title: Sodium-gradient-driven, high-affinity, uphill transport of succinate in human placental brush-border membrane vesicles.
abstract: Brush-border membrane vesicles isolated from normal human term placentas were shown to accumulate succinate transiently against a concentration gradient, when an inward-directed Na+ gradient was imposed across the membrane. This uptake was almost totally due to transport into intravesicular space, non-specific binding to the membranes being negligible. The dependence of the initial uptake rate of succinate on Na+ concentration exhibited sigmoidal kinetics, indicating interaction of more than one Na+ ion with the carrier system. The Hill coefficient for this ion was calculated to be 2.7. The Na+-dependent uptake of succinate was electrogenic, resulting in the transfer of positive charge across the membrane. Kinetic analysis showed that succinate uptake in these vesicles occurred via a single transport system, with an apparent affinity constant of 4.8 +/- 0.2 microM and a maximal velocity of 274 +/- 4 pmol/20 s per mg of protein. Uptake of succinate was strongly inhibited by various C4 or C5 dicarboxylic acids, whereas monocarboxylic acids, amino acids and glucose showed little or no effect. Li+ and K+ could not substitute for Na+ in the uptake process. Instead, Li+ was found to have a significant inhibitory effect on the Na+-dependent uptake of succinate.
authors.affiliation: Department of Cell and Molecular Biology, Medical College of Georgia, Augusta 30912.
authors.forename: V
authors.initials: V
authors.lastname: Ganapathy
authors.forename: M E
authors.initials: ME
authors.lastname: Ganapathy
authors.forename: C
authors.initials: C
authors.lastname: Tiruppathi
authors.forename: Y
authors.initials: Y
authors.lastname: Miyamoto
authors.forename: V B
authors.initials: VB
authors.lastname: Mahesh
authors.forename: F H
authors.initials: FH
authors.lastname: Leibach
mesh_terms: D001693:Biological Transport, Active / Q000187:drug effects; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism; D010920:Placenta / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D011247:Pregnancy; D012964:Sodium / Q000494:pharmacology*; D013379:Substrate Specificity; D013386:Succinates / Q000493:pharmacokinetics*; D019802:Succinic Acid
chemical_list: D013386:Succinates; D008094:Lithium; D012964:Sodium; D019802:Succinic Acid; D011188:Potassium