Characterization of solute/proton cotransport in plasma membrane vesicles from Ricinus cotyledons, and a comparison with other tissues

Abstract

Plasma membrane vesicles, purified by aqueous two-phase partitioning, were used to investigate the presence of sugar and amino acid carriers in cotyledons and roots of Ricinus communis L. and in roots of red beet (Beta vulgaris L.). Artificial pH and electrical gradients were generated across the plasma membrane, and [(14)C]acetate and [(14)C]tetraphenylphosphonium were used to demonstrate the presence of an internal alkaline pH gradient and an internal negative membrane potential, respectively. In Ricinus cotyledons, uptake of sucrose was more strongly inhibited than that of glutamine by p-chloromercuribenzenesulphonic acid, phlorizin and phenylglyoxal. The sucrose transport system showed a high degree of substrate specificity with only the presence of maltose and phenyl-α-glucoside significantly affecting sucrose uptake; in contrast, the glutamine transport system was inhibited by a number of other amino acids. ΔpH+gDψ-driven glutamine uptake showed saturation kinetics with a K m of 0.35 mol · m(-3). Sucrose and glutamine Δψ-driven uptake was pH dependent with an optimum in the acidic range (pH 6.25) and a decrease at higher pH values. Vesicles obtained from cotyledons and roots of Ricinus showed different transport properties. In the cotyledons, gDH+gDψ-driven transport for both sucrose and glutamine were observed at similar levels; however, in the root tissue, δpH-Δψ-driven glutamine transport was the dominant uptake process. Uptake rates for glucose and fructose were low in the cotyledons whereas, in the roots, glucose and sucrose transport were slightly higher than that of fructose. In vesicles from red beet tissue there was a different uptake profile, with evidence of proton-coupled cotransport systems for sucrose and glucose, but lower uptake of glutamine and fructose. The results are discussed in relation to the reported different pathways for loading and unloading of solutes in these tissues.