Active Transport of Calcium in Inverted Membrane Vesicles of Escherichia coli

Abstract

Accumulation of (45)Ca(++) was found to occur in membrane vesicles of E. coli prepared by lysis with a French pressure cell. The uptake occurs by active transport, requiring an energy source. Substrates of the electron transport chain, including D-lactate, reduced phenazine methosulfate, and NADH, stimulated accumulation, but this effect was blocked by the addition of cyanide. ATP could also stimulate accumulation, and this effect was blocked by dicyclohexylcarbodiimide. Uncouplers of oxidative phosphorylation inhibited the accumulation driven by either type of energy source. Accumulation of calcium is rapid, reaching the steady-state plateau within 1 min. Addition of phosphate to the assay buffer results in a prolongation of the reaction, allowing for the time-dependent accumulation of calcium for as long as 30 min. Vesicles prepared by lysis with a French pressure cell exhibit almost no ability to accumulate proline, while vesicles prepared by the method of Kaback transport proline but exhibit little energy-dependent transport of calcium. It is suggested that the accumulation of calcium in these vesicles, which are believed to be inverted, reflects a system that in vivo is responsible for the active extrusion of calcium from the cells.