title: Movement of NH₃ through the human urea transporter B: a new gas channel.
abstract: Aquaporins and Rh proteins can function as gas (CO₂ and NH₃) channels. The present study explores the urea, H₂O, CO₂, and NH₃ permeability of the human urea transporter B (UT-B) (SLC14A1), expressed in Xenopus oocytes. We monitored urea uptake using [¹⁴C]urea and measured osmotic water permeability (Pf) using video microscopy. To obtain a semiquantitative measure of gas permeability, we used microelectrodes to record the maximum transient change in surface pH (ΔpHS) caused by exposing oocytes to 5% CO₂/33 mM HCO₃⁻ (pHS increase) or 0.5 mM NH₃/NH₄⁺ (pHS decrease). UT-B expression increased oocyte permeability to urea by >20-fold, and Pf by 8-fold vs. H₂O-injected control oocytes. UT-B expression had no effect on the CO₂-induced ΔpHS but doubled the NH₃-induced ΔpHS. Phloretin reduced UT-B-dependent urea uptake (Jurea*) by 45%, Pf* by 50%, and (- ΔpHS*)NH₃ by 70%. p-Chloromercuribenzene sulfonate reduced Jurea* by 25%, Pf* by 30%, and (ΔpHS*)NH₃ by 100%. Molecular dynamics (MD) simulations of membrane-embedded models of UT-B identified the monomeric UT-B pores as the main conduction pathway for both H₂O and NH₃ and characterized the energetics associated with permeation of these species through the channel. Mutating each of two conserved threonines lining the monomeric urea pores reduced H₂O and NH₃ permeability. Our data confirm that UT-B has significant H₂O permeability and for the first time demonstrate significant NH₃ permeability. Thus the UTs become the third family of gas channels. Inhibitor and mutagenesis studies and results of MD simulations suggest that NH₃ and H₂O pass through the three monomeric urea channels in UT-B.
authors.affiliation: Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106, USA. rrg8@cwru.edu
authors.forename: R Ryan
authors.initials: RR
authors.lastname: Geyer
authors.forename: Raif
authors.initials: R
authors.lastname: Musa-Aziz
authors.forename: Giray
authors.initials: G
authors.lastname: Enkavi
authors.forename: P
authors.initials: P
authors.lastname: Mahinthichaichan
authors.forename: Emad
authors.initials: E
authors.lastname: Tajkhorshid
authors.forename: Walter F
authors.initials: WF
authors.lastname: Boron
mesh_terms: D002731:4-Chloromercuribenzenesulfonate / Q000494:pharmacology; D019943:Amino Acid Substitution; D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D002245:Carbon Dioxide / Q000378:metabolism; D005740:Gases / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D056004:Molecular Dynamics Simulation; D009865:Oocytes; D009995:Osmosis; D010539:Permeability / Q000187:drug effects; D010693:Phloretin / Q000494:pharmacology; D014508:Urea / Q000378:metabolism*; D014867:Water / Q000378:metabolism; D014981:Xenopus; D000096982:Urea Transporters
chemical_list: D002731:4-Chloromercuribenzenesulfonate; D000641:Ammonia; D002245:Carbon Dioxide; D005740:Gases; D026901:Membrane Transport Proteins; D010693:Phloretin; D014508:Urea; D014867:Water; D000096982:Urea Transporters
keywords: ammonia transport; carbon dioxide transport; membrane protein; urea transport; water transport
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