Adenosine transport by plasma membrane monoamine transporter: reinvestigation and comparison with organic cations
Mingyan Zhou1, Haichuan Duan, Karen Engel
1University of Washington, Health Sciences Building, Seattle, WA 98195-7610, USA.
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.
Insights
The plasma membrane monoamine transporter (PMAT) transports adenosine but prefers organic cations. This study clarifies PMAT
Area of Science:
- Biochemistry
- Molecular Biology
- Pharmacology
Background:
- The plasma membrane monoamine transporter (PMAT), also known as solute carrier 29A4, is characterized as a polyspecific organic cation transporter.
- Recent findings suggested PMAT might also transport nucleosides, proposing a dual function.
- Clarifying PMAT's substrate specificity is crucial for understanding its physiological and pharmacological roles.
Purpose of the Study:
- To comprehensively analyze the substrate specificity of human PMAT for nucleosides, nucleobases, and organic cations.
- To determine if PMAT functions as a dual nucleoside/organic cation transporter or primarily as an organic cation transporter.
- To elucidate the kinetic properties and pH-dependence of PMAT-mediated transport.
Main Methods:
- Utilized heterologous expression systems to study human PMAT transport activity.
- Tested transport of 12 naturally occurring nucleosides and nucleobases, alongside known organic cation substrates.
- Performed kinetic analyses (Vmax, Km) and assessed pH and membrane potential sensitivity.
Main Results:
- PMAT significantly transported only adenosine among the tested nucleosides and nucleobases.
- Adenosine transport by PMAT was saturable, pH-dependent, and membrane-potential sensitive.
- PMAT exhibited at least 10-fold lower transport efficiency for adenosine compared to organic cations like MPP+ and serotonin, with acidic pH enhancing uptake for both substrate types.
Conclusions:
- PMAT demonstrates a clear kinetic preference for organic cation substrates over adenosine.
- While PMAT can transport adenosine, it functions predominantly as a polyspecific organic cation transporter.
- The findings necessitate a re-evaluation of PMAT's classification and functional role in cellular transport.
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