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.

Abstract

In the dark, the photoaffinity reagent, NAP-taurine, acted as a reversible inhibitor of [human] red cell anion exchange when it was present within the cell or in the external solution. A detailed analysis of the inhibition kinetics, however, revealed substantial differences in the responses to the probe at the 2 sides of the membrane. On the inside of the cell, NAP-taurine was a relatively low affinity inhibitor of Cl- exchange (Ki = 370 .mu.M). The effects of Cl- on NAP-taurine inhibition and the affinity of NAP-taurine for the system as a substrate were consistent with the concept that internal NAP-taurine competes with Cl- for the substrate site of the anion exchange system. External NAP-taurine was a far more potent inhibitor of Cl- exchange (Ki = 20 .mu.M). It acted at a site of considerably lower affinity for Cl- than the substrate site, probably the modifier site, at which halide anions are reported to cause a noncompetitive inhibition of Cl- transport. NAP-taurine therefore seemed to interact preferentially with the substrate or modifier site of the transport system, depending on the side of the membrane at which it was present. The modifier site is apparently accessible to NAP-taurine only from the outside whereas the transport site may be accessible from either side.