Studies of the Ca²⁺ transport mechanism of human erythrocyte inside-out plasma membrane vesicles. III. Stimulation of the Ca²⁺ pump by anions

The effects of various anions on calcium transport and calcium-stimulated magnesium-ATPase in human erythrocyte inside-out vesicles (IOV) was investigated. Chloride, sulfate, acetate, and phosphate stimulated both calcium transport and (Ca²⁺-Mg²⁺)-ATPase. However, a specific anion did not stimulate calcium transport and (Ca²⁺-Mg²⁺)-ATPase to a similar degree. As a consequence, the ratio of calcium transported to ATP hydrolyzed was found to vary from 0.87 in the case of phosphate to 0.25 in the case of chloride. The molar ratios of calcium to anion transported were 0.6, 2.0, and 1.5, for chloride, acetate, and sulfate, respectively, suggesting that calcium uptake proceeds by an electroneutral mechanism. The stimulation of calcium transport by the anions was additive. Conversely, uptake of subsaturating phosphate was inhibited by the presence of other anions. This suggests that phosphate, sulfate, chloride, and acetate activate calcium transport by a similar mechanism and are transported into IOV by a similar pathway, presumably band III. Approximately 4.0 gluconate molecules were taken up per calcium transported. However, the inability of gluconate to inhibit phosphate transport or to stimulate calcium transport additively with other anions suggests that gluconate may move into IOV as an uncharged species. These results suggest that the pumping of calcium by the human erythrocyte membrane may proceed by a mechanism involving calcium/anion transport. A role for band III as the anion transporter is suggested.