Chloride net efflux from intact erythrocytes under slippage conditions. Evidence for a positive charge on the anion binding/transport site

Tracer chloride and potassium net efflux from valinomycin-treated human erythrocytes were measured into media of different chloride concentrations, Clo, at 25 degrees C and pH 7.8. Net efflux was maximal [45-50 mmol (kg cell solids)-1 min-1] at Clo = 0. It decreased hyperbolically with increasing Clo to 14-16 mmol (kg cell solids)-1 min-1. Half-maximal inhibition occurred at Clo = 3 mM. In the presence of the anion exchange inhibitor DNDS, net efflux was reduced to 5 mmol (kg cell solids)-1 min-1, independent of Clo. Of the three phenomenological components of net efflux, the Clo-inhibitable (DNDS-inhibitable) component was tentatively attributed to "slippage," that is, net transport mediated by the occasional return of the empty transporter. The Clo-independent (DNDS-inhibitable) component was tentatively attributed to movement of chloride through the anion transporter without the usual conformational change of the transport site on the protein ("tunneling"). These concepts of slippage and tunneling are shown to be compatible with a model that describes the anion transporter as a specialized single-site, two-barrier channel that can undergo conformational changes between two states. Net chloride efflux when the slippage component dominated (Clo = 0.7 mM) was accelerated by a more negative (inside) membrane potential. It appears that the single anion binding-and-transport site on each transporter has one net positive charge and that is neutralized when a chloride ion is bound.