Effect of calcium ions on black lipid membranes of phosphatidylinositol and oxidized cholesterol in presence of protein forming pore

The divalent calcium ions have been found to affect the surface potential of the membranes of nerve and of the black lipid membranes (BLM). In fact the addition of calcium to the bilayer formed of phosphatidylserine decreases the magnitude of surface potential either by screening and/or binding effects. Furthermore, calcium influences the conductance of the neutral carrier nonactine across BLM (S. Mclaughin, 1977: Current Topics in Membranes and Transport, 9, 71). We have studied the effect of this ion on the incorporation of the porin in BLM formed by phosphatidylinositol (PI) and oxidized cholesterol (OxChol). The artificial membranes were formed as previously reported (G. Monticelli et al., 1990, The Tenth School on Biophysics of Membrane Transport, School Proceedings, l: 329, Szczyrk, Poland). PI was extracted from ox brain and was chromatografícally pure; the OxChol was obtained as described by H. T. Tien et al. (Nature, 1966, 212, 718). The porin, purified by heart mithocondria, kindly given by Prof. F. Palmieri. The medium bathing the BLM was KCl 1 M and KCl plus CaCl2 1-10 mM, and porin (at different concentrations) was added at membrane black on the two aqueous bathing solutions. The conductance and the capacitance of the BLM have been measured as previously reported (G. Monticelli et al., 1990, The Tenth School on Biophysics of Membrane Transport, School Proceedings, 1: 329, Szczyrk, Poland). The presence of calcium ions in the medium makes the BLM more stable. The addition of porin to the aqueous phases of the both kind of BLM resulted in an increase of the conductance; but the kinetic is "S" shaped for the PI and hyperbolic for the OxChol membranes; furthermore the increase and the responsiveness were different, in that PI membranes needed an higher concentration of porin to determine a large increase of the conductance compared with OxChol membranes. Calcium ions (at the concentrations used), by exerting a binding and screening effect on the surface of the membrane, affect either the maximum value of the conductance and the speed at which this value is reached when the porin is present.