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 (Vmax/Km) 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.
Footnotes
This work was supported by the National Institutes of Health National Institute of General Medical Sciences [Grant GM066233].
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.110.032987.
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ABBREVIATIONS:
- SLC
- solute carrier
- ENT
- equilibrative nucleoside transporter
- NBMPR
- nitrobenzylmercaptopurine ribonucleoside
- PMAT
- plasma membrane monoamine transporter
- MPP+
- 1-methyl-4-phenylpyridinium
- MDCK
- Madin-Darby canine kidney
- KRH
- Krebs-Ringer-Henseleit
- RT
- reverse transcriptase
- PCR
- polymerase chain reaction
- h
- human.
- Received February 24, 2010.
- Accepted June 30, 2010.
- Copyright © 2010 by The American Society for Pharmacology and Experimental Therapeutics
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