CHARACTERIZATION OF SOLUTE TRANSPORT IN PLASMA-MEMBRANE VESICLES ISOLATED FROM COTYLEDONS OF RICINUS-COMMUNIS L .2. EVIDENCE FOR A PROTON-COUPLED MECHANISM FOR SUCROSE AND AMINO-ACID-UPTAKE

Evidence is presented for the proton-coupled transport of sucrose and glutamine in purified plasma membrane vesicles isolated from cotyledons of Ricinus communis. Imposition of a pH gradient (internal alkaline) across the plasma membrane resulted in a rapid uptake of sucrose and glutamine which was inhibited in the presence of carbonyl cyanide-m-chlorophenyl hydrazone. Imposition of a pH gradient plus an internal negative membrane potential stimulated uptake further. Glucose and fructose uptakes were negligible under these conditions. Sucrose uptake into the vesicles demonstrated saturation kinetics with a Km of 0.87 mol·m-3, indicating carrier-mediated transport. In support of this, uptake was very sensitive to the protein-modifying reagent p-chloromercuribenzenesulphonic acid. N-Ethylmaleimide, another sulphydryl reagent, was only slightly inhibitory. However, both reagents strongly inhibited sucrose uptake into intact cotyledons; the possible reasons for the difference between the intact and isolated systems are assessed. The value of this system for the study of sucrose and amino acid carriers is discussed. © 1990 Springer-Verlag.