Sucrose-dependent H+ transport in plasma-membrane vesicles isolated from sugarbeet leaves (Beta vulgaris L.). Evidence in support of the H+-symport model for sucrose transport

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.