Involvement of a histidine residue in inorganic phosphate and phosphoenolpyruvate transport across the human erythrocyte membrane.

Abstract

The transport of phosphoenolpyruvate across the erythrocyte membrane was compared with the transport of inorganic phosphate in resealed ghosts of human erythrocytes. Two conditions were employed: in one, the external pH (pHe) was varied from 6.0 to 7.5 while internal pH (pHi) was maintained at 7.2 or 6.2, whereas in the other, the internal and external pH were adjusted to have the same value over a pH range of 6.0 to 7.5. At a constant pHi of 7.2, the pH profile for the transport of phosphoenolpyruvate was different from that of inorganic phosphate; both pH profiles were similar to those previously observed in intact erythrocytes (Deuticke, B. (1970) Naturwissenschaften 57, 172-179; Hamasaki, N., et al. (1978) Biochem. J. 170, 39-46). In the absence of a transmembrane pH gradient, the pH profile for phosphoenolpyruvate transport was bell-shaped with the maximum at pH 6.8, and essentially the same pattern was seen with inorganic phosphate transport. The different pH profiles obtained under the two conditions suggests that one or more residues exposed at the inner surface of the membrane participate in the transport of inorganic phosphate and phosphoenolpyruvate. One of the most likely candidates is a histidine residue of the transport protein.