title: Mechanisms and kinetics of uptake and efflux of L-methionine in an intestinal epithelial model (Caco-2).
abstract: Mechanism and kinetics of uptake and efflux of L-methionine in a human intestinal epithelial model system (Caco-2) were studied to understand the transcellular transport process and to determine its rate-limiting step. The kinetic studies indicated that uptakes from both the apical and basolateral sides were saturable [for apical uptake: Km = 0.96 mmol/L, maximum flux = 673 pmol/(min.cm2); for basolateral uptake: Km = 3.46 mmol/L, maximum flux = 3480 pmol/(min.cm2)], whereas the efflux from these two membranes was apparently linear for intracellular concentrations < 6.5 mmol/L [for apical efflux, the apparent first-order rate constant = 1.01 x 10(-4) cm/min; for basolateral efflux, the rate constant = 1.78 x 10(-4) cm/min]. The results of inhibition studies indicate the apical uptake is partially active and Na(+)-dependent via a combination of amino acid transport systems B0,+ and ASC, which is somewhat different from the less energy- and Na(+)-dependent basolateral uptake. The basolateral uptake is also more dependent on system L and exhibits high counter-exchange capability. Finally, the rate-limiting step in the apical to basolateral transport of methionine is determined to be the basolateral efflux.
authors.affiliation: Department of Pharmaceutical Sciences, College of Pharmacy, Washington State University, Pullman 99164-6510.
authors.forename: J
authors.initials: J
authors.lastname: Chen
authors.forename: Y
authors.initials: Y
authors.lastname: Zhu
authors.forename: M
authors.initials: M
authors.lastname: Hu
mesh_terms: D000230:Adenocarcinoma; D000584:Amiloride / Q000494:pharmacology; D000596:Amino Acids / Q000494:pharmacology; D001667:Binding, Competitive; D001692:Biological Transport / Q000187:drug effects; D004848:Epithelium / Q000378:metabolism; D006801:Humans; D007408:Intestinal Absorption; D007414:Intestinal Neoplasms; D007700:Kinetics; D008715:Methionine / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D000596:Amino Acids; D010042:Ouabain; D000584:Amiloride; D012964:Sodium; D008715:Methionine