486 



5. QUINONES 



that the C-l/C-6 ratio rises, pointing to an activation of the pentose-P 

 pathway. Menadione is about 100 times more potent than menadiol-diP 



and stimulates the pentose-P pathway maximally at 0.001 mM (Hoskin, 

 1960 a). The relative activities of various quinones are shown in the ac- 

 companying tabulation (Hoskin, 1962; Hoskin and von Eschen, 1963), 



from which it may be concluded that this activity is dependent on a 2-CH3 

 group, and either CH3 groups at positions 5 and 6 or a ring on that side. 

 Long side chains, as in Q^q and vitamin K,, were thought to possibly prevent 

 access to the enzymes involved. Using soluble brain preparations, Hoskin 

 and von Eschen (1963) have shown that the active quinones stimulate the 

 metabolism of glucose-6-P if NADP is added, and also stimulate the oxi- 

 dation of NADPH. Some stimulation with menadione occurs at 0.001 mM, 

 is maximal at 0.01 mM, and remains constant as the concentration is raised 

 to 1 mikf , showing that even at these high concentrations there is no in- 

 hibition of glucose-6-P metabolism. The mechanism of the stimulation is 

 apparently a rapid regeneration of NADP, with autoxidation of the mena- 

 diol formed, so that glucose is fed into the pentose-P pathway at an accel- 

 erated rate. Some of the less effective quinones may be inactivated by reac- 

 tions with proteins. 



