Measurement of Transmembrane Potentials in Phospholipid Vesicles

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Abstract
Phosphatidylcholine vesicles are permeable to tempotartrate, a spin-label derivative of tartaric acid. The inside-outside distribution of tempotartrate is coupled to the inside-outside distribution of H(+), so it must be a measure of the transmembrane electrical potential difference in vesicles permeable to H(+). This prediction is borne out by the finding that the inside-outside distribution of tempotartrate is the reciprocal of the inside-outside distribution of K(+) in vesicles prepared in the presence of valinomycin. The inside-outside distribution of tempotartrate is, by contrast, equal to the inside-outside distribution of Cl(-) in vesicles without valinomycin. This is evidence that an inside-outside Cl(-) concentration gradient induces an H(+) gradient, which must be due to HCl permeation.