Abstract
The human and Drosophila serotonin transporters (hSERT and dSERT, respectively) were used to explore differences in substrate properties. hSERT and dSERT showed similar Km values for 5-hydroxytryptamine (5-HT; serotonin) transport (1.2 and 0.9 μM, respectively), suggesting similar recognition of 5-HT by the two species variants. Although dSERT cell surface expression was approximately 8-fold lower than that of hSERT, dSERT does appear to have a 2-fold faster turnover number for inward transport of 5-HT. Interestingly, another substrate, N-methyl-4-phenylpyridinium (MPP+), was transported only by hSERT. However, MPP+ inhibited 5-HT uptake in both species variants with similar potencies. Two cross-species chimeras, H1–118D119–627 and H1–281D282–476H477–638, were also unable to transport MPP+, implicating the role of transmembrane domains V to IX in the substrate permeation pathway. Based on exchange experiments, certain substituted-amphetamines also appear to be poor substrates at dSERT. Two-electrode voltage-clamp studies in oocytes confirmed that the amphetamines do not possess substrate-like properties for dSERT. Our data suggest distinct molecular recognition among SERT substrate classes that influence translocation mechanisms.
Footnotes
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This work was supported by an American Psychological Association Minority Fellowship in Neuroscience (G.J.R.) and National Institute of Mental Health Grant MH60221 (E.L.B.).
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DOI: 10.1124/jpet.103.048751.
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ABBREVIATIONS: 5-HT, 5-hydroxytryptamine (serotonin); SERT, serotonin transporter (h, human; d, Drosophila); TMD, transmembrane domain; MPP+, N-methyl-4-phenylpyridinium; HEK, human embryonic kidney; KRH, Krebs/Ringer/Hepes; MMAI, 5-methoxy-6-methyl-2-aminoindan; DAT, dopamine transporter.
- Received January 6, 2003.
- Accepted April 7, 2003.
- The American Society for Pharmacology and Experimental Therapeutics
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