Sodium and potassium requirements for active transport of glutamate by Escherichia coli K-12.

Active transport of glutamate by Escherichia coli K-12 requires both Na(+) and K(+) ions. Increasing the concentration of Na(+) in the medium results in a decrease in the K(m) of the uptake system for glutamate; the capacity is not affected. Glutamate uptake by untreated cells is not stimulated by K(+). K(+)-depleted cells show a greatly reduced capacity for glutamate uptake. Preincubation of such cells in the presence of K(+) fully restores their capacity for glutamate uptake when Na(+) ions are also present in the uptake medium. Addition of either K(+) or Na(+) alone restores glutamate uptake to only about 20% of its maximum capacity in the presence of both cations. Changes in K(+) concentration affect the capacity for glutamate uptake but have no effect on the K(m) of the glutamate transport system. Ouabain does not inhibit the (Na(+)-K(+))-stimulated glutamate uptake by intact cells or spheroplasts of E. coli K-12.