Sodium-dependent high-affinity binding sites for carnitine in human placental brush border membranes

The interaction of carnitine with human placental brush border membrane vesicles was investigated. Carnitine was found to associate with the membrane vesicles in a Na+ dependent manner. The time course of this association did not exhibit an overshoot, which is typical of a Na+ gradient-driven transport process. The absolute requirement for Na+ was noticeable whether the association of carnitine with the vesicles was measured with a short time incubation or under equilibrium conditions, indicating the presence of Na+-dependent binding sites for carnitine. The binding was saturable, with a dissociation constant of 1.37±0.03 μM. Anions had little or no influence on the binding process. The binding site was specific for carnitine and its acyl derivatives. Betaine also shared the binding site, but other related compounds failed to interact with the site. Kinetic analyses revealed that Na+ increased the affinity of the binding site for carnitine and the Na+/ carnitine coupling ratio for the binding process was 1. The dissociation constant for the interaction of Na+ with the binding site was 24±4 mM. This constitutes the first report on the identification of Na+-dependent high-affinity carnitine binding sites in the plasma membrane of a mammalian cell.