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Benzoic acid and specific 2-oxo acids activate hepatic efflux of glutamate at OAT2.
MedLine Citation:
PMID:  22981274     Owner:  NLM     Status:  Publisher    
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 (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 1mmol/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.
Till Pfennig; Beate Herrmann; Tim Bauer; Edgar Schömig; Dirk Gründemann
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-9-7
Journal Detail:
Title:  Biochimica et biophysica acta     Volume:  -     ISSN:  0006-3002     ISO Abbreviation:  Biochim. Biophys. Acta     Publication Date:  2012 Sep 
Date Detail:
Created Date:  2012-9-17     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  0217513     Medline TA:  Biochim Biophys Acta     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
Copyright Information:
Copyright © 2012. Published by Elsevier B.V.
Department of Pharmacology, University of Cologne, Gleueler Straße 24, 50931 Cologne, Germany.
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