title: OAT2 catalyses efflux of glutamate and uptake of orotic acid.
abstract: OAT (organic anion transporter) 2 [human gene symbol SLC22A7 (SLC is solute carrier)] is a member of the SLC22 family of transport proteins. In the rat, the principal site of expression of OAT2 is the sinusoidal membrane domain of hepatocytes. The particular physiological function of OAT2 in liver has been unresolved so far. In the present paper, we have used the strategy of LC (liquid chromatography)-MS difference shading to search for specific and cross-species substrates of OAT2. Heterologous expression of human and rat OAT2 in HEK (human embryonic kidney)-293 cells stimulated accumulation of the zwitterion trigonelline; subsequently, orotic acid was identified as an excellent and specific substrate of OAT2 from the rat (clearance=106 μl·min⁻¹·mg of protein⁻¹) and human (46 μl·min⁻¹·mg of protein⁻¹). The force driving uptake of orotic acid was identified as glutamate antiport. Efficient transport of glutamate by OAT2 was directly demonstrated by uptake of [³H]glutamate. However, because of high intracellular glutamate, OAT2 operates as glutamate efflux transporter. Thus expression of OAT2 markedly increased the release of glutamate (measured by LC-MS) from cells, even without extracellular exchange substrate. Orotic acid strongly trans-stimulated efflux of glutamate. We thus propose that OAT2 physiologically functions as glutamate efflux transporter. OAT2 mRNA was detected, after laser capture microdissection of rat liver slices, equally in periportal and pericentral regions; previous reports of hepatic release of glutamate into blood can now be explained by OAT2 activity. A specific OAT2 inhibitor could, by lowering plasma glutamate and thus promoting brain-to-blood efflux of glutamate, alleviate glutamate exotoxicity in acute brain conditions.
authors: Department of Pharmacology, University of Cologne, Gleueler Straße 24, 50931 Cologne, Germany.
authors: Christian
authors: C
authors: Fork
authors: Tim
authors: T
authors: Bauer
authors: Stefan
authors: S
authors: Golz
authors: Andreas
authors: A
authors: Geerts
authors: Jessica
authors: J
authors: Weiland
authors: Domenico
authors: D
authors: Del Turco
authors: Edgar
authors: E
authors: Schömig
authors: Dirk
authors: D
authors: Gründemann
mesh_terms: D000470:Alkaloids / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active / Q000235:genetics; D020134:Catalytic Domain / Q000235:genetics; D002461:Cell Line, Transformed; D018698:Glutamic Acid / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D029362:Organic Anion Transporters, Sodium-Independent / Q000235:genetics / Q000378:metabolism*; D009963:Orotic Acid / Q000378:metabolism*; D051381:Rats; D013379:Substrate Specificity / Q000235:genetics
chemical_list: D000470:Alkaloids; D029362:Organic Anion Transporters, Sodium-Independent; C490280:SLC22A7 protein, human; C088800:Slc22a7 protein, rat; D018698:Glutamic Acid; C009560:trigonelline; D009963:Orotic Acid