2 CATIONIC AMINO-ACID-TRANSPORT SYSTEMS IN HUMAN PLACENTAL BASAL PLASMA-MEMBRANES

Transport of cationic amino acids in basal (fetal facing) plasma membranes was investigated by characterization of L-[H-3]lysine and L-[H-3]arginine uptake in membrane vesicles isolated from term human placentas. At least two Na+-independent systems were present. Lysine concentration dependence data were fit by a two-system model with K(m) values of 1.0 +/- 0.8 and 223 +/- 57-mu-M and V(max) values of 0.06 +/- 0.03 and 24.0 +/- 5.8 pmol.mg protein-1.min-1. In the presence of either 10 mM L-leucine or Na+ plus 10 mM L-homoserine, the data were fit by single system models with kinetic parameters similar to the higher and lower K(m) systems seen in the absence of inhibitors. Uptake of 10 or 20-mu-M L-lysine in the absence of Na+ showed the higher K(m) system was inhibited completely by L-arginine, L-homoarginine, and L-histidine. In the presence of Na+, the higher K(m) system was inhibited completely by L-alanine, L-homoserine, L-leucine, L-phenylalanine, and L-norleucine. The lower K(m) system was inhibited completely by L-arginine, L-homoarginine, L-histidine, L-leucine, and L-methionine. Time course studies of uptake demonstrated that uptake by either system alone filled the total vesicular space. The basal membrane of human placental syncytiotrophoblast possesses two transport systems for lysine and arginine, resembling the ubiquitous y+ system and the b(o,+) system previously described in mouse blastocysts. The higher V(max) of the y+ system suggests that in utero it may mediate transfer of cationic amino acids from the syncytiotrophoblast to the fetus. The role of the high-affinity low-capacity b(o,+) system remains to be determined.