Ionophore-mediated cation translocation in artificial systems. I. A23187-mediated calcium translocation.

The inophore A23187 stimulates the translocation of calcium from an aqueous Hepes buffer into an organic immiscible phase. At saturating calcium concentrations, 2 molecules of ionophore seem to complex each atom of calcium. Consistent with such a stoichiometric behaviour, the apparent ratio of calcium-ionophore association to dissociation rate constants increases as the concentration of ionophore is raised. As a result, at low calcium concentrations, the amount of translocated calcium increases as a power function of A23187 concentration. When allowance is made for such a phenomenon, the relation between calcium translocation and concentration is characterized by usual substrate-receptor binding kinetics.