Mechanisms of Active Transport in Isolated Membrane Vesicles

An inducible transport system for d-galactose is present in isolated membrane vesicles from Escherichia coli ML 3 and ML 35. Like the transport systems for β-galactosides and amino acids, galactose transport is coupled primarily to a membrane-bound d-lactic dehydrogenase. In addition to d-lactate, the transport system is markedly stimulated by ascorbate plus phenazine methosulfate and oxygen. Succinate, dl-α-hydroxybutyrate, and l-lactate partially replace d-lactate, but are less effective than either d-lactate or ascorbate-phenazine methosulfate. ATP and P-enolpyruvate, in addition to a number of other metabolites and cofactors, do not stimulate galactose transport by the vesicles. transport by isolated membrane vesicles requires oxygen, and is blocked by electron transfer inhibitors and by other general metabolic inhibitors. However, transport is not significantly inhibited by high concentrations of arsenate or oligomycin. Galactose binding protein is totally absent from the vesicles, as is a high affinity galactose transport system present in whole cells.