Citrate uptake into tonoplast vesicles from acid lime (Citrus aurantifolia) juice cells

Citrate transport into the vacuoles of acid lime juice cells was investigated using isolated tonoplast vesicles. ATP stimulated citrate uptake in the presence or in the absence of a Delta mu H+. Energization of the vesicles only by an artificial K+ gradient (establishing an inside-positive Delta psi) also resulted in citrate uptake as was the case of a Delta pH dominated Delta mu H+. Addition of inhibitors to endomembrane ATPases showed no direct correlation between the inhibition to the tonoplast bound H+/ATPase and citrate uptake. The data indicated that, although some citrate uptake can be accounted for by Delta psi and by a direct primary active transport mechanism involving ATP, under in vivo conditions of vacuolar pH of 2.0, citrate uptake is driven by Delta pH.