ION-BINDING CONSTANTS FOR GRAMICIDIN A OBTAINED FROM WATER PERMEABILITY MEASUREMENTS

Gramicidin A pores are permeable to water and small monovalent cations . For K, Rb, and Cs there is good evidence from conductances and perme ability ratios that a second ion can enter a pore already occupied by another, but for Na this evidence is inconclusive and comparison of tr acer fluxes and single channel conductances suggests that second ion e ntries are prohibited. Partly as a result of the complications of seco nd ion entry there have been widely differing estimates for the dissoc iation constants for the first ion in the channel. Dani and Levitt (19 81, Biophys. J. 35: 485-499) introduced a method for calculating ion b inding constants from simultaneous measurements of water fluxes and me mbrane conductance. They found no evidence for second ion binding and calculated dissociation constants of 115 mM for Li, 69 mM for K, and 2 mM for T1. It is shown here that the two-ion, four-state model predic ts a dependence of water permeability on ion concentration that is dif ficult to distinguish from the predictions of block by a single ion. U sing a modified technique that allows measurement of higher conductanc es, the first ion dissociation constants have been determined as 80 mM for Na, 40 mM for Rb and 15 mM for Cs. These values and those of Dani and Levitt fall in a smooth sequence. The dissociation constant for C s is consistent with single channel conductances and flux ratios. Ther e is a discrepancy between this constant for Na and the value, 370 mM, calculated from the single channel conductances and the assumption th at a second ion cannot enter or affect an occupied pore. The dissociat ion constant for Rb is intermediate between those for K and Cs whereas tracer flux measurements (Schagina, Grinfeldt & Lev, 1983. J. Membran e Biol. 73: 203-216) have suggested that Rb interacts much more strong ly with the channel than Cs.