Proton-sulfate co-transport: mechanism of H+ and sulfate addition to the chloride transporter of human red blood cells.

Abstract

Proton and sulfate inhibition of the obligatory Cl-Cl exchange of human erythrocytes was measured at 0.degree. C to determine their mechanism of reaction with the anion transporter. The proton and sulfate co-transported by this mechanism at higher temperatures behaved as nontransported inhibitors at 0.degree. C. The data were analyzed in terms of 4 molecular mechanisms: HSO4- addition to the transporter; ordered addition with the proton first; ordered addition with the sulfate first; random addition to the transporter. The Dixon plots of 1/MCl vs. [SO4] at different proton concentrations were not parallel. Thus protons and sulfate ions were not mutually exclusive inhibitors. The slope of these Dixon plots was independent of pH above 7.0, which indicates that sulfate could bind to the unprotonated carrier and excludes the first 2 mechanisms. Protons were inhibitors of Cl flux in the absence of sulfate, which indicates that protons could bind to the unloaded carrier and excludes mechanism 3. The KI for sulfate was 4.35 .+-. 0.36 mM. The pK for the protonatable group was 5.03 .+-. 0.02. The binding of either a proton or sulfate to the carrier decreased the KI of the other by 9-fold. The only simple mechanism consistent with the data is a random-ordered mechanism with more transporters loaded with a sulfate than loaded with a proton at the pH and sulfate concentrations of plasma.