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Electrogenic Cl- Pump in Acetabularia

D. Gradmann, J. Tittor and Valentina Goldfarb
Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences
Vol. 299, No. 1097, The Binding and Transport of Anions in Living Tissues (Dec. 1, 1982), pp. 447-457
Published by: Royal Society
Stable URL: http://www.jstor.org/stable/2395787
Page Count: 11
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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 (Ip(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 Ip(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.

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