Abstract
The mechanism of sucrose transport across the plasma membrane (PM) was investigated in membrane vesicles isolated from sugarbeet (Beta vulgaris L.) leaves. In the presence of a membrane potential (Δψ) generated as a K(+)-diffusion potential, negative inside, sucrose induced a rapid and transient alkalization of the medium. Alkalization was inhibited by carbonyl cyanide m-chlorophenylhydrazone, was specific for the sucrose sugar and was dependent on the sucrose concentration with a Km of approx. 1 mM. Sucrose-induced alkalization and sucrose transport were inhibited by the sulfhydryl-reactive reagent, p-chloromercuribenzene sulfonic acid, and by the histidine-reactive reagent, diethyl pyrocarbonate. Parallel analysis of sucrose uptake and alkalization indicated that the stoichiometry of sucrose uptake to proton consumed was 1∶1. These results provide clear evidence that the saturable mechanism of sucrose transport across the PM in plants is a coupled H(+)-sucrose symport.
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