The mechanism of anion transport across human red blood cell membranes as revealed with a fluorescent substrate: I. Kinetic properties of NBD-taurine transfer in symmetric conditions.

The molecular mechanism of anion exchange across the human red blood cell membrane was assessed with the fluorescent substrate analog NBD-taurine and the method of continuous monitoring of transport by fluorescence. The efflux of NBD-taurine was studied under a variety of experimental conditions such as temperature, pH and anion composition of cells and media. The temperature profile of NBD-taurine transfer from Cl-loaded cells into Cl media resembled that of Cl self-exchange, whereas that of NBD-taurine transfer from sulfate-loaded cells into sulfate media resembled that of sulfate self-exchange. Although the pH profiles of NBD-taurine transfer from Cl-loaded cells into Cl media and that of Cl self-exchange resembled each other, the analogous transfer with sulfate replacing Cl was markedly different. These and other data were analyzed and found to be consistent with a model which comprises the following: (a) a H+-titratable group in the carrier mechanism; (b) alteration of transport sites between the two sides of the membrane (i.e., ping-pong kinetics); and (c) transmembrane distribution of transport sites which is modulated by pH. It is shown that NBD-taurine transfer represents a tracer flux of a fluorescent substrate which gives a measure for the presence of monovalent transport sites at the inner surface of the membrane. The latter is markedly affected by the relative concentrations of anions and H+ on both sides of the red blood cell membrane.