The role of chloride in taurine transport across the human placental brush-border membrane.
Taurine, a sulfated beta-amino acid, is conditionally essential during development. A maternal supply of taurine is necessary for normal fetal growth and neurologic development, suggesting the importance of efficient placental transfer. Uptake by the brush-border membrane (BBM) in several other tissues has been shown to be via a selective Na(+)-dependent carrier mechanism which also has a specific anion requirement. Using BBM vesicles purified from the human placenta, we have confirmed the presence of Na(+)-dependent, carrier-mediated taurine transport with an apparent Km of 4.00 +/- 0.22 microM and a Vmax of 11.72-0.36 pmol mg-1 protein 20 s-1. Anion dependence was examined under voltage-clamped conditions, in order to minimize the contribution of membrane potential to transport. Uptake was significantly reduced when anions such as thiocyanate, gluconate, or nitrate were substituted for Cl-. In addition, a Cl(-)-gradient alone (under Na(+)-equilibrated conditions) could energize uphill transport as evidenced by accelerated uptake (3.13 +/- 0.8 pmol mg-1 protein 20 s-1) and an overshoot compared to Na+, Cl- equilibrated conditions (0.60 +/- 0.06 pmol mg-1 protein 20 s-1). A Cl(-)-gradient (Na(+)-equilibrated) also stimulated uptake of [3H]taurine against its concentration gradient. Analysis of uptake in the presence of varying concentrations of external Cl- suggested that 1 Cl- ion is involved in Na+/taurine cotransport. We conclude that Na(+)-dependent taurine uptake in the placental BBM has a selective anion requirement for optimum transport. This process is electrogenic and involves a stoichiometry of 2:1:1 for Na+/Cl-/taurine symport.
Moyer
M S
MS
Department of Pediatrics, Yale University School of Medicine, New Haven, CT 06510.
Insler
N
N
Dumaswala
R
R
D000469:Alkaline Phosphatase / Q000032:analysis; D001692:Biological Transport / Q000187:drug effects; D002712:Chlorides / Q000494:pharmacology*; D006801:Humans; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008871:Microvilli / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D010920:Placenta / Q000187:drug effects* / Q000378:metabolism; D011506:Proteins / Q000032:analysis; D012965:Sodium Chloride / Q000494:pharmacology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000032:analysis; D013654:Taurine / Q000378:metabolism*; D013861:Thiocyanates / Q000494:pharmacology; D014316:Tritium
D002712:Chlorides; D011506:Proteins; D013861:Thiocyanates; D014316:Tritium; D013654:Taurine; D012965:Sodium Chloride; D000469:Alkaline Phosphatase; D000254:Sodium-Potassium-Exchanging ATPase