Effects of the transport site conformation on the binding of external NAP-taurine to the human erythrocyte anion exchange system. Evidence for intrinsic asymmetry.
External N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate (NAP-taurine) inhibits human red cell chloride exchange by binding to a site that is distinct from the chloride transport site. Increases in the intracellular chloride concentration (at constant external chloride) cause an increase in the inhibitory potency of external NAP-taurine. This effect is not due to the changes in pH or membrane potential that usually accompany a chloride gradient, since even when these changes are reversed or eliminated the inhibitory potency remains high. According to the ping-pong model for anion exchange, such transmembrane effects of intracellular chloride on external NAP-taurine can be explained if NAP-taurine only binds to its site when the transport site is in the outward-facing (Eo or EClo ) form. Since NAP-taurine prevents the conformational change from EClo to ECli , it must lock the system in the outward-facing form. NAP-taurine can therefore be used just like the competitive inhibitor H2DIDS (4,4'-diisothiocyano-1,2- diphenylethane -2,2'-disulfonic acid) to monitor the fraction of transport sites that face outward. A quantitative analysis of the effects of chloride gradients on the inhibitory potency of NAP-taurine and H2DIDS reveals that the transport system is intrinsically asymmetric, such that when Cli = Clo, most of the unloaded transport sites face the cytoplasmic side of the membrane.
D027321:Anion Transport Proteins; D000838:Anions / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D002712:Chlorides / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials; D008954:Models, Biological; D013654:Taurine / Q000031:analogs & derivatives* / Q000378:metabolism; D014018:Tissue Distribution
D027321:Anion Transport Proteins; D000838:Anions; D002352:Carrier Proteins; D002712:Chlorides; D013654:Taurine; C010861:NAP taurine