Symport of Proton and Sucrose in Plasma Membrane Vesicles Isolated from Spinach Leaves

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Abstract
The mechanism of sucrose transport was investigated in plasma membrane (PM) vesicles isolated from spinach (Spinacia oleracea L.) leaves. PM vesicles were isolated by aqueous two-phase partitioning and were equilibrated in pH 7.8 buffer containing K(+). The vesicles rapidly accumulated sucrose in the presence of a transmembrane pH gradient (DeltapH) with external pH set at 5.8. The uptake rate was slow at pH 7.8. The K(+)-selective ionophore, valinomycin, stimulated uptake in the presence of a DeltapH, and the protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), greatly inhibited DeltapH-dependent sucrose uptake. Addition of sucrose to the vesicles resulted in immediate alkalization of the medium. Alkalization was stimulated by valinomycin, was abolished by CCCP, and was sucrose-specific. These results demonstrate the presence of a tightly coupled H(+)/sucrose symporter in PM vesicles isolated from spinach leaves.