Asymmetry of the red cell anion exchange system. Different mechanisms of reversible inhibition by N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate (NAP-taurine) at the inside and outside of the membrane.

In the dark, the photoaffinity reagent, N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate (NAP-taurine), acts as a reversible inhibitor of red cell anion exchange when it is present either within the cell or in the external solution. A detailed analysis of the inhibition kinetics, however, reveals substantial differences in the responses to the probe at the two sides of the membrane. On the inside of the cell, NAP-taurine is a relatively low affinity inhibitor of chloride exchange (Ki = 370 microM). Both the effects of chloride on NAP-taurine inhibition and the affinity of NAP-taurine for the system as a substrate are consistent with the concept that internal NAP-taurine competes with chloride for the substrate site of the anion exchange system. External NAP-taurine, on the other hand, is a far more potent inhibitor of chloride exchange (Ki = 20 microM). It acts at a site of considerably lower affinity for chloride than the substrate site, probably the modifier site, at which halide anions are reported to cause a noncompetitive inhibition of chloride transport. NAP-taurine therefore seems to interact preferentially with either the substrate or modifier site of the transport system, depending on the side of the membrane at which it is present. It is suggested that the modifier site is accessible to NAP-taurine only from the outside whereas the transport site may be accessible from either side.