Reconstitution and partial purification of cardiac sarcolemmal Na+/H+ (K+/H+) antiporter

Most of the plasma membrane Na+/H+ antiporters that have been well characterized do not transport K+. Our previous studies showed that cardiac plasma membrane contains a nonselective alkali cation/H+ antiporter that transports either Na+ or K+. The purpose of this work was to develop a convenient assay for the cardiac antiporter to be used in the course of its purification. Sarcolemmal membranes prepared from bovine heart were solubilized with Triton X-100 in the presence of a mixture of phospholipids, and the solubilized proteins were passed through Bio-Beads to form proteoliposomes. Exchange activities were assayed either by measuring H+ gradient-dependent uptakes of 22Na+ and 86Rb+ by proteoliposomes or by recording H+ release from proteoliposomes as reported by the fluorescence of a pH probe that was trapped in the proteoliposomes during reconstitution. Properties of the reconstituted nonselective antiporter were found to be similar to those of the native sarcolemmal antiporter. Using a DEAE-cellulose column, 20-fold purification of the antiporter was achieved, demonstrating the suitability of the reconstitution assay procedures for further purification of the functional antiporter.