D-Enantiomers Take a Close Look at the Functioning of a Cardiac Cationic L-Amino Acid Transporter

Cationic amino acid transporters are highly selective for L-enantiomers such as L-arginine (L-Arg). Because of this stereoselectivity, little is known about the interaction of these transporters with D-isomers. To study whether these compounds can provide information on the molecular mechanism of transport, inward currents activated by L-Arg with low apparent affinity were measured in whole-cell voltage-clamped cardiomyocytes as a function of extracellular L-Arg and D-Arg concentrations. D-Arg inhibited L-Arg currents in a membrane-potential (VM)-dependent competitive manner, indicating the presence of D-Arg binding sites in the carrier. Analysis of these steady-state currents showed that L- and D-Arg binding reactions dissipate a similar small fraction of the membrane electric field. Since D-Arg is not transported, these results suggest that enantiomer recognition occurs at conformational transitions that initiate amino acid translocation. The VM dependence of maximal current levels suggests that inward currents arise from the slow outward movement of negative charges in the unliganded transporter. Translocation of the L-Arg-bound complex, on the other hand, appears to be electroneutral. D-Arg-dependent transient charge movements, also detected in these cells, displayed a VM-dependent charge distribution and kinetics that are consistent with amino acid binding in an ion well in a shallow, water-filled extracellular binding pocket.