title: Adenosine transport by plasma membrane monoamine transporter: reinvestigation and comparison with organic cations.
abstract: The plasma membrane monoamine transporter (PMAT) belongs to the equilibrative nucleoside transporter family (solute carrier 29) and was alternatively named equilibrative nucleoside transporter 4. Previous studies from our laboratory characterized PMAT as a polyspecific organic cation transporter that minimally interacts with nucleosides. Recently, PMAT-mediated uptake of adenosine (a purine nucleoside) was reported, and the transporter was proposed to function as a dual nucleoside/organic cation transporter. To clarify the substrate specificity of PMAT, we comprehensively analyzed the transport activity of human PMAT toward nucleosides, nucleobases, and organic cations in heterologous expression systems under well controlled conditions. Among 12 naturally occurring nucleosides and nucleobases, only adenosine was significantly transported by PMAT. PMAT-mediated adenosine transport is saturable, pH-dependent, and membrane-potential sensitive. Under both neutral (pH 7.4) and acidic (pH 6.6) conditions, adenosine is transported by PMAT at an efficiency (V(max)/K(m)) at least 10-fold lower than that of the organic cation substrates 1-methyl-4-phenylpyridinium and serotonin. PMAT-mediated adenosine uptake rate was significantly enhanced by an acidic extracellular pH. However, the effect of acidic pH was not adenosine-specific but was common to organic cation substrates as well. Our results demonstrated that although PMAT transports adenosine, the transporter kinetically prefers organic cation substrates. Functionally, PMAT should be viewed as a polyspecific organic cation transporter rather than an archetypical nucleoside transporter.
authors.affiliation: University of Washington, Health Sciences Building, Seattle, WA 98195-7610, USA.
authors.forename: Mingyan
authors.initials: M
authors.lastname: Zhou
authors.forename: Haichuan
authors.initials: H
authors.lastname: Duan
authors.forename: Karen
authors.initials: K
authors.lastname: Engel
authors.forename: Li
authors.initials: L
authors.lastname: Xia
authors.forename: Joanne
authors.initials: J
authors.lastname: Wang
mesh_terms: D015655:1-Methyl-4-phenylpyridinium / Q000378:metabolism* / Q000493:pharmacokinetics; D000241:Adenosine / Q000378:metabolism* / Q000493:pharmacokinetics; D000818:Animals; D001692:Biological Transport; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D033705:Equilibrative Nucleoside Transport Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D005260:Female; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials; D009865:Oocytes / Q000378:metabolism; D027701:Organic Cation Transport Proteins / Q000378:metabolism / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012701:Serotonin / Q000378:metabolism* / Q000493:pharmacokinetics; D013379:Substrate Specificity; D014162:Transfection; D014982:Xenopus laevis
chemical_list: D033705:Equilibrative Nucleoside Transport Proteins; D027701:Organic Cation Transport Proteins; C495988:SLC29A4 protein, human; D012701:Serotonin; D000241:Adenosine; D015655:1-Methyl-4-phenylpyridinium