NIP-TAURINE AND NAP-TAURINE BIND TO EXTERNAL MODIFIER SITE OF AE1 (BAND-3), AT WHICH IODIDE INHIBITS ANION-EXCHANGE

External iodide (I-o(-)) inhibits AE1 (band 3)-mediated anion exchange in human red blood cells by binding to a noncompetitive inhibitory si te, the external halide modifier site. External N-(4-azido-2-nitrophen yl)-2-aminoethyl sulfonate (NAP-taurine) and N-(4-isothiocyano-2-nitro phenyl)-2-aminoethyl sulfonate (NIP-taurine) also inhibit Cl- exchange noncompetitively. Increasing I-o(-) decreases the inhibitory potency of NIP-taurine in a competitive fashion; this effect is not due to I- binding to the transport site, which has little effect on the NIP-taur ine affinity. Bis(sulfosuccinimidyl)-suberate (BSSS) abolishes the non competitive inhibitory effect of I-o(-) and greatly reduces the inhibi tory effect of NAP-taurine. Together with previous work, these data su ggest that external halides, such as I-, Br-, and probably also Cl-, b ind to the same noncompetitive inhibitory site as do NAP- and NIP-taur ine and that these reagents can be used to label the halide modifier s ite. Lys-539, a probable reaction site of BSSS, lies within the same s egment of AE1 that is labeled by NAP-taurine and thus may be part of t he modifier site.