Cation-binding Capacity of Membranes Isolated from Micrococcus lysodeikticus

Abstract

A study was made of H + , Na + , K + , Ca ++ , and Mg ++ binding and ion-exchange properties of the plasma-mesosome membrane system isolated from Micrococcus lysodeikticus strain NCTC 2665. Titration curves were obtained on membranes prepared according to the method of M. R. J. Salton and further exposed to p H 4 for 4 hr (membranes-H). The dissociation coefficients and binding capacities were obtained by applying the mass law equation and the plot of G. Schatchard to the data. The membranes-H possess four kinds of dissociable groups with p K 4.96, 4.18, 3.60, and 3.09, respectively, and a total binding capacity of 0.65 meq/g (dry weight). Potentiometric titrations of cations in the presence and in the absence of membranes-H show that cations (Na + , K + , Ca ++ , and Mg ++ ) are bound by the dissociated groups of the membrane. The fall in p H value for bivalent cations is greater than that for monovalent cations. Cations of the same valency produce equal diminutions on p H. Furthermore, ion-exchange tests carried out on membranes saturated with Mg ++ or Na + and suspended in a medium containing 45 Ca show that the cations are reversibly bound.