Sodium-stimulated glutamate uptake in membrane vesicles of Escherichia coli: the role of ion gradients.

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Abstract
Membrane vesicles prepared from E. coli B/r grown on glutamate as a sole source of C and energy require Na for glutamate accumulation when energized by D-lactate oxidation. Glutamate uptake can also be driven by a prearranged Na gradient (out to in) in the absence of an energy source or a protonmotive force. Na+ are exchanged rapidly in respiring vesicles and the Na gradient may be large enough under certain conditions to drive glutamate uptake after the protonmotive force is abolished with m-chlorocarbonylcyanide phenylhydrazone. Glutamate uptake due to a prearranged sodium gradient of lactate oxidation is inhibited by monensin but not by nigericin. Transport does not occur in response to valinomycin-induced membrane potential. Apparently, glutamate transport is obligately coupled to Na transport and can only occur when there is a net flux of Na+. This flux is driven by a chemical gradient of Na that is created by the protonmotive force generated by respiration.