The maxi cation channel from the plasma membrane of rye (Secale cereale L.) roots was studied following its incorporation into planar phosphatidylethanolamine bilayers. Current recordings were made in the presence of 100-mm KCl containing quinine on both sides of the bilayer. Quinine produced voltage- and concentration-dependent blockade of the channel, reducing its apparent unitary current and open probability. The voltage-dependence suggested that blockade was effected from the cytoplasmic side by cationic quinine. Blockade was modelled using a kinetic scheme with two independent blocked states termed B1 and B2 (B1⇆O⇄B2). Rate constants promoting fast kinetics (k 1 and k −1 ) were found to be several orders of magnitude greater than those promoting slow kinetics (k 2 and k −2 ). Analysis of the fast kinetics indicated that the rate constants for blockade of the open channel at the first site (k 1 ) and its clearance (k −1 ) had voltage-dependencies (zδ p ) of 0.41 and −0.71, respectively, and that the equilibrium dissociation constant for the binding site (K d (0)) was about 1 mm. Analysis of the slow kinetics indicated that the rate constants for blockade of the open channel at the second site (k 2 ) and its clearance (k −2 ) had zδ p values of 0.12 and −1.27, respectively. The K d (0) value for the second binding site was about 10 mm.
The Journal of Membrane Biology – Springer Journals
Published: Aug 1, 1998
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