Kinetics of Ca+ adsorption and cationic selectivity with a synaptic membrane protein
Abstract A study was conducted on the adsorption of 45 Ca 2+ to a surface film of a hydrophobic protein derived from synaptic membranes isolated from bovine cerebellum. A kinetic analysis of Ca 2+ displacement from the protein by various metal and organic cations could be described by a rate law based on diffusion and displacement. The relative rate constants for the displacement of bound Ca 2+ were in the order Li + + , Rb + + + , NH 4 + . Among the alkaline earth series the sequence was Mg 2+ , Sr 2+ 2+ . Ca 2+ adsorption could be described by a theoretical formulation which takes into account an interfacial energy and potential barrier as well as the diffusional process. An attempt was made to consider the effect of energy of hydration of the cations, surface charge, and the chemical environment at the interface on catonic selectivity. The behavior of the cations in this system significantly resemble their behavior in natural membranes, particularly excitatory ones. The structural and physicochemical environment of the protein at the interface is discussed in relation to Ca 2+ binding and cationic selectivity.