Characterization of the Band 3 substrate site in human red cell ghosts by NDS-TEMPO, a disulfonatostilbene spin probe: the function of protons in NDS-TEMPO and substrate-anion binding in relation to anion transport.

NDS-TEMPO is a specific disulfonatostilbene spin label for the Band 3 substrate site (K.F. Schnell, W. Elbe, J. Käsbauer & E. Kaufmann, Biochim. Biophys. Acta 732:266-275, 1983). The pH dependence of NDS-TEMPO binding and of chloride and sulfate binding was studied in resealed human erythrocyte ghosts. pH was varied from 6.0 to 9.0. The ESR spectra from NDS-TEMPO-labeled red cell ghosts exhibited a strong immobilization of membrane-bound NDS-TEMPO. Changes of pH had no effect upon the mobility of membrane-bound NDS-TEMPO. A mutual competition between NDS-TEMPO binding and the binding of the substrate-anions, chloride and sulfate, was observed throughout the entire pH range. The maximal number of NDS-TEMPO binding sites per cell was in the range of 9.0 X 10(5) to 1.10 X 10(6) and was found to be insusceptible to changes of pH. The NDS-TEMPO/substrate-site and the chloride/substrate-site dissociation constants amounted to 1.25 microM and to 17 mM and were independent of pH from pH 6.0 to 8.0, while the sulfate/substrate-site dissociation constant displayed a strong pH dependency with a maximum of approximately 50 mM at about pH 7.0. The NDS-TEMPO inhibition constants from the chloride and the sulfate flux experiments were 0.5 microM (0 degree C) and 1.8 microM (25 degrees C), respectively, and are in close accordance with the NDS-TEMPO/substrate-site dissociation constants. Our studies provide strong evidence for the assumption that NDS-TEMPO binds in fact to the substrate site of Band 3. They show that the strong pH dependence of the chloride and of the sulfate transport cannot result from the pH dependency of substrate-anion binding, but point to the participation of ionizable regulator sites in transport catalysis. These regulator sites seem to be positioned outside the substrate site of the Band 3 transport domain.