire whether mutual depletion kinetics should ever 

 be used in the analysis of quinone inhibition. This will depend on the 

 potency of the inhibition, but obviously in some cases the potency is suf- 

 ficient for depletion of free inhibitor to occur, and indeed Hoffmann-Osten- 

 hof and Gierer (1948) noted the inhibition of catalase by p-benzoquinone 



Fig. 5-3. Inhibitions of catalase and urease by p-Q, at different 

 pH's. (From Hoffman- Ostenhof and Biach, 1948 a.) 



at 0.1 TCiM to be 40% when 3.5 units of catalase were present and only 

 15% when 11 units of catalase were used. Ackermann and Potter (1949) 

 also found typical pseudoirreversible inhibition of succinate dehydrogenase 

 by 23-benzoquinone, curves such those as in Fig. 1-3-14 being obtained. 

 The potent antimalarial naphthoquinones, such as SN-5949, are classic 

 examples of this type of inhibition; they will be discussed elsewhere. 



EFFECTS ON ELECTRON TRANSPORT SYSTEMS 



The disturbances produced by quinones in various electron transport 

 systems are frequently complex because of the different mechanisms which 

 may be involved. The recent demonstration of the functioning of benzoand 

 naphthoquinones in complete electron transport chains has increased the un- 

 derstanding of how certain unnatural quinones can interfere, and therefore 

 a brief summary of the role of the natural quinones will be presented. 



