Kinetics and ion specificity of Na +/Ca 2+ exchange mediated by the reconstituted beef heart mitochondrial Na +/Ca 2+ antiporter

Abstract

The Na +/Ca 2+ antiporter was purified from beef heart mitochondria and reconstituted into liposomes containing fluorescent probes selective for Na + or Ca 2+. Na +/Ca 2+ exchange was strongly inhibited at alkaline pH, a property that is relevant to rapid Ca 2+ oscillations in mitochondria. The effect of pH was mediated entirely via an effect on the K m for Ca 2+. When present on the same side as Ca 2+, K + activated exchange by lowering the K m for Ca 2+ from 2 to 0.9 μM. The K m for Na + was 8 mM. In the absence of Ca 2+, the exchanger catalyzed high rates of Na +/Li + and Na +/K + exchange. Diltiazem and tetraphenylphosphonium cation inhibited both Na +/Ca 2+ and Na +/K + exchange with IC 50 values of 10 and 0.6 μM, respectively. The V max for Na +/Ca 2+ exchange was increased about fourfold by bovine serum albumin, an effect that may reflect unmasking of an autoregulatory domain in the carrier protein. © 2004 Elsevier B.V. All rights reserved.