Voltage‐dependent pump currents of the sarcoplasmic reticulum Ca2+‐ATPase in planar lipid membranes

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Abstract
Sarcoplasmic reticulum vesicles containing largely Ca2+‐ATPase were incorporated into planar lipid membranes. The ATPase was activated by a UV flash‐induced concentration jump of ATP from a photolabile caged ATP. Under these conditions stationary pump currents were observed. The dependence of these pump currents on applied voltages was investigated. The current‐voltage curve of the Ca2+‐ATPase shows monotonously increasing pump currents with increasing positive potentials of the ATP containing compartment. This indicates the existence of electrogenic steps in the direction of the transported Ca2+ ions. From the extrapolated reversal potentials of the curve is concluded that less than four positive net charges are transported per hydrolyzed ATP.

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