title: Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms.
abstract: Uptake of system L amino acid substrates into isolated placental plasma membrane vesicles in the absence of opposing side amino acid (zero-trans uptake) is incompatible with the concept of obligatory exchange, where influx of amino acid is coupled to efflux. We therefore hypothesized that system L amino acid exchange transporters are not fully obligatory and/or that amino acids are initially present inside the vesicles. To address this, we combined computational modeling with vesicle transport assays and transporter localization studies to investigate the mechanisms mediating [(14)C]L-serine (a system L substrate) transport into human placental microvillous plasma membrane (MVM) vesicles. The carrier model provided a quantitative framework to test the 2 hypotheses that l-serine transport occurs by either obligate exchange or nonobligate exchange coupled with facilitated transport (mixed transport model). The computational model could only account for experimental [(14)C]L-serine uptake data when the transporter was not exclusively in exchange mode, best described by the mixed transport model. MVM vesicle isolates contained endogenous amino acids allowing for potential contribution to zero-trans uptake. Both L-type amino acid transporter (LAT)1 and LAT2 subtypes of system L were distributed to MVM, with L-serine transport attributed to LAT2. These findings suggest that exchange transporters do not function exclusively as obligate exchangers.
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Kate L
authors.initials: KL
authors.lastname: Widdows
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Nuttanont
authors.initials: N
authors.lastname: Panitchob
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Ian P
authors.initials: IP
authors.lastname: Crocker
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Colin P
authors.initials: CP
authors.lastname: Please
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Hanson
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Colin P
authors.initials: CP
authors.lastname: Sibley
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Edward D
authors.initials: ED
authors.lastname: Johnstone
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Bram G
authors.initials: BG
authors.lastname: Sengers
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom rohan.lewis@soton.ac.uk.
authors.forename: Rohan M
authors.initials: RM
authors.lastname: Lewis
authors.affiliation: *Maternal and Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, United Kingdom; St. Mary's Hospital and Central Manchester University Hospitals National Health Service Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom; Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, United Kingdom; Mathematical Institute, Oxford University, Oxford, United Kingdom; and Faculty of Medicine, and Institute for Life Sciences, University of Southampton, Southampton, United Kingdom.
authors.forename: Jocelyn D
authors.initials: JD
authors.lastname: Glazier
mesh_terms: D027182:Amino Acid Transport System y+ / Q000378:metabolism; D000596:Amino Acids / Q000378:metabolism* / Q000493:pharmacokinetics; D001692:Biological Transport; D015153:Blotting, Western; D002250:Carbon Radioisotopes; D002462:Cell Membrane / Q000378:metabolism*; D003198:Computer Simulation; D005260:Female; D005455:Fluorescent Antibody Technique; D027301:Fusion Regulatory Protein 1, Light Chains / Q000378:metabolism; D006801:Humans; D027283:Large Neutral Amino Acid-Transporter 1 / Q000378:metabolism; D008871:Microvilli / Q000378:metabolism; D008954:Models, Biological; D010920:Placenta / Q000166:cytology / Q000378:metabolism; D011247:Pregnancy; D012694:Serine / Q000378:metabolism / Q000493:pharmacokinetics; D022161:Transport Vesicles / Q000378:metabolism
chemical_list: D027182:Amino Acid Transport System y+; D000596:Amino Acids; D002250:Carbon Radioisotopes; D027301:Fusion Regulatory Protein 1, Light Chains; D027283:Large Neutral Amino Acid-Transporter 1; C420646:SLC7A8 protein, human; D012694:Serine
keywords: LAT2 (SLC7A8); antiporters; facilitated transport; overshoot phenomena