Human placental L-tyrosine transport: a comparison of brush-border and basal membrane vesicles.
1. The mechanisms responsible for L-tyrosine transport at both the maternal-facing and fetal-facing surfaces of the human full-term placenta have been studied using isolated brush-border and basal membrane vesicles under conditions where a direct comparison of the transport properties of the two membranes can be made. 2. Brush-border vesicle uptake of L-tyrosine was substantially into an osmotically active space. Transport was Na(+)-independent, N-ethylmaleimide-sensitive (half-maximal inhibition, Ki = 1.1 mM), and insensitive to pH over the range 5.5-8.5. The initial rate of brush-border L-tyrosine uptake as a function of concentration showed saturation and obeyed Michaelis-Menten kinetics with Michaelis constant (Km) and maximum velocity (Vmax) values of 54.2 microM and 1.28 pmol (mg protein)-1 s-1, respectively. Influx of L-tyrosine was stereospecific and was virtually completely abolished by L-phenylalanine, L-tryptophan, L-leucine or by 2-aminobicycloheptane-2-carboxylic acid. These properties suggest that system L is responsible for brush-border L-tyrosine transport. 3. Basal membrane transport of L-tyrosine was more complex and uptake was slower than that found in the brush border. Although, as in the brush-border membranes, uptake was completely Na(+)-independent, N-ethylmaleimide was a less effective inhibitor, there was stimulation of transport at more alkaline pH and uptake did not show marked stereospecificity. An apparent Km of 168.9 microM and a Vmax of 0.31 pmol (mg protein)-1 s-1 were calculated for basal L-tyrosine transport. There was clear inhibition by L- and D-tyrosine, L-phenylalanine and L-tryptophan. 2-Aminobicycloheptane-2-carboxylic acid was not as effective. 4. These findings suggest the existence of non-identical carrier-mediated transport systems for L-tyrosine in brush-border and basal membranes. Brush-border transport resembles that by system L; L-tyrosine transport at the basal membrane may be via system t.
Kudo
Y
Y
Department of Human Anatomy, University of Oxford.
Boyd
C A
CA
D000596:Amino Acids / Q000378:metabolism; D001485:Basement Membrane / Q000378:metabolism; D001693:Biological Transport, Active / Q000187:drug effects; D005033:Ethylmaleimide / Q000494:pharmacology; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008871:Microvilli / Q000378:metabolism; D009994:Osmolar Concentration; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D012964:Sodium / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism*; D014634:Valinomycin / Q000494:pharmacology
D000596:Amino Acids; D014634:Valinomycin; D014443:Tyrosine; D012964:Sodium; D005033:Ethylmaleimide