Quercetin interaction with the chloroplast ATPase complex

1. Quercetin, a flavonoid which acts as an energy transfer inhibitor in photophosphorylation is shown to inhibit the P-ATP exchange activity of membrane-bound CF₁ and the ATPase activity of isolated CF₁. Quercetin, affects also the proton uptake in chloroplasts in a manner similar to that of dicyclohexyl-carbodiimide. 2. The light-dependent proton uptake in EDTA-treated chloroplasts is stimulated by quercetin. In untreated chloroplasts quercetin has a dual effect: it enhances at pH above 7.5 while at lower pH values it decreases the extent of H⁺ uptake. Similar effects were obtained with dicyclohexylcarbodiimide. 3. Like quercetin, dicyclohexylcarbodiimide was also found to inhibit the ATPase activity of isolated CF₁. 4. Quercetin inhibits uncoupled electron transport induced by either EDTA-treatment of chloroplasts or by addition of uncouplers. Quercetin restores H⁺ uptake in both types of uncoupled chloroplasts. 5. The mode of action of quercetin and dicyclohexylcarbodiimide in photo-phosphorylation is discussed, and interaction with both CF₁ and F₀ is suggested.