Branched-Chain Amino Acid Transport in Cytoplasmic Membranes of Leuconostoc mesenteroides subsp. dextranicum CNRZ 1273

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Membrane vesicles of Leuconostoc mesenteroides subsp. dextranicum fused with proteoliposomes prepared from Escherichia coli phospholipids containing beef heart cytochrome c oxidase were used to study the transport of branched-chain amino acids in a strain isolated from a raw milk cheese. At a medium pH of 6.0, oxidation of an electron donor system comprising ascorbate, N,N,N′,N′-tetramethyl-p-phenylenediamine, and horse heart cytochrome c resulted in a membrane potential (Δψ) of −60 mV, a pH gradient of −36 mV, and an l-leucine accumulation of 76-fold (ΔμLeu/F = 108 mV). Leucine uptake in hybrid membranes in which a Δψ, ΔpH, sodium ion gradient, or a combination of these was imposed artificially revealed that both components of the proton motive force (Δp) could drive leucine uptake but that a chemical sodium gradient could not. Kinetic analysis of leucine (valine) transport indicated three secondary transport systems with Kt values of 1.7 (0.8) mM, 4.3 (5.9) μM, and 65 (29) nM, respectively. l-Leucine transport via the high-affinity leucine transport system (Kt = 4.3 μM) was competitively inhibited by l-valine and l-isoleucine (Ki and Kt values were similar), demonstrating that the transport system translocates branched-chain amino acids. Similar studies with these hybrid membranes indicated the presence of high-affinity secondary transport systems for 10 other amino acids.

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