valinomycin and thiomethylgalactoside

Transport of lactose and methyl beta-D-thiogalactopyranoside, a melibiose analogue, was studied in intact cells of Escherichia coli. A proton motive force could drive the translocation of these solutes via these two transport systems, but the initial rates and steady-state levels of solute accumulation increased upon initiation of electron transfer. When the absolute value of the proton motive force was decreased by ionophores the steady-state levels of lactose accumulation did not decrease as expected if thermodynamic equilibrium with the proton motive force had existed. Accumulation of lactose was also observed in the absence of any measurable proton motive force as long as electron transfer took place. Since both proton/lactose and sodium/methyl beta-D-thiogalactopyranoside symport showed the same characteristics, an explanation based on local proton diffusion pathways is unlikely.

Topics: Anaerobiosis; Biological Transport; Electron Transport; Escherichia coli; Gramicidin; Hydrogen-Ion Concentration; Lactose; Methylgalactosides; Methylglycosides; Oxygen Consumption; Potassium; Protons; Solutions; Thiogalactosides; Thioglycosides; Valinomycin

When cells of the anaerobe Streptococcus lactis 7962 are deprived of their normal fermentable energy source, active transport of galactosides is completely abolished although the membrane carriers are still capable of facilitating the equilibration of sugars across the cell membrane. In these nonmetabolizing cells it was possible to test the Mitchell hypothesis of obligatory coupling of proton movement with sugar transport. This hypothesis was supported by alkalinization of the medium observed when thiomethylgalactoside was added to a lightly buffered suspension of S. lactis cells. Conversely, addition of protons resulted in active transport of thiomethylgalactoside. Accumulation of thiomethylgalactoside to a concentration more than 20-times that in the external medium was induced by suddenly exposing cells to a medium at pH 6; no accumulation of thiomethylgalactoside was observed with cells exposed to pH 8.Active transport of thiomethylgalactoside occurred in the absence of energy metabolism when S. lactis cells were treated with valinomycin. This ionophore allowed intracellular K(+) to flow out, thus imposing a membrane potential (inside negative). This potential resulted in a proton uptake and an associated active transport of galactoside. The membrane potential was measured from the distribution ratio (inside/outside) of K(+) in the presence of valinomycin. The pH gradient was measured from the distribution ratio of [(14)C]methylamine. The protonmotive force, calculated from the membrane potential and the pH gradient, was found to be directly related to the accumulation of galactoside, in accordance with the chemiosmotic hypothesis.

Topics: Biological Transport, Active; Carbon Radioisotopes; Energy Transfer; Galactose; Hydrogen; Hydrogen-Ion Concentration; Lactococcus lactis; Membrane Potentials; Methylamines; Methylgalactosides; Methylglycosides; Potassium; Protons; Sorbitol; Thiogalactosides; Thioglycosides; Valinomycin