Anionic amino acid uptake by microvillous membrane vesicles from human placenta.
The transport mechanisms for anionic amino acids in trophoblast microvillous (maternal facing) membrane were investigated by characterization of L-[3H]aspartate and L-[3H]glutamate uptake in membrane vesicles. Uptake of the anionic amino acids was by a single high-affinity Na+-dependent K+-stimulated cotransporter that is pH sensitive and electrogenic. A second Na+-dependent transporter could not be discriminated, and there was no observable Na+-independent uptake. An outwardly directed K+ gradient (100 mM KCl inside) resulted in a 5- to 10-fold stimulation in glutamate uptake in the presence of Na+. Intravesicular KCl had no effect on transporter affinity but increased transporter velocity in a concentration-dependent manner. Inhibition of Na+-K+-dependent uptake of L-aspartate and L-glutamate (20 mM, 30 s) by 2 mM unlabeled amino acids demonstrated stereoselectivity for L-glutamate but not for L-aspartate. The neutral amino acids (L-alanine, L-threonine, L-serine, L-cysteine, L-phenylalanine) were not effective inhibitors. These data are consistent with an anionic amino acid transporter in the microvillous membrane of the trophoblast, which has characteristics qualitatively similar to the X-AG system found in other epithelia. This system may mediate the concentrative placental uptake of anionic amino acids from maternal blood in utero.
Moe
A J
AJ
Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, Children's Hospital, St. Louis, Missouri 63110.
Smith
C H
CH
D000596:Amino Acids / Q000378:metabolism*; D000838:Anions / Q000378:metabolism; D001224:Aspartic Acid / Q000037:antagonists & inhibitors; D004563:Electrochemistry; D005260:Female; D005971:Glutamates / Q000378:metabolism; D018698:Glutamic Acid; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008871:Microvilli / Q000378:metabolism; D009994:Osmolar Concentration; D010920:Placenta / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D011189:Potassium Chloride / Q000378:metabolism; D011247:Pregnancy; D012964:Sodium / Q000494:pharmacology; D013997:Time Factors
D000596:Amino Acids; D000838:Anions; D005971:Glutamates; D001224:Aspartic Acid; D018698:Glutamic Acid; D011189:Potassium Chloride; D012964:Sodium; D011188:Potassium