Potential-dependent anion transport in tonoplast vesicles from oat roots

Potential-dependent anion movement into tonoplast vesicles from oat roots (Avena sativa L. var Lang) was monitored as dissipation of membrane potentials (Δψ) using the fluorescence probe Oxonol V. The potentials (positive inside) were generated with the H+-pumping pyrophosphatase, which is K+ stimulated and anion insensitive. The relative rate of Δψ dissipation by anions was used to estimate the relative permeabilities of the anions. In decreasing order they were: $\text{SCN}^{-}$ (100) > NO3 - (72) = Cl- (70) > Br- (62) > SO4 2- (5) = $\text{H}_{2}\text{PO}_{4}{}^{-}$ (5) > malate (3) = acetate (3) > iminodiacetate (2). Kinetic studies showed that the rate of Δψ dissipation by Cl- and NO3 -, but not by $\text{SCN}^{-}$, was saturable. The Km values for Cl- and NO3 - uptake were about 2.3 and 5 millimolar, respectively, suggesting these anions move into the vacuole through proteinaceous porters. In contrast to a H+-coupled Cl- transporter on the same vesicles, the potential-dependent Cl- transport was insensitive to 4,4′-diisothiocyano-2,2′-stilbene disulfonate. These results suggest the existence of at least two different mechanisms for Cl- transport in these vesicles. The potentials generated by the H+-translocating ATPase and H+-pyrophosphatase were nonadditive, giving support to the model that both pumps are on tonoplast vesicles. No evidence for a putative Cl- conductance on the anion-sensitive H+-ATPase was found.