CHARACTERIZATION OF AMINO-ACID-TRANSPORT SYSTEMS IN HUMAN PLACENTAL BASAL MEMBRANE-VESICLES

The amino acid transport systems have been characterized in basal membrane vesicles prepared from human full-term placental syncytiotrophoblasts. Transport of amino acids across basal membranes occurred via passive diffusion and Na+-independent and Na+-dependent carrier-mediated systems. Passive diffusion was responsible for a substantial fraction of transport. l-Glutamate and α-(methylamino)isobutyrate were transported only Na+-independently, while the transport of l-alanine was dependent solely on an Na+ gradient from the outside to the inside of the vesicles. l-Methionine, l-leucine, glycine and l-proline transport were supported by both Na+-independent and Na+-dependent systems. l-Lysine transport was decreased in the presence of cations, an inwardly directed Na+ gradient was much more effective than a K+ gradient at slowing l-lysine transport. A cross-inhibition analysis of these amino acids indicates that at least three Na+-independent and five Na+-dependent carrier-mediated systems exist in the human placental syncytiotrophoblast basal membranes. One Na+-independent system interacts with all substrates tested. Another Na+-independent system carries glycine, l-methionine, l-leucine and l-lysine; it is sensitive to l-glutamate, but not to l-proline or α-(methylamino)isobutyrate. The third system is selective for l-lysine, which is inhibited by l-methionine, glycine and l-leucine, but inaccessible to l-glutamate, l-proline and α-(methylamino)isobutyrate. One Na+-dependent system carries l-alanine, glycine, l-methionine and l-leucine, and it is sensitive to l-proline. The second system mediates transport of l-alanine, glycine, l-methionine and l-proline, but is not sensitive to l-leucine. The third system carries l-alanine, glycine and l-proline, and is inaccessible to l-methionine and l-leucine. The fourth system is responsible for l-methionine and l-leucine; it is sensitive to l-alanine and glycine, but not to l-proline. The fifth system is selective for l-proline. © 1990.