Solute uptake in plasma membrane vesicles from broad bean (Vicia faba L.) leaves

A number of physiological data that led to the conclusion of the existence of a proton/sucrose symporter mediating the uptake of sucrose into the phloem cells were obtained from broad bean (Vicia faba L.) The present paper investigates the possibility of studying the uptake of solutes at the plasma membrane level in this species in repsonse to an artificially imposed proton-motive-force. Plasma membrane vesicles were purified by aqueous two-phase partitioning from the microsomal fractions of broad bean leaves. The activity of selected marker enzymes showed that the plasma membrane vesicles were at least 90% pure. The imposition of an artificial proton-motiveforce energized an active (against a concentration gradient) and transient uptake of both sucrose and valine. The maximum uptake rates were comparable for both sucrose and valine; sucrose was accumulated 6-2 times and valine 5-6 times above the diffusion equilibrium. The uptake of sucrose was saturable suggesting that the uptake was carrier-mediated. The affinity of the sucrose carrier for its substrate was 0-6 him under energized conditions. These results confirm with a new material the possibility of studying proton-coupled transport of solutes directly at the membrane level, and are compared with data available from other species.