Interactions of NIP-taurine, NAP-taurine, and Cl- with the human erythrocyte anion exchange system

N-(4-isothiocyano-2-nitrophenyl)-2-aminoethanesulfonate (NIP-taurine), a newly synthesized isothiocyano derivative of N-(4-azido-2-nitrophenyl)-2-aminoethanesulfonate (NAP-taurine), is a potent inhibitor of human erythrocyte chloride exchange. At 0 degrees C, the inhibition is reversible, but at 37 degrees C, NIP-taurine inhibits irreversibly, indicating that it may be a useful label for its binding site. When present at the outside of the cell, NIP-taurine binds with low affinity to a site that seems to be the Cl- transport site (on the basis of its affinity for Cl-) and with much higher affinity to a different site, MN, which has a much lower affinity for Cl-. In this respect, NIP-taurine resembles NAP-taurine, and an analysis of interactions between these two probes is consistent with the idea that they bind to the same two sites. The affinity of NIP-taurine for the high-affinity MN site is enhanced by about fourfold when the transport protein, band 3, is in the conformation with the transport site facing outward (Eo), as compared with the conformation with the transport site facing inward (Ei). External Cl-, but not cytoplasmic Cl-, competes with NIP-taurine for binding to the external, high affinity site. Thus NIP-taurine provides a label for an external site, at which Cl- and perhaps other anions bind, which is separate from both the transport site and the cytoplasmic modifier site at which high Cl- concentrations inhibit Cl- exchange.