Accumulation of lipid-soluble ions and of rubidium as indicators of the electrical potential in membrane vesicles of Escherichia coli.
We have studied the accumulation of dibenzyldimethyl-ammonium ion (DDA+) by respiring membrane vesicles of Escherichia coli, as an index of the generation of an electrical gradient during respiration. Nonrespiring vesicles accumulated DDA+ when K+ efflux was induced by valinomycin or monactin. By various criteria this was shown to be the exchange of one cation for another, independent of metabolism and coupled entirely by electrical forces. Uptake of DDA+ by respiring vesicles was inhibited by ionophores that translocate electrical charge and by reagents that block the respiratory chain. Oxamate and p-chloromercuribenzoate inhibited accumulation of DDA+ but did not dissipate a preformed pool; the reason appears to be that these reagents are less inhibitory to transport after lactate oxidation has begun than they are in resting vesicles. Uptake does not appear to involve a biological carrier, but requires trace amounts of a lipid-soluble anion such as tetraphenylboron, which has a catalytic role in DDA+ translocation. Respiring K+ vesicles accumulated substantially less DDA+ than did Na+ vesicles. Na+ was expelled from the vesicles concurrently with DDA+ uptake, whereas Rb+ and K+ were not. Thus, DDA+ uptake, whereas Rb+ and K+ were not. Thus, DDA+ uptake may be limited in the latter case by the availability of anionic groups. This explanation was supported by the finding that the addition of nigericin doubled the capacity of K+ vesicles to take up DDA+, presumably by providing a route for K+ to exit in exchange for H+. Parallel experiments on the valinomycin-dependent accumulation of Rb+ by respiring vesicles indicate that this process is analogous to the uptake of DDA+. Ionophores that elicit electrogenic K+ movement also induced respiration-linked transport. Proton-conducting ionophores and several inhibitors of respiration block Rb+ uptake and dissipated a preformed gradient. Preincubation of the vesicles with oxamate or p-chloromercuribenzoate inhibited Rb+ uptake, but their addition to respiring vesicles again did not cause efflux. Rb+ and DDA+ complete for uptake when present simultaneously. We conclude that the accumulation of both DDA+ and Rb+ occurs in response to an electrical gradient, vesicle interior negative, produced by respiration.[1]References
- Accumulation of lipid-soluble ions and of rubidium as indicators of the electrical potential in membrane vesicles of Escherichia coli. Altendorf, K., Hirata, H., Harold, F.M. J. Biol. Chem. (1975) [Pubmed]
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