Sodium-dependent carnitine transport in human placental choriocarcinoma cells

The JAR human placental choriocarcinoma cells were found to transport carnitine into the intracellular space by a Na+-dependent process. The transport showed no requirement for anions. The Na+-dependent process was saturable and the apparent Michaelis-Menten constant for carnitine was 12.3 +/- 0.5 mu M. Na+ activated the transport by increasing the affinity of the transport system for carnitine. The transport system specifically interacted with L-carnitine, D-carnitine, acetyl-DL-carnitine and betaine. 6-N-Trimethyllysine and choline had little or no effect on carnitine transport. Of the total transport measured, transport into the intracellular space represented 90%. Plasma membrane vesicles prepared from JAR cells were found to bind carnitine in a Na+-dependent manner. The binding was saturable with an apparent dissociation constant of 0.66 +/- 0.08 mu M. The binding process was specific for L-carnitine, D-carnitine, acetyl-DL-carnitine, and betaine. 6-N-Trimethyllysine and choline showed little or no affinity. It is concluded that the JAR cells express a Na+-dependent high-affinity system for carnitine transport and that the Na+-dependent high-affinity carnitine binding detected in purified JAR cell plasma membrane vesicles is possibly related to the transmembrane transport process.