556 2. ANALOGS OF ENZYME REACTION COMPONENTS 



Some of the enzymes inhibited by quinacrine do not involve flavin co- 

 enzymes, as emphasized by Hellerman et al. (1946) and Hemker and Hiils- 

 man (1960), so that no direct antagonism with FMN or FAD would be 

 expected. Nevertheless, reversal of the inhibition is sometimes seen and 

 may be due to the formation of a complex between the quinacrine and the 

 added coenzyme, as was shown to occur between quinacrine and FMN in 

 the inhibition of aliesterase, this presumably removing some of the quina- 

 crine from the enzyme. In any case, the degree of reversal or even whether 

 reversal occurs with FMN or FAD will depend on the way in which the ex- 

 periment is run and the relative concentrations. If interaction of the en- 

 zyme and quinacrine is allowed to occur, the chance of reducing the inhi- 

 bition by adding coenzyme is less than if both are added together, since it 

 may be difficult to reach equilibrium due to the tight binding of quinacrine. 



The effects of quinacrine on ATPases and oxidative phosphorylation are 

 interesting and perhaps important in explaining some of the metabolic 

 changes. Myofibrillar ATPase does not depend on any flavin component 

 and yet is inhibited rather well by quinacrine (Kaldor, 1960). Furthermore, 

 increasing the ATP concentration from 1 raM (where 1 mM quinacrine in- 

 hibits about 50%) to 7 vaM almost abolishes the inhibition. The inhibition 

 is potentiated by Mg++ and counteracted by Ca++, so that a quinacrine-Mg 

 complex was postulated as the possible active inhibitor. Interactions of 

 Mg++ and ATP with quinacrine were shown by fluorescence changes. Irvin 

 and Irvin (1954) had found that quinacrine forms complexes with AMP 

 and ATP at physiological pH's. The dissociation constant for the ATP com- 

 plex is 1.38 X 10"^, so that 1 raM quinacrine would lower the ATP con- 

 centration from 1 vaM to 0.67 raM, whereas it would have a negligible ef- 

 fect on ATP around 7 raM. Increasing the ATP might also reduce the Mg++ 

 available for a quinacrine complex, and if this is the active inhibitor, the 

 inhibition would be lessened. The ATPase activity in mitochondrial pre- 

 parations may be quite different from the myofibrillar enzyme and could 

 involve a flavin component. Quinacrine inhibits the ATPase of beef heart 

 mitochondria and simultaneously uncouples oxidative phosphorylation even 

 more potently (Penefsky et al., 1960), while the DNP-stimulated ATPase 

 of rat liver mitochondria is stimulated by quinacrine at lower concentra- 

 tions (0.75 mM) and inhibited by higher (3 raM) (Low, 1959 a), the P,- 

 ATP exchange reaction and oxidative phosphorylation being depressed. 

 Low felt that this provides evidence for the participation of flavin in such 

 ATPase activity, especially, as FMN and FAD can reverse the inhibition, 

 but other interpretations are possible (e.g., complexes between quinacrine 

 and ATP or the flavins). There is no doubt, however, that DNP alters the 

 response of mitochondrial ATPase to quinacrine, but no DNP effect is ob- 

 served in muscle ATPase (Pennington, 1961). Uncoupling by quinacrine 

 was first reported by Loomis and Lipmann (1948) and recent work seems 



