Titration of transport and modifier sites in the red cell anion transport system

This work demonstrates the existence of titratable transport and modifier sites in the anion transport system of human red cells. Effects of alkaline extracellular pH on chloride exchange were studied up to pH 13 at 0 degrees C. The studies revealed two sets of reversible titratable groups. One set, having a pK of or approximately 11, appeared to be identical with the inhibitory halide-binding modifier site. Deprotonation of this site stimulated anion transport. The apparent dissociation constants of chloride and iodide at this modifier site were 0.3 and 0.06 M, respectively, and it was confirmed that the organic sulfonate NAP-taurine inhibits anion transport reversibly by a high-affinity interaction with halide-binding modifier sites at the extracellular side of the membrane. Other groups, with apparent pK of or approximately 12 at chloride concentrations above 0.1 M, were named as "transport sites" because transport function depended totally on their protonation. The apparent pK decreased when extracellular halide concentrations was lowered below 0.1 M. It was dependent of the intracellular chloride concentration, and was equally sensitive to extracellular pH of 13, was fully reversible. Hydroxyl ions were not transported to an appreciable extent by the anion exchange system. The pK values of both sets of groups make it likely that they are both arginyl residues, functioning as anion recognition sites similar to the role of functionally essential arginyl residues observed with numerous enzymes.