AJP - Cell Physiology, Vol 259, Issue 1 C47-C55, Copyright © 1990 by American Physiological Society
Anionic amino acid transport systems in isolated basal plasma membrane of human placenta
S. D. Hoeltzli, L. K. Kelley, A. J. Moe and C. H. Smith
Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, Children's Hospital, St. Louis, Missouri 63110.
The placenta absorbs anionic amino acids from the maternal and fetal
circulations but does not significantly transfer these amino acids from
mother to fetus. Uptake of L-aspartate and L-glutamate by basal
(fetal-facing) plasma membrane vesicles from placental syncytiotrophoblast
was stimulated by an inward sodium and an outward potassium gradient.
Measurable saturable uptake was entirely sodium dependent and electrogenic.
Studies of concentration dependence resolved a high-affinity (microM)
system that has characteristics of the X-AG system found in other tissues
including the placental microvillous plasma membrane. Uptake of 0.2 microM
L-glutamate was inhibited by 2 mM L-glutamate, L-aspartate, D-aspartate,
L-cysteate, and L-cysteinesulfinic acid and was uninhibited by 2 mM
D-glutamate, L-glutamine, L-alanine, L-serine, L-asparagine, and taurine or
by 1 mM methylaminoisobutyric acid. The X-AG system in the two membranes of
the placental syncytiotrophoblast may mediate the concentrative uptake of
anionic amino acids from the maternal and fetal circulations into the
placenta.
