Liposomes containing 150 mM KCl and 0.48 mM sulfobromophthalein have been prepared. The internal pH was set at 6.5, a value at which sulfobromopthalein is colorless. When brought to alkaline pH a certain amount of the dye is deprotonated and can be read spectrophotometrically as external sulfobromophthalein. Upon addition of Triton X-100 the membrane is dissolved and all sulfobromophthalein present in the preparation may be measured. Addition of bilitranslocase to such a preparation of liposomes causes the internal sulfobromophthalein to leave the internal compartment. The rate of this phenomenon may be followed directly and shown to be greatly accelerated by the addition of valinomycin. The latter finding indicates that sulfobromophthalein transport occurs in response to a membrane diffusion potential created by permeabilisation to K+ of liposomes brought about by valinomycin (uniport). The permeability change induced by bilitranslocase is specific and does not reflect an alteration of the normal impermeability of liposomes to small ions such as protons or Ca2+.
