The anion-transfer system of erythrocyte membranes. N-(7-Nitrobenzofurazan-4-yl)taurine, a fluorescent substrate-analogue of the system

The fluorescent probe Nbd-Tau [N-(7-nitrobenzofurazan-4-yl)taurine] was synthesized and evaluated as a potential substrate of the anion-transport system of human erythrocyte membrane. The probe inhibited Cl- exchange in a competitive manner from either surface of the membrane, displaying Ki values in the mM range at the inner surface and in the microM range at the outer surface. Inhibition from within cells was via interaction with Cl--transport sites, whereas from it was via interaction with sites of unidentified nature. Nbd-Tau efflux from cells was monitored fluorimetrically in a continuous mode by a novel method that circumvents separation of the cells from the medium. Using this method, it is shown that Nbd-Tau efflux fulfils the following criteria of a substrate of the anion transport system: (a) susceptibility to classical and specific inhibitors of the system; (b) competitive inhibition with Cl- for anion-transport sites; and (c) temperature coefficient comparable with that of Cl- exchange. The fluorometric method is highly sensitive, versatile, and kinetically informative. With minor modifications it can be used for measuring anion transport across “ghost” and isolated membrane vesicles.