Kinetics of interfacial ATP adsorption and anionic exchange with a synaptic membrane protein

A study was conducted on the adsorption of 14C-ATP to a surface film of a hydrophobic protein derived from synaptic membranes isolated from bovine cerebellum. The adsorption of ATP to the protein film followed a rate law based on diffusion and an energy barrier to adsorption, the rate law being generally applicable to the adsorption of ions to a charged interface. Studies were also carried out on the displacement of ATP from the film by the injection of other nucleotides, inorganic phosphates, and other anions. The kinetics conformed to a rate law based on diffusion and displacement. The most important factor in the displacement of ATP was the magnitude of charge of the anion, while steric factors were relatively minor. However, from a consideration of the energy barrier to adsorption it appeared that steric factors play a greater role in the adsorption of ATP. The results are discussed in relationship to the configurational aspects of the surface film as well as their possible significance in synaptic function.