Electrogenic Cl - pump in Acetabularia

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Abstract
Measurements of the transmembrane potential difference ( V ) under various conditions have demonstrated the operation of an electrogenic Cl - pump in the outer plasma membrane (plasmalemma) of the unicellular marine alga Acetabularia . In preparations of partly purified membranes (containing plasmalemma), there is Cl - -stimulated, N , N '-dicyclohexylcarbodiimide-insensitive, vanadate-sensitive ATPase activity with a pH optimum around pH 6.5. These properties are consistent with the assumption that the electrogenic Cl - pump is an ATPase. In order to investigate electrical details of this ‘ Mitchellian ’ type of charge-translocating enzyme, steady-state current-voltage curves of the electrogenic pump ( I p ( V )) were measured in vivo under dark and light conditions and analysed by a two-state reaction kinetic model. This model with the resulting parameters predicts V -sensitive, unidirectional Cl - effluxes through the pump. The predictions of this model agree with the experimental results. Green light causes a fast decrease of V , which is explained as a disturbance of the pump cycle. Relaxation studies on this effect and reaction kinetic analysis of I p ( V ) under different external Cl - concentrations are used to develop a consistent three-state model of the pump that includes the order of and absolute rate constants of individual reactions, states of charge, stoichiometry, voltage-sensitivity and density of the pump molecules in the membrane.