The relationship between substrate dissociation constants derived from transport experiments and from equilibrium binding assays. Implications of the conventional carrier model.

A kinetic analysis of substrate and inhibitor binding, based on the conventional carrier model, leads to the following conclusions. The substrate constant derived from equilibrium binding studies is not a simple dissociation constant; rather, it is identical to the half-saturating substrate concentration for equilibrium exchange transport, which is a function of both the dissociation constant and the rate constants for carrier reorientation. In general, binding and transport constants are identical, assuming the same substrate distribution across the membrane in the two experiments. Binding studies reveal only a single substrate site--even if the carrier is unsymmetrical, with different substrate affinities on the two sides of the membrane. The binding constants for inhibitors are identical to the inhibition constants found in transport. These rules, which apply to a carrier imbedded in the cell membrane or free in solution, offer a means of deciding whether an isolated carrier retains the properties of the intact system.