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The role of chloride in taurine transport across the human placental brush-border membrane

1992; Elsevier BV; Volume: 1109; Issue: 1 Linguagem: Inglês

10.1016/0005-2736(92)90189-s

1879-2642

Mary Susan Moyer, Nancy F. Insler, Ranjana Dumaswala,

Neonatal Health and Biochemistry

Taurine, a sulfated ß-amino acid, is conditionally essential during development. A maternal supply of tarine 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 Kmof 4.00 ± 0.22 μM 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 o 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 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− ions 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.