468 



5. QUINONES 



Noncompetitive kinetics have also been found for the inhibitions of trypto- 

 phan pyrrolase (Frieden et al., 1961) and NADHiHgOa oxidoreductase 

 (Gamborg et al., 1961) by p-benzohydroquinone. Foote et al. (1949) claim 

 that the inhibitions of pyruvate decarboxylase by various 1,4-naphtho- 

 quinones depend on the presence of substrate and hence that the inhibitors 

 must combine with the ES complex, the inhibition thus presumably being 

 uncompetitive, but possibly pyruvate augments the inhibition by altering 

 the state of the SH groups involved in the catalysis. 



The potent inhibition of succinate dehydrogenase by p-benzoquinone 

 suggests that some structural factor may be involved. j)-Benzoquinone is 

 structurally related to succinate or fumarate in that two negatively charged 

 groups are separated by about the same distance in each. Although the 

 atoms in the quinone are not ionic, they possess an appreciable negative 

 charge (see page 426), and are approximately 5.4 A apart, which might 

 allow some interaction with the two cationic groups on the enzyme surface, 

 this orienting the quinone so that reaction with the SH group might more 

 readily occur. It would be interesting to know how the potencies of o- 

 benzoquinone and 1,2-naphthoquinone compare with the corresponding 

 1,4-quinones, and to know whether duroquinone and chloranil inhibit or 

 not, but I have found no data on these points. 



The rate of inhibition is fairly slow in most instances, as we have pre- 

 viously noted, and a typical curve for the development of inhibition is that 

 for p-benzoquinone acting on heart lactate dehydrogenase (Fig. 5-1) 



Fig. 5-1. Inhibition of pig heart lactate dehydro- 

 genase by 33-Q at 1 mM. The inhibition becomes com- 

 plete at near 180 min. (From Pileiderer et al., 1959.) 



