title: Benzoic acid and specific 2-oxo acids activate hepatic efflux of glutamate at OAT2.
abstract: The liver is the principal source of glutamate in blood plasma. Recently we have discovered that efflux of glutamate from hepatocytes is catalyzed by the transporter OAT2 (human gene symbol SLC22A7). Organic anion transporter 2 (OAT2) is an integral membrane protein of the sinusoidal membrane domain; it is primarily expressed in liver and much less in kidney, both in rats and humans. Many years ago, Häussinger and coworkers have demonstrated in isolated perfused rat liver that benzoic acid or specific 2-oxo acid analogs of amino acids like e.g. 2-oxo-4-methyl-pentanoate ('2-oxo-leucine') strongly stimulate release of glutamate (up to 7-fold); '2-oxo-valine' and the corresponding amino acids were without effect. The molecular mechanism of efflux stimulation has remained unclear. In the present study, OAT2 from human and rat were heterologously expressed in 293 cells. Addition of 1 mmol/l benzoic acid to the external medium increased OAT2-specific efflux of glutamate up to 20-fold; '2-oxo-leucine' was also effective, but not '2-oxo-valine'. Similar effects were seen for efflux of radiolabeled orotic acid. Expression of OAT2 did not increase uptake of benzoic acid; thus, benzoic acid is no substrate, and trans-stimulation can be excluded. Instead, further experiments suggest that increased efflux of glutamate is caused by direct interaction of benzoic acid and specific 2-oxo acids with OAT2. We propose that stimulators bind to a distinct extracellular site and thereby accelerate relocation of the empty substrate binding site to the intracellular face. Increased glutamate efflux at OAT2 could be the main benefit of benzoate treatment in patients with urea cycle defects.
authors.affiliation: Department of Pharmacology, University of Cologne, Gleueler Straße 24, 50931 Cologne, Germany.
authors.forename: Till
authors.initials: T
authors.lastname: Pfennig
authors.forename: Beate
authors.initials: B
authors.lastname: Herrmann
authors.forename: Tim
authors.initials: T
authors.lastname: Bauer
authors.forename: Edgar
authors.initials: E
authors.lastname: Schömig
authors.forename: Dirk
authors.initials: D
authors.lastname: Gründemann
mesh_terms: D000143:Acids / Q000737:chemistry*; D000818:Animals; D019817:Benzoic Acid / Q000737:chemistry / Q000494:pharmacology*; D001665:Binding Sites; D001692:Biological Transport; D004305:Dose-Response Relationship, Drug; D018698:Glutamic Acid / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D007668:Kidney / Q000378:metabolism; D008024:Ligands; D008099:Liver / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D029362:Organic Anion Transporters, Sodium-Independent / Q000378:metabolism*; D051381:Rats; D013379:Substrate Specificity; D013997:Time Factors; D014633:Valine / Q000737:chemistry
chemical_list: D000143:Acids; D008024:Ligands; D029362:Organic Anion Transporters, Sodium-Independent; C490280:SLC22A7 protein, human; D018698:Glutamic Acid; D019817:Benzoic Acid; D014633:Valine