Transport Properties of the Tomato Fruit Tonoplast : II. Citrate Transport

Citrate transport across the membrane of tomato fruit tonoplast vesicles was investigated. In the tonoplast vesicles, ¹⁴C]methylamine uptake was stimulated 10-fold by MgATP and strongly inhibited by NO₃⁻. Under identical experimental conditions, ¹⁴C]citrate uptake was inhibited by 5 millimolar free Mg²⁺, and this inhibition was reversed in the presence of ATP, presumably by ATP chelation of free Mg²⁺. No evidence was obtained in support of energy-linked ATP stimulation of citrate uptake. Citrate uptake showed saturation kinetics, and was inhibited by 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid and by other organic acids. The pH-dependence of uptake suggested that citrate³⁻ was the transported species. Our results indicate that citrate transport across the tomato fruit tonoplast occurs by facilitated diffusion of citrate³⁻. The carrier shares some features in common with anion channels in that it is relatively nonspecific for organic acids and is inhibitable by 4,4′-diisothyocyano-2,2′-stilbenedisulfonic acid.