The pH dependence of red cell membrane transport of titratible anions. An NMR study.

The titratible carrier model for erythrocyte anion transport predicts that the pH dependence of titratible anion transport should differ considerably from that for monovalent or divalent anions. The effects of pH on titratible anion influx rates have been determined for a series of related anions, the phosphate "analogs". These compounds are simply substituted phosphorus oxyacids, differing in the number and acidity of titratible protons. Influx of each analog at 60 mM concentration into Cl- loaded erythrocytes at constant intracellular pH (7.2) was measured over an extracellular pH range of 4.0 to 8.0 using 31P or 19F nuclear magnetic resonance spectroscopy. Influx rates peaked at pH values that increased with increasing anion pK. Using the Henderson-Hasselbalch equation to determine the effects of pH on the charges carried by a titratible anion and a titratible carrier, the pH at which the anion would most likely have the same charge as the carrier was found to be exactly midway between their respective pK values. The observed pH maxima for the phosphate analog influx curves are consistent with transport by a titratible carrier having a pK of 5.5.